305 or 350?

[captain carnage]

Give me 5 mins with a 350 and I can make it feel like a 4 cyl.

[jackjeckel757]

Found the casting numbers on the back of the motor, turns out I do have a 305, from an '80-'84 is what was listed.  The numbers were not too hard to see, a little bit of grime but not bad.

[Joe-NC]

I'm pretty disgusted right now and that's putting it politely.  I've been in the engine compartment 4 times, felt all over with my hands, got grease on them that won't come off for days, and I still can't find the casting numbers.

But the real kick in the butt is finding a number I didn't want to see ...GM5.0LG.  This is on the flange of the block on the drivers side.  The timing tab is also at 12 o'clock just like Grim said it would be on a 305.  The guy I bought it from told me he replaced the original 305 with a 350.  I can deal with a 305, but I don't like being lied to.  Guess its my own fault for believing him and not checking it myself.

Problem #2:
It needs a rear main seal. I spoke to a mechanic who worked on the carb before I bought it. My flywheel has a weight on it and he says its NOT supposed to and the seal is leaking because of the vibration. I thought if that were the case it would shake my teeth out. Am I wrong? It runs down the interstate at 80 mph just as smooth as can be. He says it has a little vibration at idle, but I don't feel much at all. Since I don't know what kind of engine I have, I don't know whether it takes a weighted flywheel or not.

P.S. It also sounds awful while its cranking and this mechanic says the weighted flywheel is the cause.

Question:
I like to drive faster than most people, be the first one off the line at a stop light, etc (I'm not a butt hole, I just like speed).  So if I replace the flywheel, should I go with a smaller/lighter one?  If I do go with a smaller one, how would that affect my ability to pull a small utility trailer?

[beastie_3]

the casing number is there. keep looking.

I dont know what the weight is for, but I do know that 400s were externally balanced. 400 flexplate?

[Grim 82]

As near as I know, any 305 should be internally balanced (no weights) and would also have a neutral balance dampener on it. As far as the rear main seal, if it's a 2-piece seal I'm pretty sure they are all the same regardless of displacement, 283 up through 400.
A smaller flywheel will spin up faster (less mass to get moving), but a bigger one will be easier to keep spinning if you are pulling anything (objects in motion...).
I think you will probably still have to find that casting number, go to a parts house and get the one specified for your motor, and compare it with what you have. Lots of things can cause a vibration, but if you can run 80 mph I doubt that that's your problem.
Here's where the casting number is on my 400.

[Joe-NC]

I've got someone looking at it right now.  This is what he's telling me: The flexplate is definitely not supposed to have the weight.  I never noticed, but it was shaking my rearview mirrors.  He says the casting number on this engine will be at the front near the water pump and that it probably came out of a car. The carb doesn't have the electric sensors that the original truck had, and whoever installed the engine didn't bother putting in a kickdown cable. I'll wait on that because according to him, the reason I've been having trouble with reverse is that the transmission needs a rebuild. The roaring I hear when decelerating is the rear end, so I need that replaced too. All the smog connections coming out of the manifold have been cut off and crimped with pliers.

Just stamp "NOOB" on my forehead. I've got plenty to do on my project.

[Grim 82]

Disregard what I said about flywheel sizes, I was under the impression that you had a manual transmission.

Unless it's a 400 it should have a neutral balanced flexplate.
If you're not required to have all of the smog business hooked up and functioning I would ditch those manifolds. Did he give you a diagnosis that's more specific than "the rear end needs replacing"? It could be as simple as a u-joint or bearing.