Polishing Pistons

[DSPTurtle]

Quick question for you guys... Wayne says in his book that carbon buildup on the pistons is pretty typical for the 911 engine. Has anyone torn down an engine after the pistons had been polished and seen a noticable reduction in the amount of carbon buildup on the pistons? I just wanted to get some opinions before I invest the blood, sweat and tears of polishing the tops of the pistons. 1985 Euro 930 motor with stock 96mm flat top pistons running CIS.
Thanks in advance!!!

[DSPTurtle]

So I spent some time polishing the pistons even though I have been unable to find empirical data showing that it makes a difference. My question now is how polished does it need to be to be effective?
I wet sanded with 250 then 400 and finished with 2000 grit. After that I polished with mag wheel polish. My bench grinder polishing wheel was too loaded up and ended up just putting more scratches on the surface. I am planning to go get a new buffing wheel tomorrow, but do I really need to go any further? How polished is polished?
Here are some pics that maybe you can get an idea of how much further I need to go!!!

[350HP930]

You can never avoid carbon build up no matter how much you polish them but for the sake of preventing pre-ignition getting everything in the combustion chamber nice and smooth is never a bad thing.

Its actually quite easy to de-carbon your combustion chamber at any time with nothing more than a little 'water-injection' while the engine is running under load. The water vapor getting into the carbon and turning into steam can remove all the accumulated carbon build up out of there. If you have ever taken apart an engine that blew a head gasket or had a leaky manifold gasket that allowed a little water into an operational combustion chamber its easy to see this effect first hand.

[DSPTurtle]

I have done that on other cars before. But, do we really want all that hard carbon running through our turbos?

[WolfeMacleod]

Originally Posted by DSPTurtle

.... I am planning to go get a new buffing wheel tomorrow, but do I really need to go any further? How polished is polished?

As a knife and sword maker in a previous life, it's not polished untill you can see your reflection in the surface.
Clearly.

[DSPTurtle]

Hmm... but aluminum is a pretty soft metal. I can get steel to look like a mirror, I can only get aluminum to look like a cloudy mirror. Is there a technique or compound that will bring it to a mirror finish? I just re-did them about 15 minutes ago and I can see my reflection in them ( I guess that means I am not a vampire) but nowhere near as clear as a mirror.

[Flieger]

It is very hard to get Aluminum and especially Magnesium to stay polished. I would not go crazy with a mirror finish on your pistons. I don't think it will last too long and there will not be that big of a difference between a rougher finish.

Aluminum oxidizes right away and unless you anodize them (which makes a little cloudyness in the finish) they will not stay mirror finish for long.

This is the best you can hope for:

[notmytarga]

I did my wheels - so I couldn't help myself. I didn't like the texture that was left after they were glass(?) blasted by the shop prior to balancing them. It didn't change the weights BTW.

[opticalfuel]

I was thinking of polishing the pistons... but I see that it's really not going to make too much difference with carbon build up. It just makes it feel better to put shiny new pistons in the cylinders when rebuilding... Nice for the picture gallery, but functionally it's probably going to be of little value...

Here is my rebuild action: http://www.74911RS.com/

Here is a nice shot of the pistons. The motor had stamped on it (opposite the ignition distributor: 930/09

Any idea what configuration that translates to? Are these pistons 8.5/1 or higher?



Any help is greatly appreciated. I have quite a project going on.

It's hard to make out, but the markings on this piston are: 'K6' and faintly a '2' and the '+' also.
Who knows what compression that translates to?

[opticalfuel]

Originally Posted by DSPTurtle

Quick question for you guys... ... 1985 Euro 930 motor with stock 96mm flat top pistons running CIS.
Thanks in advance!!!

Do you know what compression ratio that your flat top pistons are?

I have a 3.0 Liter CIS engine I am rebuilding, and they supposedly were higher compression Euro spec pistons, but I don't know where to find the answers for which is which.

I am trying to figure out if I need to get different pistons to use with carbs (probably PMOs) since my vehicle will be built on a 74 chassis. It is smog exempt (of course)

Any ideas?

Thanks!

[cgarr]

What you really need is dimpled piston tops, like a golf ball!

[cmonref]

Opticalfuel: 930/09 designates a ROW 1979-80 3.0L 8.5 CR, probably with the big-port heads and CIS. At least it was that way when it left the factory ...