My M20 while racing, jumped outta 4th gear. I didnt miss a shift, I was in 4th briefly under max acceleration, then it jump out. Then, I couldn't get in back in gear till everything slowed down.

What to look at?

Something tells me a synchro,but I might be wrong.I love 4 speeds,thats all I use

69-CHVL... my M20 does the same thing. Usually when mashing it, and then letting off the gas. Once it starts to coast, it pops out ( not always ). However, I can just hit the clutch and put it back into 4th no prob... it's fine during "normal driving". I've gotten used to it for now, but plan to drop the trans and try a rebuild this winter. I've been told a monkey with 2 thumbs can rebuild a Muncie...now I just need to find a monkey. I have other trans issues that warrant some tlc ( leakin', reverse is a bit cranky, etc)

You have worn syncro, hub and sliders.

Look at the sharp corners in these sliders. That's why it stays locked in gear.
http://carcraft.com/howto/p133685_image_large.jpg

and here, the edges on the gear and syncro need to be sharp with no rounded edges.
http://www.geocities.com/milesdjsquid/muncrebu/breakdown5.jpg

look how worn this slider is
http://www.geocities.com/milesdjsquid/muncrebu/junkslider.jpg

new slider
http://www.geocities.com/milesdjsquid/muncrebu/torqlockslider.jpg

new
http://www.geocities.com/milesdjsquid/muncrebu/rebuild1c.jpg

It will be the sliders, the syncro's just help get it in gear, not keep it there.

In pic 2 above, the brass ring is the syncro.

The teeth on the left side of the gear, where the arrows are, are engaged by the sliders to select each gear.

The second to the last pic, labelled "new slider" shows the new design "torque lock" sliders, look how the grooves inside it are cut like a bowtie, these lock everything up tight and hold the trans in gear very well, you want this type if you replace them.

It is true a monkey can rebuild a muncie. I did it using a posted scan of a 20 year old popular hot rodding article!!!

Copo...Jimm...thanks for the info. Are any specialty tools required for rebuilding a Muncie ( other than obvious req'd toolset ). For example, will a press be required to slide gears on/off the shaft ??...or just a BFH and block of Oak ???

If you need to get at the 1-2 syncro / slider / hub assembly, you need a press to get something around second gear (I forget what, the slider?, I dunno was a few years ago and it's fuzzy now. I needed to work on 1-2, and couldn't get it apart without a press) off the mainshaft. Other than that normal handtools will do.

I took mine apart for jumping out of first, ended up driving the mainshaft 50 miles to a kind hearted trans expert to get second off. He dropped everything and just did it while I was there and watched. That's where I learned abouyt torque lock sliders, and also about how there are 2 styles of syncro rings, my trans had 1 style on 1-2 and the other on 3-4. My rebuilt kit of course had 2 sets of the same style. The guy traded me for ones that fit, and also supplied a used 1-2 hub.

The only thing he charged me for was the new torque-lock sliders. He was a sweetheart, taught me a bunch in that hour, and I owe him big.

I chose not to pull the counter shaft. I hear itsa beetch getting it back in, something like over a hundred uncaged roller bearings, and using grease to stick em in place while putting that 20 lb shaft through a hole that's smaller than it is and pounding in a foot long steel rod without knocking any bearings out of the grease...lol. I left it alone.

Truthfully it takes considerable skill and knowledge to do one right, not something for a trained monkey at all. I had a lot of help, both from that guy and from several others on Team Camaro, and the trans worked like new when done.

Thanks fellas.

I had the trans out over the winter to replace the speedo drive gear and mid plate gasket, so I checked everything thing out as best I could w/my limited knowledge.

I did notice that the trans was rebuilt....had new sync's anyway. The sliders looked good, gears looked good. Only thing suspect, and Wally actually mentioned this, was the dogs on some of the gears looked a little worn.

Question is, even if the dogs are worn, if the trans is placed into gear and the shifter is still placing pressure on the slider...how does it jump out?

unless the dogs are worn really bad, the torque lock sliders should hold it in gear.
Without them, you end up with parts worn at angles that allow rotating torque to push the slider teeth and gear teeth apart and pop it out of gear.

As I think more about this, perhaps it was 3-4 that I needed a press for? It was about 4 years ago, and something I did once, and watchd someone else do 20 (once) years before.

While I do believe the experiences of a fellow ametuer (me in this case) are useful to another ametuer, facts from an experienced pro are invaluable.

It's 2nd gear where you may need a press. But some have the luck of holding onto the 2nd gear with both hands and slamming the end mainshaft down on a rag that's on the pavement. The countershaft is no big deal building it up. I used a dowl or broom stick the same length as the shaft and used that first then when all the washers and bearings were in place I pushed the wooden stick out by installing the metal shaft. Make note of the final rotation where the 1/2 cut end sits. There are 2 lengths of the aluminum pipe GM used in the countergear, so depending on the one you have will depend on how many washers you will need. See my rebuild section http://ca.geocities.com/mlvd@rogers.com/muncie_rebuild.htm