I had my m-21 rebuilt about 12,000 miles ago which stopped it from popping out of 3rd and 4th. I recently replaced 2 sliders and 4 sychros and now it pops out of 3rd and 4th when I decellerate for more than 5 seconds or so and when I stay in 4th for over 10 minutes or so it is hard to get out of 4th. The shifter won't move at all until I force it. The hubs appeared to be in good condition and the input shaft was worn on the teeth and the pilot rod.
Any suggestions?

thanks
Mike

Adjust the shifter. Its a common problem when the shifter is out o wack.

From what I have researched it is a common problem even if your shifter is adjusted perfectly. My M-20 would consistent drop out of 4th on deceleration.

There is at least one vendor that is producing an improved syncro slider collars, because of this inherent problem, but they cost twice as much. He has them sporadically for sale on ebay.

For my rebuild I went with brand new shift forks, a brand new slider and a brand new hub for 3rd-4th. Also, I had the choice of a key kit with a thin spring or a thicker spring key and I opted for the stout one.
I don't have the tranny in the car yet to let you know if my falling out of 4th problem is gone yet, but I'm crossing my fingers.

If you have excessive endplay in the sliders and forks, then yes, I guess it would be a common problem for ALL transmissions using and external shifter. But even with the newer and/or older replacement sliders/dogs/hubs/syncros the problem is mainly asociated with the adjustment of the shifter.

After adjusting these shifters for 15 years+, there is a sweet spot that you will only find by adjusting the shifter until it doesnt pop-out of any gear. If it continues than it would be more related to an internal part. And if you are using a Hurst 'Competition Plus Shifter' the adjustment of the throw limiters also plays a big role in this as does the adjustment of the 'Dog' springs. This is very critical to eliminating this problem.

Employing the "Double D" side cover arms and the "DD" arm spring will greatly reduce this from happening as well. (the double d is the insigna stamped onto the arms themselves; a factory GM/Muncie part and the asociated spring comes with). :D

What is a dog spring?

Mike

It is the spring that is in the inner diameter of the hubs themselves. The dog 'springs' push the dogs themselves outward to hold the slider into its position when shifted. You can open these springs more by streching- opening the diameter, being that they are shaped like the letter 'C', it puts more outward pressure on the dogs, thus providing a more positive shift.

I didn't stretch the dog springs but I did trim the tangs on the ends so they would not rub on the inner of the hub.

Mike

Make sure the input shaft doesn't need to be shimmed in deeper. They make a slider called a torque loc they are available from most trans parts places and will cure a real bad set of enguagement teeth.