73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: Monkey Uncle on February 28, 2017, 08:19:02 PM
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Sorry this post is probably going to be long, but I want to be sure I don't leave out anything important.
Bought my truck a couple of months ago from a guy who assembled it from various donor vehicles.
'85 C 10 body
'78 GMC chassis, axles, and engine; 350 V8, 4WD
turbo 350 automatic transmission of unknown year
Was told the engine had 124k and the transmission had 78k, but no documentation.
Ran well upon test drive, though I noticed a not-too-loud ticking noise. Rapid, regular, and correlated with RPMs. Seller dismissed it as "probably a lifter," although it sounded like it was coming from down low and toward the rear of the engine. The price was good and the truck otherwise appeared to be in great shape, so I took a chance. I wasn't looking for a show truck, but rather an inexpensive old truck for local hauling, probably 1,000 mi/yr or less. I figured even if I had to replace the entire drive train, I'd come out better than buying something newer. The fact that it could pass for a "20-footer" was just a bonus. Dickered down to 5 grand and drove it home. The 75 mi trip home was uneventful.
Recently I decided I wasn't comfortable just ignoring the ticking noise, so I did a little googling. Based on various old forum posts and youtube's best minds, I figured I had some sort of issue with the flex plate or torque converter (based on the apparent location of the noise). Took it to my favorite mechanic for cheap old cars. I trust him because he has a good reputation in a small town where everyone knows everyone, and because he's done work for me before for very reasonable prices. His first instinct was collapsed lifter. I was skeptical given where the sound seemed to be coming from, but I figured since he's worked on cars 6 days a week for 25 years, I'd defer to his opinion and let him have a crack at it.
It took him a while to finally get around to taking a serious look at it. I called him today and asked him what was up. He said, "bad news, it's a wrist pin." Says he listened with a stethoscope and could tell that the sound is moving up and down. He doesn't do internal engine work, but said he could quote me a price on a new crate engine if I wanted to go that route. He mentioned another guy across town who might be willing to tackle fixing the old engine, but he noted that he doesn't really know of anyone else locally who does major internal work.
So, choices seem to be:
1. See if I can find someone to confirm that it is indeed a wrist pin, and tackle the repair. If I go this route, would it make sense to just go ahead and do a re-build? Seems kind of silly to pull a 40 year old engine and tear it down just to replace one part.
2. Take Mr. Mechanic up on the crate engine option.
3. See if I can find a used engine and see if Mr. Mechanic would do the swap for me. Not sure of my odds of finding a used small block V8 that isn't just as worn out as the current one.
4. What if I just keep driving it? I presume if it really is a wrist pin, it will break eventually and the engine will lock up. Then I have to invoke option 2 or 3, plus a tow and the inconvenience of getting stranded somewhere (with my luck, probably in the middle of the woods somewhere with a full load of firewood).
As you can probably tell if you've stuck with me this long, I'm not really up to tackling the job myself. Although I understand basic anatomy and function, I don't have the experience, tools, shop facility, or time to tear down or swap an engine.
But on the bright side, labor rates are relatively cheap in this area, with independent shops running about 50 bucks an hour.
Well, that's about all I can think of at this point. Any help is greatly appreciated.
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If you were swapping the motors yourself, I would roll the dice on a $350-$500 used motor off Craigslist. My opinion is that if you are paying for the labor (as you plan to) I would go with a crate/rebuilt motor. You don't want to pay the mechanic twice if the cheap used motor doesn't work out.
You can buy the GM Goodwrench 350/260 HP motor for around $1500. A few guys on here run that motor and are happy with it.
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I'm sure you probably looked into it already but I've heard a similar clicking sound from a cracked exhaust manifold. There are some YouTube videos where you can hear the sound and that demonstrate an easy fix with furnace cement. Good luck!
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Never heard of anyone making a positive diagnosis of a wrist pin with a stethoscope.
