Piston

[126coupe]

An old German engineer who trained me to build race motors says always polish the tops of your pistons to a bright shine. not only does it make for a better combustion but when it comes time to refreshen they clean easier. This is a Mahle 89MM racing piston that will become part of my 2.5L Screamer

[Andras]

Nice!!!!!

When you say "polish", do you mean bead blast, or hand polish (which I doubt)?

The concept sounds valid; as we are about to unbutton my engine for valve guides and piston rings, this sounds like a great idea.

However, too bad the Concours judges will not be able to appreciate this........Andras

[126coupe]

Hand polish definetly!! get the carbon off by soaking in carb cleaner. its kinda tedious w older pistons, easy w new pistons. Clean and polish the combustion cham,ber on your heads too!!

[smestas]

What do you guys recomend to do the polishing of the heads and piston tops?

Wetsand? Polishing compound? Special buffing pad?

[126coupe]

Once you get all the carbon off you can start w a buffing wheel on a dremel or small drill motor. Be careful though you only want to stay on the top part of the piston. I finish them by hand w a nice terry cloth rag and some metal polish. Of coarse you want the rings out and labled

[David]

I've had the best luck with Simichrome polish. It comes in a yellow and red tube or can. It's made in Germany which can't hurt either.

[911pcars]

I would mask the ring land area with heavy tape, then polish with a buffing wheel and tripoli.

Sherwood
http://members.rennlist.org/911pcars

[126coupe]

The masking idea is a great one!

[john walker's workshop]

what are you going to gain? i can't see it.

[126coupe]

When a german from Berlin named Volkar who has engineering degrees all over europe Who worked with Vasek, Siffert and the like tells me to polish the top of the piston. I polish the top of the piston!!

[Oldporsche]

Could be the theory that it will reflect power?

I do know that the race team of Robert Yates, NASCAR, somehow experimented with ceramic coating of the combustion chamber.

This was to reflect the heat of combustion?

David Duffield

[911pcars]

Advantage? On second thought, maybe less of a tendency to accumulate carbon. Performance-wise, probably not much. People seem to covet very shiny and pretty JE pistons.

Ceramic coating has it's place in reducing heat transfer to the cylinder heads and pistons. Not sure about the current technology or effectiveness of ceramic coating the combustion chambers. If it works, there's potentially more power to push the piston down rather than heat energy lost to the cooling system. Early coating attempts resulted in major flaking.

I don't think PAG embraces ceramic coating as much as some engine builders and race teams do. The later Pcar engines have ceramic-lined exhaust ports; maybe more for emissions than for power.

Sherwood

[RoninLB]

when I was investigating EGTs I found out that if EGTs see over 1,400degF then coat the piston domes.. Some engines see 1,700degF, like in big hp turbos. I may be able to find what a dyno turbo tuner wrote if needed.
........Ron

[Jdub]

There are contrary opinions on this.

Creating a smooth surface prevents particulate from adhering to "hooks" in the metal that are essentially not there due to polishing. However, once you do build up an even covering of carbon (inevitable) it is very likely that a piece will break free, thus exposing a single part of the original metal to the heat where the rest is covered. This is a hot point on the piston and not a good thing to have.

I have heard an even, "hooked" surface is ideal. I am on the fence, as shell-blasting or scuffing piston tops sounds like trouble to me. That said, folks who hold this opinion believe that you don't build an engine in anticipation of the next rebuild, so there is no point in creating a smooth surface for cleanup down the line.

On a side note, I dead-time Triumph/Norton motorcycles using magnetos using a timing stick attached to a dial gauge (the stick touches the piston top). One oldtimer told me to never scuff the surface of a piston via the stick or I'd expose clean piston top where the rest is covered. I guess this goes along the line of the my first comment above.

John

[911pcars]

Is carbon a desired insulator in this case? If so, maybe we should apply an even layer onto the piston top before installation. :-) Just playing devil's advocate.

A few months ago, many folks tried cleaning their combustion chambers with a spray of water into the intake (engine running). It works. If the piston dome/combustion chamber is smooth, carbon should come off easier.

Sherwood

[304065]

The reason for polishing piston tops is to smooth out the surface irregularities that could cause preignition-- you don't want any sharp edges, for example, where the valve reliefs are cut in. Visualize a sharp edge as a mountain ridge-- where the metal is thin near the top, it tends to heat to a high temperature which can be a source of preignition.

I'll admit that it would have to be pretty extreme to do it, but that's what I've read.

You build an engine like this you are spending $20K, what's a few bucks more to polish the domes?