[davepl]

I think it's factory stroke.

Here's a picture of the piston. You can see the pin boss relief area does NOT interfere with the ring groove at all, so how or why the ring pops out when it rotates around to this area is the question.

Checked radial depth of the ring, groove depth, amount of barrel to the piston, bore roundness, piston to bore clearance, it all checks out.

As a last ditch attempt he's going to order an oil ring with increased radial depth so it'd have an even harder time getting out of the groove, but we're all still worried since no one can explain how or why it could pop out with the old one!