How much to safely mill from the cyl head surface

lrodri64 · 03-19-2018, 06:16 PM

Hi,
I have my cylinder heads at our local Machine shop.
Sealing surface is really pitted on all of thems.
I need to know how much can be safely milled from the sealing surface between head and cylinder.
It is a 2.4 T
Nothing fancy, just a regular engine repair.
I understand that also chain housings also have to match.
Thanks.

ed mayo · 03-20-2018, 05:31 PM

As little as possible is the answer, hopefully .005" or less. What ever is milled from the seal surface must also be milled from the fin surface to maintain adequate top fin clearance.

lrodri64 · 03-20-2018, 05:34 PM

Thanks Ed

Trackrash · Trackrash's Garage

Ed is correct. However, Porsche says 0,010" can be removed IIRC. My old motor had something like 0,020" removed.

You will need to account for the material removed. Normally thicker base gaskets will be needed. You will need to be careful about the resulting deck height, piston to head clearance and the compression ratio.

You can enlarge the holes in the cam sealing flange to account for the fact that the head is closer to the case, instead of machining the cam housing.

lrodri64 · 03-21-2018, 02:31 PM

Gordon,
Which is the cam sealing flange ?
Regards

Trackrash · Trackrash's Garage

911pcars · 03-25-2018, 12:09 PM

Quote:

Originally Posted by Trackrash

Ed is correct. However, Porsche says 0,010" can be removed IIRC. My old motor had something like 0,020" removed.

You will need to account for the material removed. Normally thicker base gaskets will be needed. You will need to be careful about the resulting deck height, piston to head clearance and the compression ratio.

You can enlarge the holes in the cam sealing flange to account for the fact that the head is closer to the case, instead of machining the cam housing.

.
Correct. Milling the cylinder. head sealing surface will result in changes to deck height and compression ratio as well as stack height (total height of cylinders + cyl. head).

To compensate for the shorter "stack", mill the chain box-to-crankcase sealing surface a commensurate amount so the cam and O-ring seal remains centered in the chain box. Ovalizing that opening as suggested invites a path for an oil leak.

Another result of the camshaft's reduced distance to the crank Is add'l chain slack. If excessive, the chain idler arm becomes very close to the chain box wall (driver side).

Nothing insurmountable; just have to understand cause and effect and take appropriate mitigation measures.

Sherwood

Walt Fricke · 03-26-2018, 10:35 PM

If you mill a little, and add that back with a thicker cylinder base copper gasket or gaskets, you should end up right back where you started, and don't need to do anything with the chain boxes or worry about the chain being too long.

Now if you want to raise compression without different heads or pistons (or maybe rods), that's different when it comes to the chain boxes.

And Ed's point about being sure that it is the sealing surface of the cylinder which contacts the sealing surface of the head that you milled, and not anything outside of those surfaces applies no matter what you are doing.

lrodri64 · 03-27-2018, 05:56 AM

Thanks for all comments.
I'm going to analyze all suggestions.
And post here my final results.
I was really worried but there are alternatives.
Regards.

cgarr · cgarr's Garage

I had an issue with one head out of the 6 had a bad spot that if cleaned up would put all the heads at their limit since you do one you do them all to match. I welded up the one bad head rather than cut them all down.

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