I'm trying to install a new o-ring seal for a TH400 dipstick tube after a rebuild and I can't get the tube to push all the way into the transmission bore hole. The tube will slide all the way into the transmission bore hole without the o-ring in the grove, but when the o-ring is on the tube it won't slide all the way in. The tube still needs to be push in about another 1/4" for the tube bracket hole to line up with the bellhousing hole. It seems as the o-ring seal isn't compressing enough in the tube grove once it enters the hole in order to push the tube all the way in.

This complete car rebuild has been one step forward and two steps back,

Is this in or out of the car? Are you trying to get it in dry? A bit of tranny oil will help it slide in. I always use some RTV sealer like Ultra Black on all gaskets and seals. It will also help lube it enough to slide in.

If you are still struggling to get it in you could remove the transmission mount and that would let you move the engine up and down a little to help align the dipstick tube. There is not much wiggle room trying to get them back in. Just make sure not to move the engine/transmission combo up and down to much. The fan and distributor are more than likely the first things to make contact. Sorry, I should have read everything before posting. Could be the wrong size o-ring. I have never had a problem with one going in.