You big dummy! Pay attention.
 

One of my winter projects is to finish the rebuild on my ‘62 Mercury engine. So having just finished cleaning the pistons up, I figured I’d break out the micrometer and feeler gauges to inspect them.
I installed a new top compression ring and checked clearance between the ring and groove, it’s supposed to be no more than.006”. I’m getting around.010”. Crap. I grab another piston and check all the way around, same thing. Man, this engine has been in impressively good shape till now. I’m checking the Summit website for replacements ($450), and noticed I had grabbed my son’s metric gauges! I grabbed my old Craftsman set and now showing.004”. Whew
I can only imagine how pissed I’d be if I ordered new pistons and they showed up and measured the same. :)

Yea........but.........how do the rings feeeeel in the lands? They got any play up& down?

4mil sounds good enough for a used slug rebuild. Racers may agonize about it though.

This happened a number of years back. I had a friend who was building a 455 for his car. He wanted to degree his cam, so I loaned him my dial indicator and degree wheel. A few weeks went by, and I asked about the tools I had loaned him. He said the cam he received was wrong, so he sent it back. About a week later, he called me and asked me what he was doing wrong because he was getting the exact same reading on the replacement cam.

I went over to his house to see what was going on. After checking his setup, I found he was measuring the lift off the cam base circle. The cam lift was specified as the valve lift using 1.5 rocker arms. Once he took that into consideration, the cam was on the money.