Compression Bump

[justinp71]

I just got a 3.0 and I need to fix some of the head studs and will get the heads rebuilt.

I thought I heard for every .010" they take off of the heads (when they fly-cut) you get .2 bump in compression. I currently have 9.3:1 pistons with CIS, so if I wanted to go to 9.8:1 I could thoeoretically have .025" taken off, correct?

Has anyone had experience with this? good/bad idea?

[304065]

Flycutting the heads as a means of increasing compression ratio makes the engine narrower, which will require the chain boxes to be machined an equal amount, and eventually cause you to need larger chain idler sprockets. But it is really not recommended because it reduces piston-to-head clearance.

[justinp71]

Yeah.. I thought about the piston valve clearance, probably a bad idea on a CIS motor.

Is there another option to clean this surface besides cutting the heads? or will you be okay if they only take off .005" or so..

[Eagledriver]

You can take .010 off the heads without causing problems if they haven't been cut before. Piston and valve clearance seems to be pretty large on 3.0 engines with stock cams. The worry that John mentions is the pistons getting close to the heads themselves. I don't think .010 inches will cause problems there either but it's good to check with a piece of clay or solder.

-Andy

[kenikh]

Just make sure you don't throw off your deck height (which you will). If you are going to rev the motor over 7 grand and have under .040" deck height, prepare to see what piston to head contact looks like.