rear main seal style

bigtizzles · Member Joined: 17 Nov 2010 Posts: 79 2938.76 points

Hi, noob question. Does my 1968 large journal 327 have a single or double lip rear main seal? It has the 4672 factory forged crank. Block casting number is 3914678 with 7178 main caps. Also, why are 7178 main caps "desirable"? I have heard people say that.

Big Dave · Posted: Sat May 19, 2012 2:05 pm Post subject:

A 1968 SBC block has a two peace rear main seal. The one piece rear main seal didn't come out until 1986. Your block has a machined single raised ridge to hold a single lip neoprene seal. I have not heard of a double lip rear main but I see no reason why some adventurous enterprising huckster could not have one made to sell you one and claim it is twice as good. Much older (pre 1964 blocks) used a rope to seal the rear mains.

Chevrolet casts the main caps out of the same grey iron they use for the blocks. Steel (forged or mild) is stronger in tensile strength than cast iron but it doesn't expand at the same rate. In a drag race engine where there are violent loads applied suddenly and are gone seconds later it makes sense to use steel main caps.

For a mass production engine, or an endurance race engine, you want the same metal in the caps as is used in the rest of the block so that everything stays together. If not your bearings will be expanding at the parting line which could result in a bearing failure or at least an oil leak (noticable as a drop in oil pressure from when you built it new).

Chevy did make a stronger high nickel content nodular iron block. They are used in all BowTie blocks and in truck blocks that have four bolt main caps (and in two bolt blocks cast by GM of Canada). Those caps will expand at about the same rate as grey iron, but any time you change a cap; the caps and block have to be align bored and finished honed which puts slack in the timing chain (so it adversely affects both cam timing and ignition timing).

Big Dave

bigtizzles · Member Joined: 17 Nov 2010 Posts: 79 2938.76 points

Thanks Dave. I knew it was a 2 piece, I just wasn't sure why they make a single and double lip. So if I have a forged factory crank, then the 7178 main caps are just common cast caps. I tried to do a search on the cap casting and people would talk about they had the "desireable 7178 main caps. But I couldn't figure out why they would be any different. So I guess I will buy a single lip 2 piece rear main seal.

af2 · Posted: Sat May 19, 2012 8:56 pm Post subject:

bigtizzles · Member Joined: 17 Nov 2010 Posts: 79 2938.76 points

so what are you trying to say about the 7178 main caps? My motor came out of a truck according to the suffix code. I'm pretty sure these caps came on some stock 327 (probably 350 too) I don't know if they went on the forged crank motors or what the deal was. So I need a single lip 2 piece rear main, just to confirm before I buy one.

af2 · Posted: Sun May 20, 2012 5:15 pm Post subject:

I would guess they were made for the endurance. The 2 piece seal is the same for all SBC with a large journal.

bigtizzles · Member Joined: 17 Nov 2010 Posts: 79 2938.76 points

so single or double lip????????

af2 · Posted: Sun May 20, 2012 9:30 pm Post subject:

http://www.summitracing.com/parts/FEL-2900/