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If you've eliminated the flexplate as the culprit and in fact are hearing a sharp knock you can waste time trying to determine which cylinder it is or if it's mains but regardless the engine needs to be rebuilt or replaced. Where do you live that $50/hour is the labor charge? If it's that cheap a new engine won't cost you much to have installed.
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I'm sure you probably looked into it already but I've heard a similar clicking sound from a cracked exhaust manifold. There are some YouTube videos where you can hear the sound and that demonstrate an easy fix with furnace cement. Good luck!
Yeah, I'm pretty sure it isn't an exhaust leak. I've had those before with my previous truck and am familiar with what they sound like. It's hard to describe in words, but the sound quality is different. The current noise has a sharp, hard tapping quality to it.
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Never heard of anyone making a positive diagnosis of a wrist pin with a stethoscope.
I don't know that he was 100% certain, but that was his conclusion based on the fact that he said he could hear the sound moving up and down within the engine block. I wasn't there when he listened with the scope, so I can't vouch for the validity of the conclusion.
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If you've eliminated the flexplate as the culprit and in fact are hearing a sharp knock you can waste time trying to determine which cylinder it is or if it's mains but regardless the engine needs to be rebuilt or replaced. Where do you live that $50/hour is the labor charge? If it's that cheap a new engine won't cost you much to have installed.
Rural WV. You certainly won't get $50/hr at a dealership, but it's possible to find small independent shops that will work for that price.
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Good luck with that.
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1)The engine is actually 39 years old, not 40. :)
2) Is the engine otherwise smoking and/or using oil and/or using coolant?
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1)The engine is actually 39 years old, not 40. :)
2) Is the engine otherwise smoking and/or using oil and/or using coolant?
It smokes just a little on startup, but not while running or idling. No drop in the oil level, despite a small leak somewhere near the back of the engine. But I haven't put many miles on it, couple hundred at most.
No drop in coolant level.
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Just tried the spark plug wire test. Several random people on the internet say that a wrist pin will stop making noise if you cut the power to the affected cylinder. So I pulled the spark plug wires one by one to see if the noise would go away. I went through all 8 cylinders twice, but the noise never went away. It varied a little in intensity over the course of the test, but it does that anyway as the engine warms up. And the intensity never made an obvious, consistent change with any particular cylinder.
This makes me think the noise is not coming from the inside of a cylinder. But I don't really know what I'm doing; what do you think?
And just as an aside, I could hear a clacking noise coming from the top end whenever I had a wire loose. It would immediately go away as soon as I reconnected the wire. I presume something is going on with the valves or lifters whenever a cylinder loses power? Not that it has any bearing on the real problem; I just thought it was odd.
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Tearing it down is the only way to know exactly what it is. Speculating is virtually a waste of time.
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Do you have AAA?
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Do you have AAA?
No. Are you suggesting just continuing to drive it until it blows?
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Well, now I'm thoroughly confused. :o
Took it for a drive today, out on the highway to test the odometer (another issue that I won't drag into this thread). Drove it a few miles at 60+ mph and then around town for a bit. Noise decreased in intensity as the engine warmed. After it was thoroughly warmed up, I made a couple of stops and noticed that I could not hear the noise at all from inside the cab when I was sitting still with the transmission in D or R. When I shifted to P, I could immediately hear it again, although not as loud as when the engine was cold. This behavior continued consistently until I got home and put it in the garage.
I know no one is going to definitively diagnose anything from what I'm writing here. But I can't help but wonder whether a major internal engine problem would behave like that. I really don't want to sink a couple grand into an engine swap or re-build if I don't have to.
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Could be worn pistons, which often are noisy when cold.
Maybe start socking money away, and if and when it blows up, throw in a rebuilt motor.
It will probably last for years, still making the same noise like that.
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Without hearing the "knock" it's pretty hard to do anything but confuse the issue. Measure the crankshaft end-play; should be 0.002" - 0.006". Carefully recheck the flexplate for cracks. You may need to unbolt the torque converter and slide it rearward then bar the engine over with a flywheel spanner and look for rust tracks using a bright flashlight. Look closely across the entire face of the flexplate.
