[steve25]

Here’s what I came up with.
I used both a .030” over motor and a .060” over 400.
Your target total CC volume to have an 9.5 compression are 97.79 for the .030” motor and 99.20 for the .060” motor.

Here’s the info / numbers I used .

Stock 6x4 chamber size = 89 CCs which now may be less or about the same due to the bigger 1,77” exh valve, but then again the valve job done on the 6X heads also sank the intake valve some.
Let’s call it a wash and use the stock 89 CC number.

I set deck height at 0

I used 3.35 CCs which is assuming two valve notch custom pistons instead of 4 valve notch pistons.

Head gasket volume 9.4 CCs

Ring land volume = 1.5 CCs


Using these numbers and a .030” over bore and the 6X heads your compression would be right at 9 to 1 because your total CC volume is 103.29, not the needed 97.79.

Using a .060” bore you would have a compression of 9.16 to 1, and once again your total CC volume is 103.29.

If you mill the 6X heads .042” you would end up with a 9.6 to 1 compression.



Assuming your #62 heads are 75 CCs then a .030” over 400 would have 10.3 to 1 compression.

Seeing as the 6X heads are set up for street gas those are the ones I would choose to run.
Both heads will make the same power and easily achieve 375 hp as long as HP manifolds are used.

Even with the small .406” lift of a factory 068 cam this 375 hp can be had, and headers bolted to the motor will make at least 15 hp more.

A aftermarket Cam with .500” lift or maybe even .525” lift will get you to 400 hp or better.