I have a 1979 Delta 88 stock 350 cheapy rebuild.

I'd like to measure how much Thousandths off the head was milled off from stock so I can figure out if I can mill more, compressed head gasket thickness needed, etc.

How can I measure this? or a machine shop??

Can't seem to find a spec or reference on this..


Need to keep stock compression ratio..

Thanks
Fred

 

I would think you can use a precision measuring device to measure the head thickness before milling off any material. I would hope yhe shop doing the work is capable of doing that. There is no "spec" for head thickness unless its on a blueprint.
How much variation in compression ratio is tolerable ?

 

How many thousandths the head is milled is only part of it. You can calculate your compression ratio via measurement.

So, volume of the chamber plus volume of the gasket thickness plus volume of clearance plus volume of the piston dish is your compressed volume.
Volume of the displacement of the cylinder plus all the above is the uncompressed volume. Compression ratio is the second divided by the first.

All the little volumes can be done by measurement and math. The combustion chamber volume can be measured by closing some valves, greasing the head, putting a piece of plexiglass on it, and putting a metered amount of fluid through a hole into it, filling the dish. You either figure by volume, or by weight divided by density. Remember that combustion chambers are in cubic centimeters, and the rest of the stuff is in cubic inches, so convert when necessary.

In short, the amount of material gone does not directly matter, it's the geometry that's left that matters, in my opinion.

 

I'll bet there was a blueprint spec thickness that had a measurement from say the valve cover plane to the head plane.. what the machinists in the factory would have to mill to after the casting was made, I'm sure these surfaces were not smooth from out of the mould..

 

There was a fixture to quickly clamp the head casting to that allowed machining one or more sides of the head simultaneously. The head casting was probably located on the roof of the combustion chamber to keep combustion chamber volumes close to the same from one end of the head to the other. A specialized machine would then cut rocker arm, intake manifold, and exhaust sides in one pass through the machine. The next step would mill the head gasket side.
Since most cylinder head combustion chamber volumes from the factory fall within a range of 2 c.c.'s from one head to another, it would seem there gasket thicknessa production height tolerance of .010" to .012".

What your capable machinist needs to know is the OLD head gasket thickness and the NEW head gasket compressed thickness is. He will machine the amount of the head gasket difference from the heads.

 

Ask your machine shop to look in their AERA specifications. The number is there.

 

Mondello’s engine book had an image and measurement pertaining to this as well. IMO, CCing the chambers is the only accurate way to evaluate but I’m no expert.

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I would CC the chambers. The shop I worked at used to stamp the amount milled off on each head along with a WO number for future reference if needed. That used to be common practice in the industry.

 

I doubt I would use the spotface for head bolts as an accurate place to confirm if the OLDS heads had been milled. The most accurate dimension would be from the head gasket surface to valve cover/rocker arm surface. Believe what you want to believe from Joe Mondello's "manual", but ask yourself how head gasket surfaces are cut at your local machine shop.
If I recall, Cutlassefi had a dimension for the overall head thickness.

 

Ralph, wouldn't they set it up in a fixture and run across it with a dial indicator and adjust till they got it level and then make a clean up pass, then zero off that level and cut down the desired amount, meaning that what the machine shop initially measured from is now gone for future measurement?

 

John, who is "they" ? Are we talking manufacturing or local resurfacing ? Using head bolt spot faces takes time and does not provide a lot of rigidity for milling.
1. Slap the head down on the mill table, clamp with "toe clamps". You can run an indicator across the head gasket surface to record "out of flatness" and the high spot if you desire. You can also depth mic down to the mill table and record this dimension.
2. Or you can touch down the the milling cutter and take a "skin cut" to clean up. Depth mic down from the head gasket surface to the mill table and record.
3. Adjust the cutter height and mill across the head gasket surface and you are done. Depth mic to the table and that is your final height.

 

Thanks, that's what I thought. I was asking about best practices in general, no specific party in mind.