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^ This
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Do you have AAA?
No. Are you suggesting just continuing to drive it until it blows?
Pretty much. But make sure you have a back-up engine....or definitive plans/budget to get another engine. Or have a back-up car.
i mean, you said you didn't want to sink a couple grand into a new engine...... This may be the most economical way.
i kind of concur with what Skinny G said.
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Thanks for all your thoughts, guys. I talked to Mr. Mechanic again yesterday, and he says he's pretty sure it is a wrist pin. Says he inspected the flex plate and torque converter closely and didn't see any issues there. I'm going to talk to another guy today who supposedly does re-builds and see what my options are there.
Still thinking about the just-keep-driving-it approach. It's not my primary transportation and I don't put many miles on it, so although having to tow it from it's death site would be inconvenient and an added expense, I'm not too worried about it being out of commission for a while. The big thing that worries me about driving it til it blows is the potential for additional damage. If it goes catastrophic, I probably lose the block, which means an extra few hundred on the replacement engine due to not having a core to trade in. Also, I'm thinking the forces generated by a catastrophic failure could destroy other parts of the drive train, or even present a safety hazard due to flying parts.
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That was my reason for asking if the engine was smoking, or losing oil/coolant? If not, then that is another reason, in my opinion, to just keep driving until the engine quits.
While it isn't good to lose a block, a lot of the gm replacement engines---like the crate 350 don't have a core charge.
As far as the engine exploding and causing a safety hazard. In my opinion, not likely. Unless the crank comes out of the engine----which is unlikely.
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This is free and you can do it yourself:
Take your valve covers off and tighten all 16 of your rocker arm nuts 1/4 turn. This will determine if you have a loose lifter or any other valve train issue. This does not sound like a wrist pin or rod bearing to me.
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This is free and you can do it yourself:
Take your valve covers off and tighten all 16 of your rocker arm nuts 1/4 turn. This will determine if you have a loose lifter or any other valve train issue. This does not sound like a wrist pin or rod bearing to me.
Thanks for the suggestion. If I remove the valve covers, do I automatically have to replace the gaskets?
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This is free and you can do it yourself:
Take your valve covers off and tighten all 16 of your rocker arm nuts 1/4 turn. This will determine if you have a loose lifter or any other valve train issue. This does not sound like a wrist pin or rod bearing to me.
Thanks for the suggestion. If I remove the valve covers, do I automatically have to replace the gaskets?
No, but the odds are better than not that they will leak afterward. You can use RTV to reseal the valve covers, but that is the Bubba way. If you buy new gaskets then you will have peace of mind. Just don't over torque the valve covers. It's a mistake that is easily made on the perimeter bolt type.
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I agree with ENGINEER all the way, especially the valve covers, which also could be causing the
Oil leak on rear of engine. You could try some 20/50 wt oil. Thicker oil may make a diff.
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just getting in but did you eliminate the flex plate like vile and BD suggested? did you unbolt the torque converter and push it back away from the flexplate then start the engine? did you make sure your bolts were tight?
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just getting in but did you eliminate the flex plate like vile and BD suggested? did you unbolt the torque converter and push it back away from the flexplate then start the engine? did you make sure your bolts were tight?
I did not do that personally, but the mechanic who looked at it assured me that he thoroughly inspected the flex plate and torque converter and eliminated those as possibilities.
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the same mechanic who says you need a new wrist pin?
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the same mechanic who says you need a new wrist pin?
Yup. Believe me, that thought has crossed my mind.
I don't think he'd take me for a ride, given past experiences with him. He once did an exhaust system patch to fix a doughnut gasket leak on my last truck, when he could have easily told me that I needed a new manifold. He also did a very reasonably priced timing belt and water pump replacement on my son's car. But I guess you never know.
