[Tim Corcoran]

Hey guys how much can you safely mill a 6X head. I know about milling the intake side too for proper match up with intake manifold. I need to mill it about .070 to get the compression up to 9.5:1

[Vckline313]

Looking for same info

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[PAUL K]

.070 shouldn't cause a problem. We used to mill HO heads around .090 and that didn't cause an issue. We would mill the intake side of the head the same amount and leave a step at the bottom to seal the valley pan.

[hurryinhoosier62]

Originally Posted by PAUL K

.070 shouldn't cause a problem. We used to mill HO heads around .090 and that didn't cause an issue. We would mill the intake side of the head the same amount and leave a step at the bottom to seal the valley pan.

Paul, what about coolant erosion? I think I would have to have an ultrasound on the head deck before going more than a .040 cut.

[Brian Baker]

I had a set of 6X-8 heads on my pump gas friendly 455 20 years ago that were "shaved" .060" with no issues.

[Formulabruce]

some exhaust hits too I have found. Now its a 1 off engine basically..

[steve25]

Let’s nail down some important details here!

If you do Mill off .070” then you have lost that same amount of piston to valve clearance.

Do you have that to play with now, or in the future if you ever go with a greater lift cam , or higher lift rockers?

Since it sounds like your working with the -8 heads they have a bigger combustion chamber in terms of width then the -4 heads.

This means that per every .006” wacked off the deck you are removing 1.2 CCs of volume.

This means that instead of a mill of ,045” taking off 7.5 CCs you are removing 9 CCs.

Since your heads heads are apart for the milling work and it sounds like your fighting for every reduction in CCs you can get I would step up to 1.77” exh valves which can be set in the chamber proud and pick you up 1.5 CCs almost.

Whatever amount you do Mill the heads do not give a potential crack any place to start so be sure to break / roll over all the sharp edges on the deck.

Be sure when you do that roll over work around the chamber that you do not undermine the combustion chamber fire ring of the head gasket.

[PAUL K]

Originally Posted by hurryinhoosier62

Paul, what about coolant erosion? I think I would have to have an ultrasound on the head deck before going more than a .040 cut.

The general rule is .060 is considered safe. At .070 as you know is only .010 more, hard to believe .060 is "safe" and .070 gets you in trouble. The HO heads we did were around .090 and never had an issue. We also milled a pair of SD heads a bunch more than .070. We did several sets (HO) for race engines (making over 600hp NA) and some nitrous motors. My biggest concern was the head gaskets giving up but that never happened.

Now with affordable custom pistons and unaffordable labor rates, we rarely mill the heads anymore than what's nescessarry to achieve a target combustion chamber size.

Also negative on exhaust clearance issues. However most of the engines were built first, installed, headers fit then exhaust attached. I can see if the engine used exhaust manifolds and the pre-bent exhaust might not be happy sealing. However that wasn't an issue with my SD car and those heads have been milled .060

Piston to valve clearance was never a concern, any head we milled that much had enough valve drop to require a minimal valve pocket if any at all.

[Skip Fix]

Here is an old post. If you look at the depth of a 6X head I doubt you will have enough lift and duration to even touch the piston as the head just won't support it.

"Cleaning out a drawer and found this chart I made. I thought I would repost it. For you 6X guys the 6X-4 chamber is slightly smaller diameter as well as slightly shallower since it was originally a 350 head. These measurements were from the deck surface of the head to the closest edge of the intake valve where it will be closest to the piston .


#614/SR 0.073

72cc E head 0.104

#16&62 0.137

#670 0.140

6X-4 0.225

6x-8 0.282

4X-1H 0.345 "

The 6X-4 heads on the 400 in the 81 were shaved 0.050. Had to trim the valley pan a little as well as bottom of intake runners as it was tight there.

[Tim Corcoran]

Thanks for all the info, I wish these heads were 6X-4 but they are not so mill them I will to get 9.5 or close to it. By the way they already have 1.77 exhaust valves. The heads were milled once before and the chambers now measure 100cc's. It's a 400 with a bore of 4.195 and a 4" stroke, if I had know how low the compression was I would have put in a crank with a longer stroke or had a small dome put on the pistons when I ordered them,

[77 TRASHCAN]

Wish you were closer... you’d have a set of -4’s....
If this is a street engine, getting to at least 9:1 will do, but I don’t like to leave anything on the table, either.
Looking forward to other details of your build!

[steve25]

Here’s what I get by using these numbers.
1) Chamber 100 CCs
2) Head gasket volume 9.4 CCs
3) Valve notch volume 6 CCs
4) Ring land volume 1.5 CCs
.025” deck clearance .3455 cid

1 thru 4 added up is 116.9 CCs, this converted to cid =7.13.
Your cylinder cid is 55.28.
Putting this info thru the rest of the math I get 8.4 to 1 compression.

Milling the heads .045” gets you to 91 CCs, this computes out to 8.97 to 1

If you have zero deck and not the .025” like I used then your compression would be 9.4 to 1.

I hope this helps some.

[Skip Fix]

My original 78 6X-8s were 97.5cc. I have some ported ones I picked up that had the ridge in the chamber smoothed that go 105cc. Picked up some used mini pop ups for them.