Here are a few possible causes of the air coming out around the bolt(s):
1. The head could be warped, except that you said it was milled flat, so I'll
assume we can skip this one.
2. The deck (top surface of block) could be warped.
3. There could be some deck (or head) erosion between cylinders from a former
leaky head gasket. This is really fairly common, especially on the narrow bridge
areas between cylinders.
4. When you (or someone else) cleaned the head or deck, they could have left
some grooves in the surface from improper use of scraping tools.
5. The head could be unevenly torqued, allowing the head gasket to leak. I'll
assume that you torqued the head in increments, following the proper bolt
sequence each time.
6. The head bolts could be incorrect (allowing you to "torque" them, but without
tightening them against the head).
7. The head could have been milled so far that some bolts either bottomed out or
the grip area (the unthreaded part) ran into the top of the block. When you
remove the bolts, check for any signs of bottoming or running out of grip range.
8. The head gasket could be incorrect.
9. There could have been some debris on the head or deck when you assembled it.
10. The head or block could be cracked, as you suspect.
11. Too much or incorrect gasket cement. I like to use a thin coating of the
Permatex high-temperature copper spray-on cement.
12. Or, it could be poltergeists, in which case I couldn't help you much. Call a
representative of your chosen denomination.
As to the compression problem, well, it could be due to the above items, or it
could involve valves, rings, pistons, etc. It could also be a matter of
incorrect valve position during your stationary readings (valves not fully
closed). Of course, for static compression testing, just pressurizing the
cylinders is not really a correct method for reading compression, since you need
a controlled flow rate at a given pressure into the cylinders for a static test.
Nevertheless, if there is a large difference like you got, it does seem to
suggest a problem.
Next time you pull off the head, you need to check it carefully for cracks,
deposits, discoloration, etc., both before AND after you clean the mating
surfaces, since cleaning can often fill small cracks or other defects. You could
also take the head to a shop and have it tested with dye penetrant or via some
other method, depending on the shop's equipment.
After cleaning the surfaces VERY thorougly, you need to check the deck and
recheck the head for flatness. There are several ways to do this. Drop me an
e-mail and I'll outline some approaches in more detail. If the surfaces are not
flat, they will have to be properly resurfaced by a machine shop. If the head is
cracked, I'd pick up a used or NOS head. They aren't hard to find. If the block
is cracked, I'd seriously consider getting another engine, although they can
often be welded and sleeved for a price. This is NOT a do-it-yourself project.
Alan
JASON GREEN wrote:
> Believe I found out why my M37 will not start. Its compresson reads from
> 1-6 as: 30,30,79,30,30,70. Thought this might of been the valves but I now
> suppect the head as the culpret.
>
> Here is what I have done to the head:
> 1. New Head gasket
> 2. Had Head Milled Flat
> 3. Torqued all bolts to 75lbs/ft.
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