Rear main seal - stock vs bbc

[ijames]

I know that most stage II motors use a big block chevy rear main seal. Why? What has to be changed, compared to stock (block, crank, both)? Is the sealing surface on the crank a different diameter? Is the od different that stock? Thanks.

 

[EightSecV6]

The ID is the same. The reason the S2 block uses the BBC (very early production and S1) rear is because the ID IS THE SAME and at the time of it's (block) creation, a rope seal was the only rear main available for the V6.The early design blocks used the stock type rear main, the BBc rear main was an improvement added later just as the cross bolted mains, etc.etc. There was never an on center block made to fit the stock type seal.We all know from experience how long a rope seal lasts in a street engine, it would never hold up the the abuse of a competition engine.
A stock block cannot be modified to use the BBC rear main. the block has a groove the seal sits down on, the S2 blocks (most of them) have a ridge or shoulder the seal sits on top of.

 

[Lee Thompson]

seal

Hate to disagree with Eight Sec V6 but the dia. of the crank is different. I have a block and crank of each style. The crank that takes the stock seal is larger in dia. where the seal rides than the one that takes the BBC seal. Not a lot, but some difference. Without going out and measuring, around .020. I don/t know if interchanging them would work or not as far as sealing or burning the seal. Maybe Bill has done this.

 

[ijames]

Thanks, guys. This came up in the Eagle crank thread in the lounge and whether if they made a stock dimensioned crank would it drop into the ta perf. block. Nick posted that all of those were machined for the bbc seal, but it sounds like Eagle either needs to use the stock diameter and let people have that surface turned down for the bbc seal, or offer both diameters if they want to sell to both the stock block and ta perf. block markets. [Or Lee and Bill are both right - the diameter is slightly different but in practice it works anyway? ]

 

[EightSecV6]

It is ok to disagree Lee . It depends on who machines the crank. Most aftermarket forgings or Billet pieces will be a smaller diameter, as minute as it sounds, the smaller diameter sealing surface will create less drag on the seal which equates to more power. If you install either crank in the same block using the same seal, the smaller diameter piece will noticably turn easier by hand, .020 sounds like alot but I guess it is possibleMeasure them both Lee and tell us what you find.I know Moldex machined forgings as well as units machined by BHJ use the stock diameter, Crower is definately smaller.I am positive you will see differences between cranks but the end result is that they are absolutely interchangable.

 

[turbodave231]

Whitey at Moldex will ask you which rear main seal you plan to use and machine your new crank to the proper dimension. I had him machine two raw forgings last summer. One was for a friend's on-center block with the BBC seal and one was for my offcenter block with a stock type seal. They are different!

All stage 1 blocks and early offcenter S2 blocks were designed for stock seals. Later S2 offcenter blocks were changed over to the BBC design. Every Oncenter block I have seen always had the BBC seal (I've owned 3 OC blocks).

 

[EightSecV6]

As I posted above Dave, it depends on who machines it but the end result is still the same, either will work with either crank.

 

[NEARING]

Eight secV6,
Do you happen to remember the cam specs for Aaron's (well mine now) car?
Thanks.

 

[Lee Thompson]

crank

Doing a quick measurement with a set of caliper
BBC ---2.631
Buick--2.655
The BBC seal crank is well used (actually a 2 piece crank)

The Buick seal crank is virgin

Turbo Dave, I may have that other crank from Moldex. (3.625)
The story Whitey told me was someone brought in 2 forgings. He turned them both and the guy who brought them in took one with him. He had the other one for sale.
Altho it is turned for a Buick seal.

 

[TurboDave]

Re: crank

Originally posted by Lee Thompson
Doing a quick measurement with a set of caliper
BBC ---2.631
Buick--2.655
The BBC seal crank is well used (actually a 2 piece crank)

The Buick seal crank is virgin

Turbo Dave, I may have that other crank from Moldex. (3.625)
The story Whitey told me was someone brought in 2 forgings. He turned them both and the guy who brought them in took one with him. He had the other one for sale.
Altho it is turned for a Buick seal.

Hmmm, interest is "peeked"

 

[Lee Thompson]

crank

Guess I should have refered that to turbodave 231.

 

[turbodave231]

Some one else must have had two forgings turned. Both of the cranks I had done are in engines now.

S2 for sale

DR

 

[gnxtc2]

Bill

How much did Whitey charge to finish a raw forging? I have a raw forging that I want to have done, I am in no rush for it.

Billy T.
gnxtc2@aol.com

 

[lazaris]

He finished mine for $975