My 1980 Slant is loosing compression in #5 due to a receeded exhaust valve seat, so I am going to replace the head with one that has valve seat inserts. The engine is otherwise stock. The specs I have indicate the factory compression ratio is 8.4:1. I would like to bump this up. For a dialy driver/workhorse, how does 9:1 sound? Does anyone have the chamber cc specs or a formula to calculate how much I would need to mill the head? The shop manual suggests pulling the head and manifolds as an assembly. Will this work best? Will I be able to reuse the manifold mounting hardware or should this be replaced? Any other tips?

Thank you

Mitch

m5black@aol.com

"specs indicate the factory compression ratio is 8.4:1... how does 9:1 sound?"

Pretty good from where I sit.

I'm assuming this is a bone stock engine. I've run the numbers through my handy dandy calculator. Here's what I got:

Assumptions:

Combustion chamber 54cc
Bore 3.400
Stroke 4.125
Piston head volume 0 cc
Deck clearance 0.140
Head gasket thickness 0.045
Gasket bore 3.500

These numbers give me 8.493:1 CR

This makes a bunch of assumptions, not the least of which is the head gasket thickness AND the existing head CC. Since your pulling the head anyway, I'd spend the money to check AT least 1 chamber to confirm the volume.

Doc has an article on this site at:

<A HREF="http://www.slantsix.org/articles/stokin ... ing.htm</A>

The spec for milling works out to 0.0066 per 1 cc.

Based upon this, I'd suggest cutting 0.035". You can cut these heads up to 0.100", so this is should be no problem. This will give you a THEORETICAL combustion chamber of 48.7cc and a 9.012:1 CR. Please consider that this assumes a "perfect" engine. Manufacturing variations may move this around a little, but nothing significant.

"The shop manual suggests pulling the head and manifolds as an assembly"

They have to come apart sometime. I usually pull the manifolds before pulling the head.

"Will I be able to reuse the manifold mounting hardware or should this be replaced"

I usually end up breaking something when I pull exhaust manifolds off, so I plan on replacing the hardware right from the start.

Here's a link to a very handy engine calculator I like to use...

Hope this helps,

Gilly



Engine calculator
tlg225ccr@aol.com

This helps a lot. Thanks Tom.

Mitch

m5black@aol.com