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M20 trans not shifting into 3 and 4

I have an m20 trans from 1967 I believe. I don’t really have much of a history of it and before my engine blew it felt great. When I took it out I tried to go through the gears with just the small levers and it did fine. Now it’s having a lot of trouble going to 3 and 4. It can go into reverse, 1 and 2 just fine. When I try and pull the 3/4 lever by hand it makes a grinding sound. Note this is all on a workbench out of the car. Does this mean I need new forks or something a lot more drastic? Do I just need it to line up or something? I’m pretty new to all of this so I’m not sure where to start looking. Thanks for any help.

I would think the M20 was full synchro by this point in time? I’m pretty sure thats correct. Difficulty going into those particular gears could be a synchro issue inside the trans. The synchros are what basically spin up the next gear until stasis is reached between the two…so that you can enter the next higher up gear. The synchros come into play literally between gear shifts…as you exit 3rd…and start to push into 4th…the sych spins up 4th to a speed allowing easy gear mesh. When they fail they dont spin up the next gear…so its difficult to enter it.

This diagnosis changes if you literally cannot get into 4th whatsoever tho… That would be a fork issue if you can never ever get into 4th

Maybe one of our Trans guys will correct me, but that’s my take on the shituation.

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An M20 trans is a bit coarse overall. Gears, that is. The trans is designed to shift into 3rd and 4th while rotating. If you can have someone turning the input shaft while you are doing this you might find it drops right in with limited clash.

If I had any doubt, I’d open the trans up and take a look… and maybe take it apart for a better look.

BTW, you are putting the 1-2 lever into its neutral position between 1-2 when you try for 3rd, right?

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If I am not mistaken, @Mustangman wasn’t the M22 called the “rock crusher” ? In reference to being course

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No luck while turning the input. Yeah it’s in neutral. I can still shift the 1/2 and the 3/4 is still stuck. It will move a few millimeters towards 3rd I believe but won’t actually go into gear and it will make a metal on metal rubbing noise if I turn it while trying to push the lever.

It’s not seeming to want to move at all. It will move a millimeter or two towards 3 and none towards 4. Still goes into 1 and 2 though

If this is a transmission sitting alone by itself with no input shaft movement like @Mustangman man was referencing… that could and would surely assist gear selection.

If it does not… Its time to open it up and view what damage was done to the shift forks…

Its gonna be something like that. Dog legs n shift forks or some shite like that inside the trans…

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Yup, time to open 'er up. Remove the shift levers and remove the side cover and take a look at the shift forks, the sliders and see if all is freed up or what might be in the way.

I’m not really sure what I’m looking for to tell if somethings broke but the forks do look worn. I don’t have any measurements or tools to measure anything but they do look worn down. Could that affect it enough to make it not go in gear?

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If the forks and shsft move freely they are not the problem. Forks wear but as long as you can’t feel a sharp edge on the parts that fit in the slider, they are OK.

Try and move the selector sleeve back and forth by hand.

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So they forks move but look like they are a little sharp if I understand what you mean. While spinning the input I was able to get it into 3 and 4 but it was pretty hard to put it into gear. I had to put a lot of force into it to get it into either gear. Especially more than it takes to go into 1 and 2. Is this supposed to happen or is something else wrong.

Mount the shift lever on to the transmission and see if it shifts normal.

It’s kind going into 3 and 4 now but it’s still putting up a fight in both gears.

Which was in reference to the shift forks… the answer is YES absolutely

You need to pay close attention to where the shifter interfaces with the tops of the shift forks… The bottom of the shifter fits into the forks like a ball and socket…the relationship of that fitment is very important…if there is damage to the top of the 3-4 shift fork OR the bottom of the shifter…something may be locking up on you due to some defect of the ball and socket type relationship of the shifter and fork…or the shifter may be trying to move both the 1-2 and 3-4 all at the same time due to a defect of the shifter bottom or top of forks.

The shifter bottom needs to leave the 1-2 fork top and enter the 3-4 shift fork top cleanly…with no hangups in between…otherwise it will be trying to shift both forks simultaneously. @Mustangman referenced moving the forks by hand (no shifter) and this is very likely the shituation he was trying to have you suss out…and its very very important… and could save you a lot of headaches if you spot it.

I would rather suspect something simple like the shifters relationship to the top of the shift forks…rather than gear tooth problems or defects… Move those shift forks with something else, another tool, or by hand…was there a change in shift quality? Turn the input shaft also during this…Did this help? Report the results.

If we move past shift fork issues and suspect gear problems, a study of the drained gear lube would also have been very helpful…to look for metals in the lube…did you drain and save that fluid? Still got it?

What is the back story of the engine failure you had? Did it seize or something violent like that? I’m after what kind of loads this trans experienced during the engine failure…which is sort of a back history of the trans itself by proxy

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The gas pedal stuck to the floor and then it just blew itself up. I realize the correct thing to do in the situation was go for the key but I was about to crash so I threw the clutch in at about 5k and then that’s about the end of that. I don’t know much about the history of the trans. I bought it with the 1987 350 and Muncie in it but all the info I had was that the engine had around 60,000 miles on it. I’ve never taken a look inside it or changed the oil since I only drove it like 10 times. When I drove it the thing felt great and even when I took it out of the car I was able to shift it around. However when I was shifting it around I didn’t turn the input shaft. This started to happen when I was turning it. So I’m able to get it into 3 and 4 now but it takes a lot of force to do it. Way more than it ever has and way mode than it does for the 1/2. When I pull the side cover and just move the forks around there they feel fine. When I tried playing with the selector sleeve it was basically the same story, 1/2 was fine and 3/4 was hard to move. I guess this could’ve been because I wasn’t spinning the input shaft enough or something like that. When it goes into either 3 or 4 the input will turn pretty smooth but will sometimes feel like it runs into something or gets caught up on something. I’ll try and get something clean to drain the oil into and go from there.

@mio.da.bomb_157143 You have to understand that the engine puts enormous inputs through that shaft… far more than you ever will even if you eat your spinach.

In the process of the engine doing this…the force on the gears and the loads it sees on the output shaft…all of this shifts things inside the trans in a way you never will be able to recreate out of the vehicle.

This may all be just how it behaves with no engine…no loads…no rpm… and best yet…no lube and no heat !

You might be finding problems that are not there and it may be perfectly normal and function just the way it did prior. With the history given and considering the “bench test” parameters (which are severely lacking things manual transmissions favor for shifting)…I see no reason why it would not.

Your engine issue very likely did nada to that trans.

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Alright thanks. I think I’m just overly worried about everything

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It happens… Personally I think that trans will be all right… One way to find outski…

so it would seem