[one70gt37]

Quick question guys, my 1970 m-21 muncie does not want to stay in 4th gear, wants to pop out of gear under even light throttle, the trans is rebuilt ,clutch,throwout bearing, roller pilot bearing are all new. Shifts up & down fine in all gears & all linkage moves freely & goes in fine while running through the gears on the hoist,& no odd noises any thoughts on what to check out?
Thanks Gary

[Skip Fix]

Tranny to bell housing loose or bad pilot bushing can do it.

[boater bill]

This sounds like worn gears or syncros.
That's been the case for me everytime the trans would pop out of gear.

[bobzdar]

Happened to my cousin, the stops on the shifter had to be adjusted as it wasn't allowing one of the gears to fully engage. If you had the trans out, go through the shifter adjustment procedure and make sure there's not slop. That or you have bad synchros.

[Tom Vaught]

Unless they replaced 4th gear in the trans, once you beat up the teeth that engage the slider collar (by power shifting) there will not be enough engagement to keep it in gear. You can try and adjust the shifter rod to force the lever farther into 4th gear vs 3rd gear and that might keep it in gear.

Either linkage rods adjusted wrong or beat up engagement teeth on the 4th gear.

Did you put a pin in the shifter slot on the trans and then adjust the rods in neutral from that point?

Tom Vaught

[Half-Inch Stud]

Circa 1989, I had a M21 HD that I rebuilt with NEW blocking rings.

I vote for it needing NEW engaging teeth...that probably means a new input shaft.

That was over 20 years ago so I could be wrongk.

[PONTIAC DUDE]

Synco rings and or hub teeth, 4th gear synco ring on shaft and keys are shot.

[69Customs]

Bad sync ring. Won't hold it in gear. Have to hold by hand. Pops out worst on hard acceleration or fast deceleration.

[andyleonard]

Make sure - even though you like the shifter rod adjustment on the lift - that the lever isn't hitting anything when the engine is moving around on its mounts. Do you have an old rubber surround that could be bunching up and nudging the lever out of 4th? Are there shiny places on the lever or the metal surrounding it at the floor or console?

[400 4spd.]

Originally Posted by 69Customs

Bad sync ring. Won't hold it in gear. Have to hold by hand. Pops out worst on hard acceleration or fast deceleration.

That's been my exp., esp. on deceleration.

[JoePapa]

Start with the easy stuff - check the stops on the shifter to make sure you didn't have something come loose that is preventing the full shift into 4th. I've had this happen before when the jamb nut came lose.

[lust4speed]

Isn't the syncro ring just along for the ride once the gears are meshed? Seems that the syncro ring could be left out entirely, and once the gears are engaged it wouldn't make any difference.

I'd vote for partial engagement being the problem.

[AG]

Shift it into fourth gear (car not running) go underneath and remove linkage. Check if you can move the shift lever on the trans further into fourth gear to make sure you are getting complete engagement with your linkage.

[one70gt37]

Thanks for all the replies! we'll check everything again on the outside before we take it back apart, the trans. rebuilder replaced everything including the main shaft forks,syncros,reverse idler gear, 4th gear etc. the hurst shifter has all new bushings too, also has new mounts, bellhousing to trans are tight. Will keep you posted to what we find. Thanks again!

[Half-Inch Stud]

Originally Posted by one70gt37

Thanks...rebuilder replaced everything including the main shaft forks,syncros,reverse idler gear, 4th gear...what we find. Thanks again!

Thought I replaced the Blocking rings too. Yet your Builder replace all items that would be involved with wear there.

My thoughts are on those 3 tabs under the blocking rings//Shift collar, as if the assembly of the input shaft onto the gear train has a method for the 3 tabs.

[Stuckinda60s]

The book method to adjust the shifter is with a pin in the hole and the shift levers, in neutral, just falling into the holes in the levers. I always turned 3-4 one turn toward 3rd and 1-2nd one turn toward 2nd. That helped with power shifts, keeping you from beating the gate.

The most common causes of jumping out of fourth are rounded or chipped peaks on 4th gear and the slider. As someone said, the stops may be misadjusted. The way I do that is to put the trans in 4th with the linkage disconnected. Then, with the shifter in 4th, as well, drop the linkage into the hole and adjust the stop so it just touches the shift lever. Back it out one flat and lock it with the locknut.

Really, unless the stops aren't adjusted wrong, the problem almost has to be the slider or 4th gear. Although it's not impossible that you have a bad motor mount or that the trans isn't lined up in the bellhousing, is it factory?

You can inspect the teeth on 4th gear and use a mirror to get a good look at the teeth on the slider.

[one70gt37]

Update on what we found was wrong, the shifter rod was out of adjustment for 4th. gear, it was not pulling it all the way in. It was a easy fix & now works good. Rowing all 4 gears now!..Thanks for the replies & tips guys!

[Half-Inch Stud]

WOW. You the fortunate one.

[one70gt37]

Originally Posted by Half-Inch Stud

WOW. You the fortunate one.

Well i was fortunate for a while, lol, it worked a little better but is now acting up again, grinding bad going into 4th, we have checked everything out on the outside, linkage bolts, shifter alignment etc,, it's coming back out to see whats wrong s***!. In the meantime i'll put back in my m-20 wide ratio.

[Somanycars]

When you get it apart I would look closely at the keys and springs in the hub assembly. maybe binding ?or a 63-65 synchro used with a later model hub.The hub and synchro have to be from the same family.63-65 uses non shouldered brass and 1-1/8 wide hub.66-74 shouldered brass 1 inch hub.You can use either matched assembly in either years of transmission but synchro must match hub.