polishing piston crowns and connecting rods

samlee · 08-29-2006, 07:57 PM

This is my first post here. I'm actually rebuilding two motors...though not Porsches. They are BMW motorcycle engines (one is a R1100s, and the other is a R1150 RT police bike motor). They are similar to 911 motors in that they are horizontally opposed (air and oil cooled) and are German;-) Only two cycliners though.

I have heard conflicting thoughts. Should I polish my pistion crowns to a highly smooth finish?

My connecting rods are steel and rough casted. I'm going to balance the big and little ends and the overall weight. I was also thinking of grinding off all the casting marks and roughness and polishing them smooth. Any thoughts to this? I have no idea of BMW shot peened to remove the stresses through the conrods or not. So what are your thoughts? These motors are in quite the high state of tune as they are.
Thanks,
Sam

88-diamondblue · 08-29-2006, 08:11 PM

Sam,

Welcome to the board. Do a search in the rebuild forum as I asked this question a while ago so I know there are threads about it. I ended up with new JE pistons so they were already nice and smooth.

samlee · 08-29-2006, 08:41 PM

Thanks Michael. I'll do a search.

911pcars · 08-29-2006, 09:58 PM

I'm sure some Bimmer engine rebuilder has gone through this before and may have suggestion for strategically removing metal from the factory con rods, pistons and piston pins w/o sacrificing strength.

Not sure you gain much by polishing the piston crowns. Removing sharp edges would be good insurance against localized hot spots and resultant pre-igntion. Some builders apply a ceramic coating to the piston tops to reduce heat transfer; add low-friction coating to the skirts to reduce friction, but make sure the vendor has a track record. Swain Tech is one such company.

In general, removing casting flash minimizes stress risers and should increase fatique life. If the rods were shot-peened previously, have them treated again after grinding, smoothing, polishing and balancing.

In general, anything you can do to blue-print an automotive engine is fully applicable to a motorcycle engine. the overall cost is less too since you're only counting two cylinders.

Hope this helps,
Sherwood

samlee · 08-30-2006, 07:32 AM

Thanks for your input Sherwood. I am going with a bigger valve combination that I know works, but am sure that a ceramic coating to the postion tops would be compatiable. I'll have to check clearances once I am further along.

I am a fan of Swain Tech, btw. I used them to coat my exhaust headers on my 1961 Daimler SP250 vintage race/street car. Great product. for that application.