Purple Loctite (222 or 221) weakest, made for small hardware.
Blue Loctite (242), Medium strength, we use on 1/4 or larger bolts.
Red Loctite (262 or 272) High strength, if you never want to remove the screw ever again. actually yah i will come out..just neet to heat it too 500+F to get it off.
Or Green loctite (620) a retaining loctite used to help fit loose fit bearings, would work great holding on the knob on without a screw. and with a little heat (hotter then the motor) you can still remove without destroying the knob.
been working on Aircraft for about 17 years now...so loctite is a friend to me. Especially on a pusher prop aircraft...all fasteners seem to find its way to the prop.
gavineli, I used the blue Loctite originally (knob fell off) and later on, had to take the knob off for easier access before doing a valve check. It came off easily and I re-installed it with another dab. Never has come loose but is real easy to remove when necessary. Sid