[kimrobertson]

Actually I assumed the compression increase from the 78 L-48 (8.5:1) to the L-82 (9.1:1) was due to the different piston design. I didn't realize there was a thinner head gasket. I did some research and I've read the L-82 originally came with .018 gaskets... Eric may be right about .015 also.
But the .026 gaskets interest me. Maybe this is a dumb question but should I have a detonation problem with the Summit cast iron cylinder heads and .026 gaskets or should I stick with the .039 gaskets bringing it to about 9.1:1??

 

[kimrobertson]

I also read that back in the 70's Chevy did not sell .018s for replacements for L-82s but rather listed thicker .026s ... perhaps because they figured that any head work would also include shaving the heads for straightness. So they sold a thicker gasket to compensate for shaved heads to keep compression ratio around 9.1:1...

 

[Ericnova]

You're actually correct on the original shim gasket being .018" from Chevy originally.....took me a minute to remember back to using them and the uncoated replacement you could buy was .018".
The modern Fel-Pro coated version today is .015".

Personally, I'd rather have the tigher quench, it'll probably be less detonation sensitive even with the higher compression ratio that results....at least, that's been my experience. The qunch effect is more important.

 

[kimrobertson]

You're actually correct on the original shim gasket being .018" from Chevy originally.....took me a minute to remember back to using them and the uncoated replacement you could buy was .018".
The modern Fel-Pro coated version today is .015".

Personally, I'd rather have the tigher quench, it'll probably be less detonation sensitive even with the higher compression ratio that results....at least, that's been my experience. The qunch effect is more important.

Eric-- Thanks, you have been very helpful. Just curious, why do you think it would be less detonation sensitive even with the higher CR? Because of the chamber design? And... I guess detonation can always be addressed at least to some degree with less advance--right?

 

[Ericnova]

Tighter quench helps to eliminate the dead spots in the chamber....those dead spots in the burn are where detonation gets it's start.
Tons of info out there on why you shouldn't use thick head gaskets to lower compression because defeating the quench effect actually makes the engine more detonation prone, even though you lowered the compression ratio.

I know, it seems counter-intuitive.

 

[kimrobertson]

Tighter quench helps to eliminate the dead spots in the chamber....those dead spots in the burn are where detonation gets it's start.
Tons of info out there on why you shouldn't use thick head gaskets to lower compression because defeating the quench effect actually makes the engine more detonation prone, even though you lowered the compression ratio.

I know, it seems counter-intuitive.

Okay, thanks. I think I will go with the .026 gaskets... Thanks for your help!

 

[kimrobertson]

There is a .026" thick composite gasket available if you want to tighten up your quench a bit more than the .039" will have.
Mahle/Clevite Original/Victor-Rienz #5746, it's a stainless core graphite faced gasket, works really well even with aluminum heads. .026" x 4.100" bore
Mahle Original, Head Gasket, Chev SB, 4.100" Bore, .026" Compressed, Each - Competition Products

Eric--My engine is stock 4" bore 350. I notice 4.1" bore on these gaskets. Will they work with 4" bore? Also, I have seen others at 4.125" Do all these work with 4" stock bore?

 

[Ericnova]

Eric--My engine is stock 4" bore 350. I notice 4.1" bore on these gaskets. Will they work with 4" bore? Also, I have seen others at 4.125" Do all these work with 4" stock bore?

Yes, gasket bore is always actually bigger than cylinder bore....both to allow for the lead-in chamfer at the top of the bore and for overbore sizes.
Smallest gasket bore I've ever seen for a 4.00" bore SBC were 4.040" offered....but I don't remember who, maybe SCE or Cometic. Pretty much a racer thing to maximize compression ratio.

The standard Fel-Pro dimension for their common composite SBC gaskets for 4.0" bore SBC is 4.166", if that tells you anything....basically made to fit nearly everything possible with fewer part numbers needed to stock.