Lapping uppers

[Osmosis]

I have two uppers I plan to use for builds this summer. One is a slick side from AP and the other is an unknown mfg I purchased from RTG a while back. I wanted to share results of lapping the uppers.

I can't quantify how rough the face of these uppers were but I could see a slight ridge on both the inside and outside edges where the barrel flange would mate, and I could detect some roughness by feel. So I got the PTG lapping tool and some valve lapping compound and gave it a shot. Both uppers still show some of the original finish in some places so that will tell you how little material I took off. In other places there is bright metal in a ring so clearly some metal was removed. They look and feel perfectly flat now.

The technique I used was to NOT mount it in a vice block as shown in various videos on the subject, but to just slide the uppers onto the tool and hold them in a gloved hand. I did it this way because I wanted to see if the uppers and the tool were coaxial or not, which I figured I could only tell if I held the upper. The tool fit was very tight on both uppers and I used lots of motor oil to get them in. I used a low-rpm drill and did not detect any wobble. Otherwise I followed the advice to check my progress often and apply the valve compound in such a way that it would not get into the inside of the uppers.

I am glad I did it based on the before and after feel. I will follow up with a report about how off-center the mounted barrels are by comparing a bore sight with the mounted sights, which will take some time.