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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: gmiles on November 20, 2007, 12:13:38 am
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The trans I have for a 74 Chev Stepside C10, is a TH400. It was bolted to a 454. The flexplate I have for the 305 engine (one piece rear main seal) has a 10" bolt pattern and a 9 3/8 bolt pattern measured center to center of the bolt holes. The torque converter bolt pattern is 10", it's 13" in diameter. The trans has a 30 spline shaft. When I bolted the trans to the engine, I noticed the torque converter was about 1/4 " from the flexplate, even when the trans was fully seated against the engine block. I don't know what the trans originally came out of.
Are there variations in the way manufacturers measure the bolt pattern? there are only 3 bolts used, so it seems a no brainer to measure from one hole to the next. I have searched and searched trying to cross-reference parts on summit, jegs and kragen. I cannot seem to find a torque converter with a small bolt pattern (10") for a TH400. Everything is advertised as either 10.7" or 11.5" bolt pattern. The torque converter that is in the trans is a high stall. Is the converter supposed to be pulled back that 1/4" to meet the flexplate or is it supposed to seat completely on the shaft? I don't particularly want a 3000 stall converter (the truck is for my son). I find it odd that the converter in the trans has the right bolt pattern but I cannot find a replacement for it. Unless it's a matter of how the manufacturers measure the pattern. Most places don't stock a bunch of GM converters for me to go measure so I'm at my wits end with this. Can someone explain what the heck is going on?
The first flexplate I bought at Autozone was for a 74 C10. The bolt pattern was too big to bolt onto the crank so I went back and looked at a bunch of different ones until I found one that fits. It's for an 86-97 SBC and has the smaller crank bolt pattern.
I just googled the part number for the flexplate (FRA-159) and read it has a dual bolt pattern of 10.5" and 11.5" bolt pattern. It has to have something to do with how the measurements are made.
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I am confused on what you are working on. Please clarify. I think you say you acquired a TH400 from a donor truck and you are putting the TH400 behind a 305 engine.
When you say it is 1/4" off are you meaning that far away from the flexplate or the boltpattern is off that much so it will never bolt up.
Pretty much all Chevy motors have a standard flexplate size. I don't mess with autos much, but as far as I remember there is only 2 torque converter patterns...
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I don't know what the TH400 came out of. When I bolted the engine and trans together, the torque converter was about 1/4" away from the flex plate. The converter will slide back against the flexplate, but I don't know if it's supposed to do that. The bolt pattern will match up, but the converter is a 3000 stall. Also, the trans was bolted to a 454, and I don't know if a BBC and a SBC use the same converters or flexplates. Admittedly, I'm not a Chevy guy, so my knowledge is limited. I even bought a couple of Chevy books to offset my Ford library, but they didn't help much.
So, does the converter slide back against the flexplate (1/4" or less)? Does a BBC and a SBC use the same torque converter/flexplates?Are there torque converters with different heights/lengths?
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As to the gap, yes that is normal. The torque converter is specific to the trans, not the engine, so as long as it is for the transmission your are good. Flexplates are different, though. They are counter wieghted for most big blocks. Good news is, they are pretty cheap if you need one.
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how did you determine it is a 3000 stall? Trans aren't engine specific. Flexplates ARE specific in relation to BBC vs SBC.
as far as bolting it up, if the converter is seated you just pull it to it and bolt it up.
converters come in different sizes based on stall speed
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The guy I got the trans from said it was a 3000 stall. He had the TH400 bolted to a 454 to go into his 69 Camaro for 1/8 and 1/4 mile runs.
Based on what I found on google, I have the right flexplate, but for clarification, is a 305 externally balanced and would the flexplate have a counterwieght on it? The balancer has holes drilled in it which would seem to mean it was externally balanced and the crank has a pin that only aligns the flexplate a certain way.
Thanks again for all the help.
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The one-piece rear main small block engines were externally balanced(easy to identify by a large aluminum plate on the back of the oil pan) and require a special flexplate, like 40 bucks from the parts store or 10 from the junkyard. These include 350's and 305's. In the two piece rear seal engines, the 400 is the only externally balanced small block. The 454 is externally balanced. The 366, 396, 402, and 427 big blocks are all internally balanced.
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Thank you all for the help. My son and I bolted the engine and trans together today and dropped it in the truck. When that happened, of course more things came up. I thought I read somewhere that GM changed motor mount positions at some point (on the block). It looked like I might have to fab some brackets, but we had the engine too far forward. ???
I had to move the trans cross-member back, too. But, it all fits.
Thanks again.
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There are 2 motor mount locations, 8 cylinder and 6 cylinder. They are marked on the frame stands. If a V8 was previously in there, just use everything off it on the donor.