case 621 zf transmission issues cold weather

it has been real cold here lately and I usually put on a magnetic heater on the transmission and that helps the trans is sluggish until you run it for 30 minutes or so then it works fine I want to take out the drain plug and install a heater the one in the picture is 5" long and has 1/2 npt threads any input would be great I was thinking of using #18 on the print



also has any one ever had the forward-reverse switch apart are there micro switches in there or are is it a circutboard

The immersion heating element seems like a very good idea. Putting it in the oil sump is the problem, maybe even a new hole is necessary...

The 621 *is* cold blooded, ours takes a long warm-up before it'll move in 1st or 2nd gear.

I don't believe that the plug your referring to is pipe thread. You may not want to put your heater there anyways, as the suction tube will be rather close to your heater element, and you don't want to restrict the suction tube.

I have only seen others install the glue on heater pads on the transmissions.