Technical 1939 ford 3" shift fork help. W/pix

Hi all, I have a question concerning the measurement on a 91a-7230 shift fork, the 3" fork for 40/48 gears in the '39 box. What is the measurement from the front face of the fork to the mount pin hole.
I'll attach a pix of what I am wanting. Pix will not load??
So the measurement wanted is from the high gear outside face back to centerline of pin hole. Also how thick is the fork tip compared to the groove where it enters the syncro hub?



Thanks, Doug.

 

If you have a fork from a 40 up trans, just weld it to the back of the 2 7/8 fork and grind the old fork for clearance.

 

Andy, do you have a pic of this conversion?

 

I did a good set of measurements, but I never welded one up. I found a good fork so no need. It would be an intersting experiment with some junk parts.

 

I have been studying the 39 box and earlier forks for a while now.

I have my own theories, but will not get back into the measuring and modifying for a bit.

But an insight is that I think the distance of shaft detents need to be changed.

So far I have turned an earlier shaft over 180 and ground new detents in it and widened the fork and I think I have got it close.

Now I have the correct 39 tower with 3 inch fork , I can delve some more.

I just do not think there is enough throw between the detents to fully home in both 2nd and 3rd.

I want to fully study why there is only 15/16ths to 31/32nds between detents, yet 1 1/4 inches that synchro ring can (and maybe should ) travel .

 

The rail detents do not need to be moved. If you move the detents, the slot for the shift lever will move. I had a 36 shift top that someone put a 3" fork in . Everything was in the right place and it was that way for at least 40 years. I found a 38 top with the double detents and the 2 3/4 fork. I switched the forks. The 36 top was back stock and I got a 81A top with the 91A fork. The distance needed to move the actual fork is the thickness of the forks. If you clamp a sideshift fork to the back of a 2 3/4 fork, it will be in the right place. Weld it up and cut away any interference from the old fork. The only consideration is to make sure it is square after welding.

 

In my last experiment, I read of some here redrilling a new hole in the fork and moving that.

This would be much easier than welding and grinding.

And I found it easier again to knock out the pin and flip the shaft over and grind all 3 detents further along the shaft and use a previously ground out 2 7/8ths fork.

This was a 38 non double detent tower.

I am still of the opinion that the distance of end detents is not far enough apart, and this is why a slightly worn box jumps out of second.

I am going to pursue this when I get back onto it.

 

BTT, With the pix of what I was talking about.

 

Doug, I get 1.540 face to hole.

.385 groove.

.340 fork width.

Wait for others to chime in to average out figures.

 

Took some pics today of 2 identical same number double detent towers..... one with a 2 7/8 fork and the other with a 91A fork for comparison.

I am eyeballing it to very close to half a fork width difference in positioning..

 

Pooch2,
Excellent, question answered. A picture, or several, beats a thousand words.
Best wishes, Doug.

 

I am curious.

Does the 39 toploader suffer from jumping out of 3rd gear too?

There does seem to be a gap when 3rd is pushed right forward by the fork rather than being fully in.