Piston Slap: Overheating at any speed?

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Dan writes:

I have a ’64 Corvair with a six-cylinder engine. It runs okay after I rebuilt the carbs and seems to be mechanically fine. However, it overheats.

I have locked the air discharge flaps at the lower rear of engine compartment open to allow air flow. Engine cooling fan seems to run properly—belt is tight. It will overheat whether it sits idling for a few minutes or is driven a couple of miles.

Your suggestions?

Sajeev says:

My response shall be brief, as Hagerty’s resident Corvair expert chimes in once again so we can learn from his personal experience. Considering how it overheats, my best guess is that you have an engine timing issue, or maybe a vacuum leak. But whatever: Now let’s hear from the authority!

Kyle answers:

This is an interesting conundrum. A few things come to mind that I think warrant investigation.

The first is to level-set. You say the car is overheating and, not knowing whether you are only using the dash-mounted temp gauge as reference, I always recommend a second opinion. Infrared thermometers have become quite affordable, and this is a perfect use case for one. The temp gauge on a Corvair is mounted in the cylinder head, underneath near the exhaust port of the number six cylinder. It is not easy to manually check temp at that point, but an infrared temp gun can be pointed up through the air flaps to get a read on the head from underneath. Check and see whether your sensor or gauge is giving you misinformation first.

Second, I would check for vacuum leaks and proper timing. If the carbs have been off recently—for instance, when you rebuilt them—it is easy to end up with a vacuum leak between the carb base, the phenolic spacer, and the cylinder head. A vacuum leak would make the engine run lean and thus hot. Too much timing advance would also cause the engine to run hot. A simple check with a timing light will let you confirm your base timing and make sure your mechanical or vacuum advance is not sticking or working erratically.

Lastly, it’s worth taking the time to inspect your cooling system. It might sound like a joke at first, but there is a lot more going on than the fan spinning on top and the flapper doors underneath. Debris and grime can build up on under the “turkey roaster” (that’s the fan shroud on top of the engine) and cause uneven airflow— or worse, no airflow. De-flashing (removing the excess metal left over from the casting process) the cooling fins of the cylinder heads makes a significant difference; it’s worth the trouble of disassembly to remove the turkey roaster to clean and de-flash everything underneath. Also, make sure the fresh air outlet by the number five cylinder is covered and sealed. Any opening in the upper shroud should be grommeted or sealed appropriately. Same goes for the perimeter seals between the engine sheetmetal and the body.

Overheating can be frustratingly simple to solve on Corvairs, but it’s important to take a methodical approach. Follow these steps and feel free to reach out with the results. I would be interested to hear exactly what solves your particular issue.

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