any tips on replacing the glass before i ruin a new paint job?

I just did one Friday. I heated up a 16 penney nail (large) with a tourch and made a hole in the plastic sight glass. Then used a hook probe to pull it out by the perforated metal behind it. The bike was a 1976 DT400, not sure if the older models actually used glass?

When installing I put a little injector oil on the seal edges and hole. Just push it into place.

I just now installed a new one on my AT2 oil tank. I let it sit in some hot water to soften the rubber up a bit and carefully pushed it in place. You just need to be a little careful getting the last seal lip in place so the sight glass seats properly, as the previous one I did went in a little too far,but no harm done

sorted, found a suggestion to put it in the freezer for a couple of hours, little oil and pushed it in, was still tight tho. thanks for the replies.