Hot Rods Muncie pops out of 4th gear

I bought my transmission [1967, 2:20 low, M-21 close ratio] at a swap meet several years ago and looked inside before I bought it. Looked for broken/missing syncro rings, chipped teeth etc...looked ok to me. Installed it with 90 weight and a new Hurst competition plus shifter with new levers and rods. Installed a new pilot bearing too..I adjusted the stops as needed and gave it a go. It worked great except for a whiney 2nd gear [at first....no louder than any other gear now after 1500-2000 miles] and it occasionally pops out of 4th gear on deceleration. I readjusted the shifter and watched while the trans was run through it's gears while parked....goes all the way into 4th gear alright.
I'm planning to pull the engine and trans to fix some leaks, clean everything up and address this 4th gear problem.
For the experienced Muncie guys: Does it sound like I have a bent shift fork, a bad syncro or anything else?

 

Rocky
I've never had any Muncie 4 speeds pop out of gear, but I had a BW T-10 pop out of 2nd gear when decelerating, I recall it has to do with the brass syncro rings wearing and riding too deep on the steel cones.
Never heard of them doing it in 4th gear though.

Here is a Google search.
https://www.google.com/search?q=mun...j0l3.9249j0j8&sourceid=chrome-mobile&ie=UTF-8

 

I had a Muncie 4 speed in a Chevelle that would pop out of 2nd on deceleration IF I didn't accelerate momentarily after shifting. On a downshift into 2nd I'd usually just stab the throttle then immediately let off, that would fully engage the gear and it would not pop out. But if I just let the clutch out after a shift from 3rd to 2nd while decelerating it would pop out every time.

 

If you would not drive it so much, you wouldn’t have worn it out already.


-Abone.

 

4th gear popping out is rare [there are 3 causes of this ]

1: Dog teeth on the input [you can get Torque-Lock sliders to fix this]
2: Worn slider [or hub] which engages the dog teeth.
3: Bell housing misalignment [which causes the input shaft to "break it's back"]

 

How about 4, Excessive clearance between input and main shaft?
Or 5 , loose , worn bearing between the input and main?

 

Pull it out and check your bearings, synronizers etc. it’s not going to heal it self. Muncie s are easy to work on and are somewhat forgiving.



Bones

 

Sounds like back-driving in 4th causes the synchro hub to slide back to neutral. Wear or loose fit in the region?

 

Dial indicate the bellhousing. The diameter that locates the trans front bearing retainer should be within about .008 of concentric with the motor's crankshaft, and the flat surface that the trans bolts up against should be parallel with the back of the block within about the same. Many people overlook this step, not understanding how important it is.

 

C spring in the syncro could be broken and not holding the dogs in place. Worn dogs or slider. D

 

If the input by was changed and not properly shimmed , it may not be installed far enough into the case , could just be a bad synchro , look inside the slider for worn teeth , if the counter gear shaft is leaking , the case may be worn and need to be bushed, there's other possible reasons...

 

You are correct here!
But generally these 2 issues cause the gearbox to whine in all the lower 3 gears [getting louder in 3rd] as the gears and countershaft try and climb apart with torque applied.

Pulling it apart is the only true way to diagnose it.
Without doing that , start with bellhousing alignment

 

A badly worn pilot pushing can also cause this problem.

 

had one that jumped out of fourth (it was in a chevelle, too)....finally figured out that someone had installed a truck bellhousing, with the big center hole. So the trans was not centered on the bellhousing. Replaced with a 621 bellhousing and all was well.

 

My fix was replacing the shifter forks.

 

Yeh, visual inspections through the side cover opening are great but that usually is only good enough at swap meets to see if there are major issues, of course the initial cost dictates whether it is a good deal or not.
I'm into mine for $250 with all new consumables.

From my post here:
https://www.jalopyjournal.com/forum/threads/muncie-input-shaft-no-good.1100438/page-2

There is a lot the naked eye can't see just looking in the side cover opening.
My M-20 was built by a trans shop owner from his stockpile of hard parts and rebuild components, there was a hairline crack in a synchro cone that he missed, fixed it but took two R&R"s. If youre feeling lucky añd don't mind wasting a day or two of your time just stab it in and take your chances.

 

What did you put into it for oil?
I was checking out oil types for my st10 and came across a lot of odd complaints with incorrect oil... just a thought.

 

Lube should be GL-4 rating.

 

Exactly, what 4th gear is there? Input and output need to be on a straight plane.

 

The answers's, so fa
are , "all over the map:, so-to-sleak.

 

Thanks, all. Gear lube is GL4. I'll have the trans out and on the bench in a few weeks. I appreciate all the input so I'll be checking all the suggestions. Your input is exactly what I was needing.
Gotta tell ya......when I had the trans out last July to fix the flywheel bolts, it went back in place sooooooooo easy I immediately wondered to myself "I wonder if I REALLY put the pilot bearing in place......it didn't require any jiggling to get the trans up against the bell housing. That NEVER happens in my world. It's been in the back of my mind ever since. I'll know for sure in a week or so. But we're supposed to have temps in the 50s the next 2 days. I'm going to put the plugs back in the sides of the block, screw the radiator drain closed and fill the cooling system with water.....take the car out and drive the shit out of it tomorrow and Friday!

 

yup, there are lots of possible causes of jumping out of gear.

 

And I'll investigate each suggestion...thanks

 

Verify that the bellhousing locates the trans concentric and parallel with the motor. This is time well spent whether you go into the trans or not.
Pull a flywheel bolt to give you a spot to attach a mag base. Dial the hole and the mounting surface. The bellhousing in the attached pics is an old two piece Offy, it was out of concentric .049, for a TIR of .098. It required a .028 cut on the mounting face to square it up. This much misalignment had been causing the trans to shift hard and pop out of forth, it had trashed the input bearing and the pilot bushing.
Never assume any bellhousing is accurately machined. Some trans manufacturers warn you to do this before installing a new trans, or they will not warranty it.

 

Checking the bellhousing with a dial indicator as discussed is a
good thing to keep in mind ESPECIALLY if it is an aftermarket
bellhousing. Another possibility is wear on the shift rails or maybe
a weak detent ball spring or detent ball or rail wear. Unlikely, but
I have seen it. Now if you just happen to have another dressed side
cover you can can pop on to try, that would be easiest...

 

shift rails?

 

Just for the halibut... back out the 4th gear stop a few turns and see if it gets better.

Before I broke my M20 back in the spring, it shifted up and down nicely, no abnormal noises. I had a local guy go through it, he was amazed that I said it worked so well. The reverse idler was junk, out of another box that had something let loose inside it. He replaced what was broken, plus a couple of synchros, the reverse idler and changed the counter shaft from 7/8" to 1". It works well again, maybe a little noisy in 3rd when I let off the gas. I can live with it.

 

Muncies don't have a detent ball in the side cover...nor do they have shift rails, so you can stop looking there

 

"Just for the halibut... back out the 4th gear stop a few turns and see if it gets better."


Thanks....first thing I did. [covered in 1st post] No Joy there. Also checked to see if lever was moving all the way into 4th gear. It is.

 

Rocky
Ten extra points for using the Muncie, you too @bobss396