Says he listened with a stethoscope on the block and could tell that the noise was coming from the driver's side, but he couldn't localize it to a particular cylinder. When I told him about my spark plug wire test, he said you can't really tell anything from that because the parts are still moving up and down regardless of cutting the power to the cylinder. IDK; like I said, I know the basic anatomy, but I don't really have the experience to second guess his diagnosis.
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If you are trying to isolate a noise association with a particular cylinder then a a power balance test will certainly help you do just that. You generally want to bring the engine RPM to where you can hear the noise steadily. Short one cylinder at a time to find the culprit.
I would suggest you disconnect the torque converter from the flexplate and see what happens and follow bd's suggestions.
I have lots of "best mechanic" stories I can tell where they've overlooked the obvious time and time again.
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When you have eliminated the impossible, whatever remains, however improbable, must be the truth ~Sherlock Holmes
its in my Signature
your mechanic maybe very right but imo theres more dishonest ones or ones that just guess at your expense then good ones. my mom took her van to a shop that had been in business for a while. the old man past a few years before and the daughter took over. she was aways there when he was around and i think she was a good person but with no one honest in the shop to keep the guys straight they were doing some shady stuff.
moms van had a misfire on one of the cylinders, the van only has two coils so i swapped the coils thinking if the misfire followed it was a bad coil. well it didnt follow, she then took it to the shop they "tested" the coil the misfire was on and it "tested" bad. so they replaced it and it didnt fix the issue (spark plug boot had loosened up and arced the plug to the point it couldnt make good contact). there was also an issue with the EGR passages they said they cleaned. but when i found the messed up spark plug and boot i also found they never took the throttle body off to clean the EGR port. got the guy to admit that he cleaned it and when i asked how did you do it without taking the throttle body off he said he has a special tool that can do a 90° turn and clean out carbon. i then pointed out the fact if this tool did exist he just knocked carbon down into the valves. he got this dumb look on his face and turned around and walked out.
sorry for the story but it was to make a point, a shop can be nice and charge a good price on a timing belt but a bigger job like changing the engine they can make more money off you especially if its a crack flex plate or something simple.
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Look. There isn't a technician out there who hasn't identified the wrong cause of a symptom. We've all done it more than once. Anyone who claims otherwise is lying. Honest professionals will dust themselves off and learn from the experience, refocus and persist to find the problem, or direct you to someone who can. Those who only claim to be professional will continue to live the lie.
I do not imply that your mechanic is "taking you for a ride." But, neither am I, or any of us, convinced he has sufficiently pursued the diagnosis to arrive to the correct conclusion with any degree of reliable certainty. More likely, he's reaching for an answer, because he simply has run out of options... or willingness. He said it himself, he thinks it's a wrist pin, but has no way to confirm it. So, essentially, he's guessing. That in itself doesn't make him right or wrong. His diagnosis simply is incomplete.
If you trust this particular mechanic, I'm certain you have good reason based on the routine services he's provided in the past. But, some of the assurances you claim he's made are in question when faced with extensive training and experience. Candidly, I think he's at the edge of his comfort zone. A power balance test is a tried and true method of diagnosis that works hand-in-hand with other diagnostic procedures and direct observation to come to a viable conclusion.
Although wrist pin failures are uncommon, they do occur. However, there are other failures that can easily be confused, sharing similar (not identical) symptoms. Since none of us are there, we also are guessing based on what you've written. But, our guessing is more to the point that the diagnosis is inconclusive. At this point the best approach is to be methodical and start with the basics in a new unit of time, making no assumptions and "leaving no stone unturned" so to speak.
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the same mechanic who says you need a new wrist pin?
My thoughts exactly.
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Although I am relatively new to the board I can tell there are quite a few people on here that know a lot about what they are talking about. I am no expert but, I have been doing this kind of thing since I was 16. I am 52. Consider the years of experience in this one thread alone. We are only trying to help and we are all basically on the same page with our thoughts and comments. Consider our advice.
