Fair enough Brian. I haven’t tried lapping a big piston.
which confirms my thinking that the fit of the piston isn’t critical for a piston with rings. There are thousands of engines being mass produced every day without lapped pistons.
Hi Tim, the piston manufacturers work to 0.0002 mm when machining, but the maching of measured pistons to bores needs better than that so the pistons and bores are measured, given size grades, and matching grades are selected on the assembly line. This applies to bores, crank journals, gudgeon pins to pistons and con-rods, rings, bearing shells to cranks, con-rods and crank-case journal mounts. A big part of the longlevity of the engine, so it can be driven from new as if it has been run-in already. And then last 100 of thousands of miles before it needs a re-bore or crank re-grind.
In the mid-1960s cars needed a re-bore at 20, 000 miles (I did dozens!), and crank re-grind at 40,000miles. In the 1970s I had a well-tuned motorcycle that wore the rings so I had compression loss after 3 ~5000 miles! Nowadays, that would be quite a low performance engine! As standard I changed the rings at 10,000 miles, to regain good compression. Poor by today's standard, but that is the same technology and machining capability achieved by our model workshops...
It was a revolution in machining in the1970s that made engines to better tolerances, partly driven by the fuel crisis and public and national interest in economy, partly following the small wealth boom of industries that survived the 1960s, who invested in new machinery with better capability. Yet in the late 70s I worked with 4 other engineers, and we all had new Company cars, built in the same week, and with the "same" engine. Top speed varied fromantic about 90 mph indicated to nearly 110mph, and a clear difference in performance as driven, and fuel economy. - We compared then together over 12 months and 20 - 30,000 miles per car. As we were working on a job 350 miles from home we did the same commute and long trips. One was showing definite worn engine symptoms at 30,000 miles (losing some compression, and burning oil), one just seemed like new. Very variable quality! Not in the industry today.
Enjoy,
K2