Quote Originally Posted by petertha View Post
I'm intrigued by your jig. In a former life I used to race RC models 'pylon racing' (200 mph, 0.40CI 2S methanol engines, 30K rpm). So shaft TIR is a similar big deal, especially after an unfortunate 'deep dirt injection' & you are now wondering if the shaft is still good. It is indeed a tricky object to measure accurately, worse on our engines because there is no stub shaft on the other end, just the counterweight/crankpin & annular gas passage hole. So even less to reference off of like end centers.

But doesn't your entire principle rely on the main shaft being 100% true? For example if orange line is 100% concentric axis & red line is 0.001" (bent) deviation, isn't it just translating that deviation to your dial indicator as it rotates through the fixed bearings?

Attachment 23016
Of course. If the main shaft is bent the indicator will show a problem. That is what it should do.