Machining heads

[jweir22]

How much is safe to take off 3100 heads with valve to piston clearances.Is .010 thou enough or can you take .020.

[StudlyCamaro]

im lookin at a 3100 head right now... eek thats gonna be tight.. even .01 is gonna be close, why do you wanna take em down that much? raise the CR using new pistons.. its much safer

[v6h.o.]

ive taken off a heapload of this one set of heads i have on the car right now..

it started as a .020 mill job to raise compression, with a .040 thick iron headgasket..

i had an incident with a coolant hose blowing off and lost all coolant and the car overheated... warped the heads,
this was a week before the final drag race day of the season so to "band-aid" it and patch it back together for the drags i milled them again (yeah you can slap me now) another .020 because they warped .016 on me....

so i have a set of heads with .040 taken off them and running .020 thinner head gasket with .469 inch valve lift. no clearance issues. none at all.

however the heads are junk now from the extreme amount of material taken off and im working on a new set.

[StudlyCamaro]

wosers, thats a whole lotta material removed...

[jweir22]

Well maybe ill try machining .005 thou off them then just clean them up and get a little more compression.

[v6h.o.]

you wont gain really much at all from .005 in terms of compression.

but to clean them up, yeah anywhere from .005-.010 off them will clean them up nicely.

[Compufreek]

How far would you have to go to increase compression?

[v6h.o.]

you'd have to measure cc's before and after of the chamber to know how much your increasing.

an old set of heads i did one time, i milled them .015 and went from stock 3.1L (gen2 3.1L) 8.9:1 to about 9.3:1. i measured the combustion chamber cc's to figure that out. but then again i did polish the chambers on those heads so i might have ruined some gain that was to be had.

thats why i said .005 will gain or benefit very little. you might see .1 - .2 compression change ??? i cant say 100% what you'd get for an increase as ive never measured each change cc's of the chamber as per each aspect of milling the head.

[Compufreek]

You didn't say what heads you machined all that off of above. I lost a gasket in my top end so I have to take the heads off. I figured I would bring them down a bit. Would .015 be safe for clearance in a 3100 head with stock gaskets. I am not looking for much but maybe .5 CR increase to hit the 10:1 mark.

[v6h.o.]

all Gen 2 and Gen 3 cyl heads have the same combustion chamber size.
they were both off of 3100 style heads though.

as stated with the 3.1L hybrid (3.1L vin T mpfi block, 3100 top end) the distance between my pistons and valves has been reduced by .060 and i run a cam with .469inch lift.

you'll be very safe with the amount you want to shave off.

[jweir22]

So if im going to shave 5 to 10 thou off my heads what would i need to take off to change compression a bit. 20 thou would that be safe on a 96 3100 gen 3 roller rocker heads.It might give me a little better gain cause i was thinking im taking 5 off anyway better make it worth my while.But i also would like to be safe too.

[v6h.o.]

.005 -0.010 will change compression, just the more off the more the compression change, .005 will result in barely noticable differences though.

not much else you can do to change the CR besides going to different pistons.
i'd take .010 off if i was you.

[jweir22]

10 thou it is, good call,thats what 'ill do.I'll get some pictures going of my works in progress.