I was looking at 4.25 cranks. Some say there for 1 piece rear main seal and others say there for 2 mains. Not sure what the means....

well one is one piece and the other is two
I'm just kidding
if i'm not right i'm close 1985 on up blocks use the one peice seal might be 1986 under that year use of the two peice seal is used ..different oil pan , crank shaft etc.....
you can by a conversion kit to make the early block make use of the newer 1 piece seal crank

86 went 1 piece rear, 87 went 1piece roller.

The one piece rear had a seal that is one piece. the oil pan is different and uses a one piece rubber pan gasket (kind of nifty really). Cranks are different on the backside for the seal. Thus a diffent flywheel/flexplate bolt sytle/pattern for both.

If you are talking about a BBC, 1991 was the first year for the one piece rear main seal on the Gen V and 1996-later Gen VI engines. Both the rear main cap/block are machined differently as is the crank!

You can put a crank designed for a 2-piece rear main seal in a later block by using an adapter plate which are available from many sources including GMPP.

Dave

Couldn't find what I was looking for in the search and this is as close as I got.

Are 1 piece and 2 piece 454 BBC flexplates interchangable? Will they physically bolt up?

Thanks, Jeff

ttt

no they will not two different bolt patterns or so Iam told.

Thank You! By different bolt patterns are you referring to crank bolts or converter bolts?

Jeff

Thank You! By different bolt patterns are you referring to crank bolts or converter bolts?

Jeff
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Crank
They are completely different deals, different bolt pattern, different balance weight
Nothing is the same between the rear of the two cranks

The 1-pc RMS looks exactly like the front seal, except a little bigger. It's a meta-clad lip seal. It goes in a retainer piece that bolts on the back of the block. The crank flange is perfectly round and smooth, like the end of the cam.

The bolt pattern for the flywheel on the 1-pc is smaller, because the seal is smaller.

Since it's round, it doesn't have the extra little bit of balance weight on the flange any more, they had to move it to the flex plate. Which causes endless confusion about "internal" vs "external" balance. 305s and 350s for example, are still very much "internal balance" motors; but that last little fragment of "internal" weight is located "external" to the crank.

I'd suspect that the reason the factory made the change, was the assembly cost; not, tendency to leak, or longevity. They didn't really have any problem with either of those things, since they generally make it out of the warranty successfully. Rather, it takes somewhere around ¼ as much time to assemble the 1-pc setup as the 2-pc. Probably a cost savings of something around $10-15 per motor; which, over the course of a few million motors a year, adds up to some significant money.

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Crank
They are completely different deals, different bolt pattern, different balance weight
Nothing is the same between the rear of the two cranks



Thank you very much Mike!! I searched all over and could not find this out for sure. I kinda figured this but was not sure and have never seen a 1 piece BBC yet.

Jeff