Problem is caused by the fact that most people don't have the right socket to tighten the countershaft sprocket nut.  Most just use a wrench on it and you can't get it tight enough.  I had a socket cut in half in a lathe and had a piece of pipe welded in between so it fits over the main shaft. Did the nut up TIGHT and it never leaked again.  If the nut is not tight enough, the countershaft gear (going by a bad memory, and I'm too lazy to look it up, but it's the gear that sits opposite the sprocket inside the tranny) has slight side movement on the mainshaft, this pumps oil down the main shaft, through the bush  and through the seal and leaves that lovely little oil dot on your shed floor.  By getting that nut up tight it pulls the boss of the main gear against the tranny case main bearing and seals it up. The slightestt of play and it becomes an oil pump. 

 

Make sure there is no play in the bush and your seal are good.  Your countershaft nut can be tight and it still leaks.  It has to be torqued to factory specs. 

I posted this before Isaw the other thread on 4 speed leaks and I can't work out how to delete this one, so sorry if this was covered in that thread.

 

 

 

 

 

Yeah okay, thanks for clarifying that. I was going by a foggy memory and I've forgotten all the technical stuff. (Been 20 years since I tore apart a 4 speed) Probably would of been wiser to omit it. My gear box leaked, replaced seals a hundred times, replaced bushes. Everybody that I knew, had gearboxes that leaked. In the '70's and 80's most people (I knew) just lived with it. Getting the sprocket nut tight fixed my ten year headache. So providing seals and bushes are good I assumed that it may of been the cause of a large percentage of the leaks. Mine never leaked since.