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righty tighty lefty loosey
(http://i.imgur.com/ySTRobY.gif)
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Thanks very much for all your thoughtful input, guys. I really appreciate your willingness to help out a newbie. And your willingness to stick with a thread that doesn't seem to have any prospects for a clear resolution. Maybe someday I'll know enough to help out someone else here.
I hear what you're saying about the possibility that my mechanic is merely guessing. Unfortunately I'm not any better at guessing than he is. About the only tool I have left in my arsenal at this point is checking out the flex plate and torque converter myself. Two questions on that:
(1) Unbolting the torque converter and sliding it backward - does that involve dropping or partially dropping the transmission? Or is there enough space to just slide it back?
(2) Would I need to use a torque wrench when re-attaching the torque converter? Loc-tite also?
I guess the other tool I have in my arsenal is taking it to another mechanic for a second opinion. That might be interesting.
Your thoughts on the power balance test inspired me to do some more googling. I'd never heard that name applied to the test, so using it in the search dug up some stuff I hadn't seen before. This explanation in relation to engine noises suggests that piston and wrist pin noises actually can increase (or stay the same, but not decrease) during a power balance test:
http://www.autozone.com/repairinfo/common/repairInfoMain.jsp?targetPage=productsHowToInspect&leftNavPage=productsHowTo&pageId=%090996b43f80a011e8&subtitle=inspect (http://www.autozone.com/repairinfo/common/repairInfoMain.jsp?targetPage=productsHowToInspect&leftNavPage=productsHowTo&pageId=%090996b43f80a011e8&subtitle=inspect)
There are several different types of piston noise resulting from cracked pistons, piston slap, excess piston pin clearance, and other causes.
•Retarding the spark timing will generally reduce the intensity of piston noise.
•Grounding out the plug during the power balance test can increase a piston noise. This is the opposite result from the way a bad connecting rod reacts to ignition grounding.
Piston Pin Noise
•Noise from excessive piston pin clearance makes a "double click" sound at idle or fast idle.
•Pin noise often becomes more intense after the installation of new piston rings. The noise will gradually become less as the rings wear.
•With the engine running at the speed where the most noise occurs, grounding the plug wire will sometimes increase the noise even more. The noise level might not increase, but it will not become less. Piston inertia causes the noise, which is why the noise does not go away when the the spark plug is shorted out.
•The noise usually becomes less or goes away when the engine warms up.
That description seems consistent with my experience. But again, more guessing.
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righty tighty lefty loosey
(http://i.imgur.com/ySTRobY.gif)
;) ;)
Excellent advice!!!
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As to your question on the flexplate/torque converter.......just drop the dust cover and unbolt the flexplate from the torque converter. Nothing to do with dropping the transmission at all. There is space enough to slide it rearward so it is away from the flexplate. There is a torque for every fastener. Personally I don't know what the torque for the t/c bolts off the top of my head. Also I do use loctite on torque converter bolts but, not everyone does. I am not sure what the service manual calls for but both of those questions can be answered by looking it up in the service manual. There are some to look at here on this site in PDF format. I would download the one for yours and do some reading.
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As to your question on the flexplate/torque converter.......just drop the dust cover and unbolt the flexplate from the torque converter. Nothing to do with dropping the transmission at all. There is space enough to slide it rearward so it is away from the flexplate. There is a torque for every fastener. Personally I don't know what the torque for the t/c bolts off the top of my head. Also I do use loctite on torque converter bolts but, not everyone does. I am not sure what the service manual calls for but both of those questions can be answered by looking it up in the service manual. There are some to look at here on this site in PDF format. I would download the one for yours and do some reading.
Thanks.
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Also can you make a video of the engine running and post it?
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I would like to hear it also.
You are quite welcome Monkey uncle.
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O.K., haven't posted video before, so this might be a little clunky.
