Piston Deposits

[Mr Brown]

Hello, I am looking for some insight into what I found on the pistons of my engine. Below is a photo of one of the pistons from my aircooled 2.0. Would this be considered normal for about 12000 miles of running? There was no oil consumption to speak of. Pistons are Brazil Mahle with what I believe are the standard Mahle cast iron rings and three piece oil ring. It looks like oil deposits to me except for the greyish spots which I can't explain. All the other pistons looked pretty much the same. If these deposits are out of the ordinary and caused by mixture, timing, temperature or other factors, I would like know so I can get those issues sorted out before the new engine goes in. Any input would be appreciated.

[pioneer1]

Hi: Heavy black deposits on the piston dome are unburned carbon accumulated because of too low temperatures resulting from light load operation or too rich a mixture. Drive it harder
Here's my source http://www.theultralightplace.com/pistons.htm
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[tencentlife]

The gray deposits look to be ash, that's not a big problem but can be lowered by selecting an oil with lower ash content (look at mixed-fleet, aka diesel, oils).

Spattering may be due to detonation, when you see that kind of peppering on spark plugs that's what it indicates.

The rest is carbon black and is unfortunately normal and essentially unavoidable; the material quoted above is relevant to 2-stroke aircraft engines that would run at a steady rpm and at peak torque a great deal of the time. An on-road automobile engine spends the majority of its time at part-load and cool temps so carbon blacking is unavoidable. If you want clean piston tops, you have to drive it like a race car. Or install water injection.
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[Howesight]

Hey Mr. Brown:

The thing that caught my eye about your post is the low mileage since this engine was installed, then TenCent mentioned the low-load running and . . and . . .

The air-cooled Type IV engine has a pull-cable and a pulley wheel that are part of the thermostat system. It might be a good idea to check upon the re-assembly of this engine that you have this correctly adjusted and that your thermostat is actually working. The Bentley manual has a test procedure for the thermostat and the procedure for adjusting the thermostat.

If the thermostat was not closing off the cooling air on cold starts, the engine would not warm up properly, and carbon deposits would be bad asnd your oil would become diluted with condensed fuel.

[IdahoDoug]

Yeah, I was also wondering about the miles on this piston. Was that a typo, or does that piston really only have 12k miles on it? That changes things a bit, so clarify.

DougM
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