Open Road

Hey all, was having a very interesting conversation with my machine shop yesterday. He was explaining to me about the a various clearance areas on the piston skirt that need to be considered in piston design. He showed me using a 928 piston how the clearances change depending on where on the piston.

There is the primary clearance area that the WSM mentions to measure.

Then there is the area just below the oil ring that requires more clearance to accommodate for the high heat. This extra clearance is alow required towards the sides (but I cant remember why).

Then there is the lower part of the skirt that experiences less heat but is important to help control rocking so the clearancesis less in this area because it experiences less heat.

I am wanting to understand these piston dimensions better.I am wondering if there folks on this forum with this understanding or can point me to a resource where I could learn more.

Also, I'd like to better understand the the ring channel depts and how they influence rock and other factors . The machinist briefly covered this but my eyes were pretty well glazed over.

Do we have any engine gurus here?

Piston skirts are barrel shaped. When viewed from the top, the piston slightly narrower along the pin axis than it is between thrust and antithrust skirt surfaces. The diameter is further reduced above the top land. These are done to account for asymmetrical growth of the piston when the engine is running. There are other miniscule dimensional details for noise and durability that oems & more likely, their suppliers, don't publish, including taper in cylinder walls for the same reasons.

Thanks for this additional insight. Super interesting stuff to learn about.