Z  Tech Tips  Cracked Head

Overview 

Early Zcar heads are prone to crack due to their length and thermal expansion when overheated. Luckily proper timing, cooling and fuel mixing should prevent this problem.
Unfortunately for me, my car was delivered with a cracked head.... and an exhaust manifold that glowed!

Cracks usually appear between the valves or between the plug hole and a valve. Cylinders 3 & 4 in the middle of the head seem to suffer the most from cracks.

Severity of cracks and location will result in different symptoms. Sometimes compressed intake stroke or explosive exhaust gas will be forced through the crack and into the coolant system. Extreme pressure at the radiator and bubbling usually result. Often the sheen of and oil slick in the coolant can be seen under the radiator cap.. Other cracks may allow coolant to enter the oil system and cause it to turn milky. Cracks allowing coolant into the combustion chamber result in white smoke from the tail pipe and radiator fluid loss. As well, crusty deposits will end up on plugs and in the chamber.

Compression tests and leak down tests as well as cooling system pressurizing tests can detect cracks or bad head gasket symptoms. In my case, the compression tests constantly showed cylinders 3 & 4 to be lower than the others.

My specific symptoms were:

• low compression test results for 3 & 4
• 3 & 4 spark plugs always crusted
• random loss of coolant (sometimes it would stay filled for 2 months, other times it would drain in 15 minutes)
• white smoke intermittently from exhaust
• no significant drop in idle when oil cap removed
• Cylinder four spit water randomly when doing compression tests

The weird symptoms I had were:

• Car seemed to run fine and pull well
• Vacuum gauge was good at ~ 19" Hg
• I had a radiator shop chemically test the coolant for hydrocarbons and none were detected. They also pressure tested the system fine
   

The bottom line for me was that I has a crack that would open and close randomly.

 

Pictures

Here are my plugs in mid summer 2003 when I got my car. Clearly #3 and #4 have an "issue".

Here are my plugs in late summer 2003. 3 & 4 are the same.


2003 plug 3 with caked on crust.

Here are my plugs last week (June 2005). The are hotter "5 temp" NGK's and they self-clean much better. You can see that 3 is still bad and 4 has some crusting.

 

In 2004, I noticed water drops coming out of the number 4 plug hole when doing a compression test and turning the motor with a remote starter.
Note that water in combustion chambers is supposed to wash them clean... not on my car! Don't believe everything "they" say.

So I finally pulled the head for the first time hoping I simply had a blown or leaking head gasket. Here is what I found:

 

Cylinder 4 was coated with a crust (click to enlarge)

You can easily see the crust difference and thickness when compared to cylinder five.

Cylinder 4 walls had white deposits on the exhaust side indicating coolant dripping.

Another view of the same. The other bits are debris that fell from the head when removed.

When the gasket was lifted, evidence of water migrating from cylinder 4 to cylinder 3 is evident.

Cylinder 4 actually affected 5 & 3

The exhaust liner for cylinder is showing a crusty build up.

The valve stem also is crusted

The exhaust valve seat is under there some where!  Can you see the crack?

My head has 3 cracks so far. Here is the biggy in number 4 cylinder
Note: This travels along the direction of the exhaust liner's anchor.
 

Here is a small one in number 3

Here is the crack in chamber 6 between the exhaust and intake valves.
Note: Some claim cylinder 6 is the hottest due to its distance from the radiator however the coolant first enters the head below # 6 so I question this claim.

So much for my 2mm shaved high compression dished head dished piston experiment