This is underneath when the engine is cold.
https://youtu.be/gamzysJv11E (https://youtu.be/gamzysJv11E)
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That seems to have worked, so let's try a few more:
cold engine from above
https://youtu.be/9OB5iDZqhww (https://youtu.be/9OB5iDZqhww)
cold engine from inside cab, trans in park
https://youtu.be/yqBAR7nILok (https://youtu.be/yqBAR7nILok)
cold engine from inside cab, trans in reverse (sitting still)
https://youtu.be/Vq_5-FRSNzc (https://youtu.be/Vq_5-FRSNzc)
warm engine from inside cab, trans in park then shifting to reverse
https://www.youtube.com/watch?v=IX0-8YyEUsg (https://www.youtube.com/watch?v=IX0-8YyEUsg)
warm engine from underneath
https://youtu.be/VySNsYp9Cac (https://youtu.be/VySNsYp9Cac)
warm engine from above
https://youtu.be/ApCATqp3gsQ (https://youtu.be/ApCATqp3gsQ)
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links don't work........
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I needed a gm crate 350, got one from summit. $1200, no interest. Runs awesome.
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links don't work........
Not sure what happened there. Seems to be fixed now - try again.
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Sounds like you have exhaust leaks and lifters out of adjustment/worn cam/lifters. I do not hear a wrist pin knock, main bearing or rod knock.
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I don't hear a deep knocking from the engine either. Some of the clips I couldn't hear anything abnormal. I don't think that we could hear on video what you are hearing up close. I wasn't sure that we would be able to though. The ne from underneath seems to have the most noise to it.
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I don't hear a deep knocking from the engine either. Some of the clips I couldn't hear anything abnormal. I don't think that we could hear on video what you are hearing up close. I wasn't sure that we would be able to though. The ne from underneath seems to have the most noise to it.
Yes, it's definitely more prominent from underneath. Pretty difficult to hear it from up top. That may be partly due to fan/accessory noise, but even trying to listen through that background noise, I don't get much from above. That's why I was suspicious of the lifter hypothesis from the beginning - I'm just not hearing anything emanating from the top end. When I did the power balance test, I could hear clattering in the heads (not sure why, but something obviously was going on there when I shorted out a cylinder). That makes me think that if the subject noise was lifter or valve-train related, I should be able to hear it from above. (Also, the mechanic initially suspected lifters, but ruled that out based on his diagnostic work.)
For some reason the sound didn't come through too well on the clips that were taken inside the cab. It is definitely audible from inside, though.
You're both correct that the sound is not a deep knock. It's best described as a tick or a tap.
I don't think it's an exhaust leak, though. I've experienced those before, both on my own vehicles and other people's vehicles. From what I've experienced, exhaust leaks do have a ticking quality, but they also have a "pffft" quality to the sound - like rushing air. I'm not hearing the "pffft" noise at all. Also, in my experience, exhaust leaks get noticeably louder when accelerating or when the engine is under a load. This noise doesn't do that. Previous exhaust leaks I've had haven't been audible at all at idle; this noise is most audible at idle. Everything behind the manifolds is brand new with no rust, so an exhaust leak would have to be coming from the manifold gasket, a cracked manifold, or an improperly installed doughnut gasket. The first two should be audible from above. And finally, the mechanic does lots of exhaust work (lots of road salt around here; everyone's exhaust system rusts out eventually). He's heard a lot of exhaust leaks and should have caught it if that's what it was.
Vile - I'm not disrespecting your experience at all; just passing along some information that I probably should have provided before you listened to the video clips.
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I am not disrespected at all so no need for that. I would suggest you buy a mechanics stethoscope and do some investigating yourself. Take the wand off and run the straw around the exhaust manifolds and you'll know it if there is an exhaust leak. Use the stethoscope probe to pinpoint where it is the loudest.
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Run it until it quits.
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I agree with stew grif, burn it out, then start fresh with a new crate 350. $1200, no interest, no tax, free shipping. I got my engine one year ago this month. I just paid it off yesterday. Not one penny interest. Haha, jokes on summit. (Not really, the house always wins)
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I agree with stew grif, burn it out, then start fresh with a new crate 350. $1200, no interest, no tax, free shipping. I got my engine one year ago this month. I just paid it off yesterday. Not one penny interest. Haha, jokes on summit. (Not really, the house always wins)
Summit's gone up since you bought yours. Seems like the cheapest 350 crate is now in the $1,500 - $1,600 range.
