I'm building (or buying) a new motor for my '86 Firebird with a 700R4 and a 305. Can I build a 400 SBC and drop that into my firebird? I've heard comments, rumors if you will, that the '86 f-bodies and up had a 1 piece rear main seal and that prevented motors that have two piece rear main seals from being swapped because of incompatability with the transmission? Is this true? Will I have to go with a motor that has a 1 piece rear main seal or am I fretting over nothing? You're input would be appreciated.

John

 

Any SBC tranny will bolt up to either rear main design.

 

Thank you, now I can sleep better....

 

Just get the right teeth on your flywheel.

 

It's not the teeth that are different. It's the bolt pattern.

When GM decided to use a 1 pc rear main seal, they lost the rear counterbalance as well as the diameter for the bolt pattern of the 2 pc rear main seal crankshaft.

Two things happened. They had to make the bolt pattern smaller and they lost the counter balance on the back of the crank. They had to balance the flexplate/flywheel instead and of course, reduce the diameter of the bolt pattern on the flexplate/flywheel.

So, i.e., the one pc rear crank is similar to a 400 crank in that it has to have a flexplate/flywheel that is counter balanced. Although the 1 and 2 pc cranks still use a nuetral balanced harmonic damper.

That's the only difference.

 

So now I'm a little confused. Will I have to buy a special flywheel to be able to allow a two piece rear main seal motor work with my 700R4?

 

with a manual, you need to have the correct FLYWHEEL to match both the engine and tranny.


with a automatic, you need the correct FLEXPLATE to bolt up the tranny. you just need a two piece rear main seal flexplate, and it will bolt right up.

 

That makes sense. Thank you.