Head cracks: this may be a Cotten question...

[joe G]

List,

During my quest for heads and cases, I didn't take long to realize that a lot of heads have a crack from the spark plug hole to the valve seat and a lot of cases have cracks from the cam hole to the pinion race, but why?

Are these seemingly common failure modes a result of abuse, poor engine rebuild practices, or just what you get when a moderately stressed porous casting goes through a zillion thermal cycles?

Just wondering.

Joe

[Cotten]

Joe!

I'm a carb and manifold student, but I will give you my opinion on the cracks anyway.

First: The head cracks:
Heat is always the culprit, but there's always more than one way to light the fuse.

Prevent cracks by preventing advanced timing, vacuum leaks, pre-ignition, vacuum leaks, low octane fuel, vacuum leaks, lean mixture, and vacuum leaks.

If the compression was 10:1 to begin with, then they appear easily.
Especially if the headgasket surface was not dressed, the headbolts were "grunted", or the manifold "felt tight".

But, dismissing all-too-common instances of cheater-bars on the sparkplugs, miss-fitted pistons, dirt and grunge left over from previous service, and all of the other obviously avoidable complications, the quickest and easiest way to make heat is a vacuum leak.

A sloppy kick at closing time where a knee hits the aircleaner can crack a head. Think about it.

On to the cam-to-main cracks:

Tappet rollers became much-shorter-lived units when they introduced the "needle" version, at the same time as the Pan head. They became a fuse upon the camchest ,as well as the fuse upon many lower case detonations.

Another common cause of stress on that portion of the right case is when a hydraulic unit sticks or 'collapses". The early design had problems filling right off the bat. Although dirt or over-revving can obviously cause any hydraulic to stick, the cause in modern times is too often the Andrews "A" grind.

Tell them I said so.


...Cotten

[steve_wood]

Cotten:

I assume by "grunted" headbolts you mean overtorqued or unevenly torqued. True?

But what do you mean by "headgasket surface was not dressed" ?


steve

[Cotten]

Steve!

An improperly torqued head, or one that is not dressed flat to begin with, are both possible sources of air to the combustion chamber. Not only do the headbolt inserts tend to protrude after years of service, but the surface adjacent to each port tends to recede.
Most blown headgaskets occur after the temperature has risen enough to warp the head by the other causes I mentioned.

And I must correct myself on head cracks:

Besides heat,...
Many, many heads have been cracked by hardened seat installation.
Usually it is from the intake seat to a headbolt insert to the manifold nipple, however.

....Cotten

[King]

Hi Joe

I'm with Cotten on the cause; air leaks=heat=cracks. And these cracks (valve seat to plug hole) are almost ubiquitous with Pan heads. I have three cracked out of four and every head I have looked at in swap meets seems to be cracked. The good news is they are easily repaired. See Headhog when the time comes. He can fix the cracks and do anything else you need to bring a head back from the junk pile.

King

[PanPal]

People pulling big hills with the timing retarded for low end torque has been another cause of these cracks.

[Cotten]

I agree, PanPal.

No matter what the Owner's Manuals say, it does not produce more power.

The Indian world still believes in it, and I cringe every time I hear of it.

....Cotten