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

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also has any one ever had the forward-reverse switch apart are...

case 621 zf transmission issues cold weather

case 621 zf transmission issues cold weather