[dhcarguy]

What would be the bore size of my 66 389 block that is .030 over?
Also what thickness are stock head gaskets?
What thickness can I get to lower my compression a little?

Thanks, Dave

[78w72]

according to wallace racing, a .030 389 is 4.0925 bore size.

common size for stock stype felpro gaskets is .039. you can get thicker gaskets .050 & bigger to lower compression a bit.

[ponyakr]

Butler sells the Cometic gaskets that are .051 thick. But, they ain't cheap.

https://butlerperformance.com/i-2617...tegory:1234803

[Tom Vaught]

389 is 4.0625" as posted above.

A 389 .030 over is basically the same bore size as a stock 421 piston but the piston pin is in a different compression height location. Compression height location is the distance from the top of the piston (flat top pistons NO DOME) and the center of the wrist pin in the piston.

Pontiac Engineers designed the Engine blocks and displacements pretty much in .030" bore increases. Same deal with the strokes of the crank 3.75, 4.00, 4.210 except a larger difference.

You could actually make a good "421 cid Smokey Yunick" engine by using a scat 4.0" stroke crank with 3" mains, a 4-Bolt splayed cap on a 389 block, and a .030" overbore to the 421 engine piston size. But there are much easier ways to get displacement.

Tom V.

[tom s]

Butler also sells their own gaskets for 389-421 that are 46 thou.Tom

[dhcarguy]

Thank you

Dave

[Schurkey]

Originally Posted by dhcarguy

what thickness are stock head gaskets?

Wouldn't they be steel shim gaskets, about .020 or close to that?

Originally Posted by dhcarguy

What thickness can I get to lower my compression a little?

If you kill the quench/squish, you may have detonation problems even with less compression.

[Tom Vaught]

Stock head gaskets for pontiacs are not steel shim gaskets of .020 like a SB Chevy is.

They are more in the .042" thickness stock.

Tom V.

[Schurkey]

Originally Posted by Tom Vaught

Stock head gaskets for pontiacs are not steel shim gaskets of .020 like a SB Chevy is.

They are more in the .042" thickness stock.

Thanks for that. I've never pulled a '60s Pontiac engine apart.

The bit about sabotaging quench/squish distance still applies, though. It's entirely possible to lower compression ratio and have MORE trouble with detonation due to slow, lazy burn.

[Tom Vaught]

Originally Posted by Schurkey

The bit about sabotaging quench/squish distance still applies, though. It's entirely possible to lower compression ratio and have MORE trouble with detonation due to slow, lazy burn.

Agree, there have been several topics out there on Quench distance and when you fall off the cliff and the engine goes into detonation.

There is a range where the engine has a lower compression ratio and as you posted have a lot more combustion issues.

And that issue starts with about a .030" piston deck height and a .042" compressed head gasket height and gets worse as the .072 dimension gets larger. I have seen .060 head gaskets and .035 deck heights (.095" quench height) that were not happy at all, even under light load driving.

Tom V.

[TedRamAirII]

Seems that thinner gaskets were part of the Royal Bobcat package. Do you know what they used?

[Tom Vaught]

Originally Posted by TedRamAirII

Seems that thinner gaskets were part of the Royal Bobcat package. Do you know what they used?

I do not know the actual head gaskets used.
I do know that they cut the cylinder heads .030"

So all I would have to go on would be Wangers advertising, and bless his heart, I really do not believe anything he put in print in the 60s.

Tom V.

[hurryinhoosier62]

Originally Posted by TedRamAirII

Seems that thinner gaskets were part of the Royal Bobcat package. Do you know what they used?

Ted, according to the late Roger Huntington, Royal Pontiac used .022-.025 steel shim head gaskets to raise the compression ratio.