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Warped head ?


zrex

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  Finally got around to pulling the cylinder head off my truck. The head gasket was intact and not blown out. However using a straight edge I was able to get a 13 thousands feeler gauge under the bar. Oil is good and no coolant in the cylinders. Truck over heated when a core plug failed. Zero compression on cylinders two and three. Very little one and four (30 lbs) With the head so warped how did the head gasket not blow ? I’ll be taking the head and block to the machine shop soon. 

 

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There's no heat gauge in your 720? Gotta keep on top of things.

 

 Zero compression in the middle two cylinders is almost for sure trading back and forth between them even if the gasket looks ok. When the aluminum head is over heated the metal lifts or puckers away from the block. Also if over heated there was only steam left in the head. If there was water it would only have been boiling temperature and not enough to warp it. The head is heated to 400F in an oven to expand the valve seat seats before the interference seats are pressed in. That doesn't hurt them.

 

I had a U67 given to me at Canby and years later took it out to maybe use, and checked the flatness. It was 0.015 high between 2 and 3. I got a glass door from a stereo cabinet and glued sheets of 180 to it and pulled the head back and forth. Every 20 passes I blew away the aluminum dust (it builds up and acts like little ball bearings) and traded ends.

 

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It's clear where the high areas are. As the area gets smaller it takes longer and longer to remove more and more metal.

 

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Even this would work, probably only a thou to go.

 

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Done and it took between 1/2 and 3/4 hour by hand and finished with 220 grit. Cam was out so the valves were all closed. I don't know how much to have done at machine shop but saved that, plus two trips taking it in and back and don't have to wait over a week. Cost was about $5 and ....

 

 

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I did the same thing as Mike but used steal wool to get most of the ugly stuff off and finished with 300 grit emery wrapped around a 10X3" machining surface. Run it back and forth on a 45 degree diagonal across the width of head in overlapping passes moving from front to back. Then rotated 90 degrees and run back to front to create a crosshatch of scoring. Look for low spot voids and repeat until the surface looks uniform. With aluminum, the paper loads up fast and I ran through a pack of 25 sheets doing one head. Still cheeper than a machine shop and made me feel like a hero..

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I had previously copper koted the head, then thought I had better check the flatness as the head was a give away. You could lightly spray paint it so the low spots are obvious. The head is heavy enough to bite into the emery cloth so no need to push down. In fact that would be bad. Just push and pull. 

 

When done a 0.002 feeler gauge would not pass under it. That's less than half the thickness of newsprint.  

 

I used an Alvin 1109-36" stainless steel straight edge..... https://www.amazon.ca/Alvin-Tempered-Stainless-Straightedge-1109-36/dp/B000HF6UVQ

 

p1HtENZ.jpg 

 

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