Just realized I've been handling stuck helicoids two different ways and I'm not sure which is better

Last week in my home shop, a Canon FD 50mm 1.4 came in with a completely seized focus ring. I tried the classic method of soaking the threads in lighter fluid overnight, which usually works. This time, after 12 hours, it only budged a millimeter. So I switched to a 50/50 mix of acetone and automatic transmission fluid, applying it with a needle every hour for three hours. That finally did the trick, but I'm worried the acetone might be too harsh on old plastics inside. Has anyone else compared these methods on vintage lenses? Which do you think is safer long-term?

4 comments

4 Comments

the_henry2mo ago

Wait, you put acetone on a lens with plastic parts?

charlesj462mo ago

My buddy learned that the hard way, totally wrecked his lens, and @matthewperry was right about lighter fluid.

jordan6532mo ago

Holy crap, did they actually do that?

matthewperry2mo ago

Whoa, hold on a second! That acetone mix is way too strong for anything with internal plastic, you'll melt the aperture ring or worse. Lighter fluid is the safer bet, it just needs more time sometimes. I've had helicoids take a full two days of soaking before they finally let go. Patience is key with the old stuff.