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Why not to let your head be belt surfaced/sanded

The subject head is a Mitsu 4G63T from an Evo VIII
This head was in very good shape to start.
The head was pulled due to a bottom end problem.
It was not overheated.
It was taken to another shop, NOT mine.
The customer was told it was surfaced .003-.004
It was then run and the bottom end failed, Then came to my shop.

The first thing I did was to check the thickness of the head.
I know a belt surfaced head will not be parallel nor square along with not having the proper surface finish for a MLS head gasket.

HN# Date
IV stock New thickness 5.200
EV stock Min thickness 5.185 (AREA SPEC)

If you look at the 4 corners, you will see how the head gasket surface is not parallel to the valve cover rail.
It is close, but still off

This is the surface finish left by a belt sander

This is how the surface looked after I milled appox .002

It did cut most of the surface, but it was still low at one corner, and it would happen to be right around the oil feed port to the head.

I made another pass of about .002
This is how it looked.

I made another .002 cut
And was to this

So one more pass to clean the gasket surface, and proper clamping force will be kept on the head gasket.

I measured the head again, and it was 5.183 at all 4 corners
When I cc’ed a combustion chamber it was 44cc
I also did “oil port mod #1” before I surfaced the head.
This head is now .002 below AREA min spec, but is still a useable head.
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