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About a month ago I reported that the distributor on my '36 seized, causing the cam shaft to freeze, thus allowing the crank gear to tear up the cam gear. When the distributor shaft froze, it caused two violent backfires before the engine failed. The backfires actually blew my muffler off the exhaust pipe!
I had the cam gear replaced and put the engine back together. While the engine was on blocks, I decided to check the compression.
To my surprise, I had no compression in all cylinders except #5 which was normal. At first I thought I did not line up my crank and cam gears, so I took the cover off, and found that I had installed them correctly. I then removed the rocker arms and blew compressed air into each cylinder through the plug holes. In each hole I was able to move the crank, but I could hear air coming out the exhaust ports. Blown head gasket?
I took the head off, and it looked good, all the valves were seated, and there were no signs of burning. I took the head to my mechanic guru, and he asked to see the head gasket. Without knowing, he pointed to gasket leaks in all but cylinder #5.
He resurfaced the heads and checked the valves for leaks, and the valve stems for straightness. They were fine. I replaced the head with a new gasket, and put the rocker arm assembly back together. I now have good compression in all cylinders.
My question is how did the engine failure cause the head gasket to blow in 5 cylinders? Or are they two coincidental incidents?
My mechanic guy suggested that the head may have gotten too hot due to air cavitation , and suggested that I drill a 1/16" hole in the thermostat to allow air, which may have been trapped in the head, to escape. My temperature gauge showed 180 degrees at the time of the engine failure.
Any thoughts?
I drill a couple of 1/16" holes in the thermostats I use in my old cars. Sounds like a lot of coincidental failures all at once.
Trapped air could be an issue. Drilling a small hole certainly won't hurt. 180 isn't abnormally hot. My 38 usually runs ~180 during the summer and when first slowing down after driving on the highway. On a hot day, especially when first rolling into town from a highway drive, it often runs hotter than that, sometimes close to 200. Keeping the idle up to pull air through the radiator usually cools it down soon. I have a 160 in the car.
7046 old site posts
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1938 Master Business Coupe
1953 210 Sedan
Thanks Tiny,
I’m still frustrated by the events that occurred and the damage that resulted.
the frozen distributor and the damaged cam drive makes sense, but I don’t understand how the blown head gasket plays into the scenario.
Is it possible that the head gasket failure caused the other problems?
This is all general theory since I'm not there to look and wasn't there when it happened. First, I don't think cavitation had anything to do with it. Air entrapment and cavitation are totally different things. I imagine the backfiring is what took out the head gasket. Someone with more experience than me will have to correct me if I'm wrong but I think it played out this way. 1: The distributor/cam bearing failure cause a fatal ignition timing issue that resulted in the backfires. 2: The backfires caused the head gasket to fail by creating substantially higher cylinder pressures than it was designed to handle. The engine temp seems pretty normal to me but drilling the hole won't hurt.
7046 old site posts
Save a life, adopt a senior shelter pet
1938 Master Business Coupe
1953 210 Sedan
Thanks Guys,
I have been away for a week.
Tiny, I agree with your analysis. It's all back together and ready to install. I also found that my vacuum advance was not working when I took everything apart.
It is at the Filling station for repair. I wonder if that had anything to do with the failure.
If anyone has a working vacuum advance for this car ('36 Chevy), I would be interested in a back up.
Steve
Rockauto says they have them.
7046 old site posts
Save a life, adopt a senior shelter pet
1938 Master Business Coupe
1953 210 Sedan
Steve,
If all else fails, a '37/'38 (and maybe more) style advance can be easily modified to work on your '36 distributor. These are easier to find. The advance characteristics might be a bit different but it is unlikely that it would be noticeable. Feel free to contact me for details.
Mike
Many Miles of Happy Motoring
3469 Posts on Old VCCA Chat
The ones at Rock auto are definitely "after Market".
Thanks for all your assistance on this issue. (from the beginning)!
Steve
Your vacuum advance was more than likely damaged by the backfire getting into the intake side via a intake valve open at the wrong time.
Mike B
Mike I agree.
it may also have been damaged when the distributor rotated after the shaft froze, plunging the arm into the diaphragm.
engine is back together and running. This week, reattach the drive shaft and see what happens.
thanks for your input.
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