Thickest head gasket

[bubbastrick]

Hello Folks:

I am going to put a set of 305 heads on my 350. In order to have a more sane compression ratio, I want to use the thickest head gasket I can.
Do any of you guys have access to the specs for a thick head gasket that I could find at a place like Autozone? I would like to have the part number and the compressed thickness.
Thanks for your help

[passthebuck]

When I was building up my 327 with 64cc heads, I wanted to get the compresion ration as close to 9.1 to 1 as i could. i found some thick head gaskets in the GM Performance parts catalogue that did the trick. I THINK they were stock on the ZZ1, ZZ2, or ZZ4 engine. They work great & I have zero (knock on wood) head gasket problems.

[Sierra]

That GM head gasket is part #12557236 and should be available at any GM dealer. Here's more info:

http://www.sdpc2000.com/cart.asp?act...id=576&pid=722

[bigjimzlll]

If you have an engine that has 10,5-1 cr and a .040 quench...it will detonat less than an engine with 9.5-1 CR and a quench of .060...just some food for thought

[Stroker]

I second the advice of BigJim. Increasing the quench to lower compression is likely to do do more harm than good with a wedge style combustion chamber. This is what Speed O Motive has to say about lowering compression and quench distance in their engine building tech tips.
http://www.speedomotive.com/building%20tips.htm

f you are building an engine with steel rods, tight bearings, tight pistons, modest RPM and automatic transmission, a .035" quench is the minimum practical to run without engine damage. The closer the piston comes to the cylinder head at operating speed, the more turbulence is generated. Turbulence is the main means of reducing detonation. Unfortunately, the operating quench height varies in an engine as RPM and temperature change. If aluminum rods, loose pistons, (they rock and hit the head), and over 6000 RPM operation is anticipated, a static clearance of .055" could be required. A running quench height in excess of .060" will forfeit the benefits of the quench head design and can cause severe detonation. The suggested .035" static quench height is recommended as a good usable dimension for stock rod engines up to 6500 RPM. Above 6500 RPM rod selection becomes important. Since it is the close collision between the piston and the cylinder head that reduces the prospect of detonation, never add a shim or head gasket to lower compression on a quench head engine. If you have 10:1 with a proper quench and then add an extra .040" gasket to give 9.5:1 and .080" quench, you will create more ping at 9.5:1 than you had at 10:1. The suitable way to lower the compression is to use a dish piston. Dish (reverse combustion chamber), pistons are designed for maximum quench, (sometimes called squish), area. Having part of the combustion chamber in the piston improves the shape of the chamber and flame travel. High performance motors will see some detonation, which leads to preignition. Detonation occurs at five to ten degrees after top-dead-center. Preignition occurs before top-dead-center. Detonation damages your engine with impact loads and excessive heat. The excessive heat part of detonation is what causes preignition. Overheated combustion chamber parts start acting as glow plugs. Preignition induces extremely rapid combustion and welding temperatures melt down is only seconds away!

[bubbastrick]

So you guys say that maybe I should go with a little tighter quench distance and that would allow me to run higher compression without detonation?
What is the usual compressed thickness of a stock smallblock head gasket?

[67chevemall]

Stick with the stock part #'s!
$0.02

[bigjimzlll]

the piston is down on a stock engine around .025. GM used to use shim head gaskets all the time..they are .015 thick. This will give you a quench of .040..perfect... stock rebuilder(fel-pros) are around .041 thick. I use the shims all of the time....the caveat is the block deck and head needs to be flat and true. Mr. Gasket makes a shim that has a little more coating on it..it is .018-.020 thick. If the head and block has not milled..you can paint the shim with copper gasket paint..this will seal any imperfectons.

[Stroker]

bubbastrick, what are the specs for the rest of the engine? And how will the truck be used? A usable compression ratio could be anywhere from 8/1 to 10.5/1 depending on the camshaft, induction system and usage of the truck. It may turn out that the small chamber of the 305 head (58cc?) isn't compatable with the rest of your equipment. If so, trying to reach a more reasonable compression ratio by using thicker head gaskets just creates additional problems that cause the same end result as the high static compression.

[bubbastrick]

Heres the deal:
I have the 350 pulled out of my S-10. I need the heads off of it to go on the 383 I am building for my '69. The only other set of heads that I have that aren't cracked are the 305 heads. So in order to get both engines going at the same time I will have to use the 305 heads on the 350.
The 350 is a .060 over 350 with all of the other dimensions (deck, etc.) being stock. The cam is a Crane Energizer with .450" lift and 218 degrees at .050". It will be using a 600 CFM Edelbrock Performer carb, and an Edelbrock Performer RPM intake. Also I am using Hedman headers.
So until I can buy a set of Vortec heads I am stuck with this setup and using a thicker headgasket appears to be the only way to get an accepable compression ratio with the heads I have available.

[Stroker]

The thickest Composition Head Gasket I know of is .051. I believe you can also get the copper gaskets up to about .080. Here's a GM performance parts head gasket that is used on the aluminum head crate engines like the zz4 and also the vortech head 350 HO engine. It has a .051 compressed thickness PN 12557236.

http://www.flatlanderracing.com/gmsbheadgas.html