640 trans work needed

[dale9n]

I hope one of you guys have worked on this tranny. it"s a 4 speed off of a 640. I bought a good 641 rear and tranny with a bad gear. My ring and pinion was shot, so I"m wanting to put the rear off the 641 on my 640. Now. the problem is the shaft out of my good 640 tranny is the old course spline and the 641 is the fine spline. we thought we might be able to take the shaft out of the 641 trans and other than the spline it would be the same. So there it is. Anyone know if I"m on the right track here? I would love some pointers on this one. Thanks, Dale in VA.

 

[Larry NCKS]

Not sure on that one, but you could have a machine shop cut the two couplers in half and weld the course front on the fine back.

 

[dale9n]

if all else fails, we'll do that. thanks

 

[old]

If your talking about the clutch shaft its easy to change the clutch disk and be done with it. No cutting no welding etc. I had to do that when we put in another sherman in our 841
Hobby farm

 

[Larry NCKS]

Doesn't sound to me like he's in the front of that thing at all, Old.

 

[dale9n]

I'm not up there. I'm at the rear of the tranny. trying to put the rear end on. thanks. Dale in VA.

 

[john_Bud]

If the main shaft is good from the 641, you can put it in the 640 case. Then all will be well.

jb

 

[dale9n]

thanks jb, do you know if I can get it out from the top openings, or is there a need to break trans from engine too? Dale

 

[john_Bud]

I've only taken them out with the trans on the bench. You might be able to after taking off the output shaft seal. It would be worth trying!

I know they are a PITA to get out as you pretty much come to the conclusion that they were in place before the casting mold was filled with iron, then suddenly you find the orientation that allows it to slide out slicker than deer guts on a door knob.

The only caution is that the preload needs to be adjusted. There are thin steel shims that go under the input and outputs. If you are too loose with zero rear shims, you will have to pop the front off too.

Get a feel for spinning the main shaft in the case in N. That will be typically the loosest that it is supposed to feel like.