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Curious to know if the dust cover has been dropped yet and t/c slid back and motor ran. If you are sure you have no exhaust leaks and it has not been done yet, then that is the next logical step along with the other steps outlined by BD and VZ referencing the power balance test.
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Curious to know if the dust cover has been dropped yet and t/c slid back and motor ran. If you are sure you have no exhaust leaks and it has not been done yet, then that is the next logical step along with the other steps outlined by BD and VZ referencing the power balance test.
Discovered that I don't have the right size socket to fit the dust cover bolts. 9/16 is too small and 5/8 is too big. Is it possible GM could have used 15 mm metric on a TH 350 dust cover? 5/8 will fit the two bolts that attach the support braces to the dust cover, but is too big for all the others. The bolt heads look a little worn, but not so much that a properly sized socket shouldn't grab. Of course I don't have a metric socket set.
I know this is very basic stuff for you guys, but anything beyond an oil change and spark plugs is new territory for me.
As noted above, mechanic said he removed the dust cover and inspected the flex plate and torque converter and found no issues. He didn't say whether he disconnected the TC and ran the engine.
Crawled around under it and listened some more yesterday; confirmed again that the noise is most prominent down low and toward the back of the oil pan/dust cover area, or even the transmission pan area.
Here's something I haven't brought up before, because I didn't think it was relevant. But as long as we're being thorough, may as well fill you in. At some point in the past, the tip broke off of the old transmission dipstick (I stuck a new one in the tube so I can check the fluid level). Every person and info source I've consulted says that the broken off tip will rest harmlessly in the trans pan until the next time the fluid is changed. But is it possible that movement of fluid in the trans pan could cause that broken piece to tap rhythmically against the pan, in perfect time with RPMs? Seems pretty far fetched to me, but as long as I'm grasping a straws...
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even if it was to tap theres not enough mass or kinetic energy to make the sound audible over the engine
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Converter cover bolt heads are 15 mm (FYI - 10, 13, 14 and 15 mm are common metric sizes used on vintage GM products; 16 mm and 18 mm are less frequently used). The dipstick "tip" is not the source of the noise you hear.
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Thanks, guys.
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I finally found the combination SAE/metric socket set that I had forgotten I have (Doh!). Dropped the dust cover, but was not able to break the TC bolts loose. Either my 3/8 drive ratchet is too short, or I didn't eat my Wheaties this morning. Probably should have tried tapping the ratchet with a hammer, but I'm always a little nervous about forcing things. Is there any particular trick to keeping the engine from rotating while trying to turn the wrench? I could hold the flex plate still with my left hand and push on the ratchet with my right, but couldn't get enough force to break the bolt loose.
I did turn the engine all the way over and take a good look at the flex plate all the way around. It looks brand new, except for the one chipped tooth that apparently happened when I was getting re-acquainted with how to start a carbureted engine. Didn't see any cracks. If the flex plate isn't cracked, and the torque converter bolts aren't loose, is there any way a rhythmic tapping noise that correlates with rpms could be coming from the torque converter or something in the front of the transmission?
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It appears my saga has finally concluded, so I figured I would give an update. I had the engine replaced with a stock remanned 350 (different shop; the first guy couldn't do it because his engine swap guy was having health problems).
Had to take it back in immediately for various leaks (coolant leak at intake gasket, oil pan gasket leak due to re-using the old oil pan, valve cover leak) and a bad starter (the old starter).
About 100 miles in, the new engine started making more noise than the old one, so I took it back in to have the valves adjusted. Two of the push rods were bent. Got those replaced under warranty.
She's purring like a kitten now. Only lingering issue is the idle appears to be set a little too high. Maybe I'll try adjusting that myself - might be a good learning opportunity for me.