Hey gang. here is the deal. I have just finished an install on my daily (off topic) car. it is a muncie m-21. fresh and clean inside and out. here is the problem. it grinds into 3rd, and unless I can keep a foot on it to hold the stick, it will pop out of gear...but only in 3rd. after adjusting, re-adjusting, and adjusting the shifter a 14th time just for practice, i am bbeginning to think I have a lame trans. my questuions are simple. once I pull the side cover off, what am I looking for here? I am a "do it yourself" kind of guy, and have never taken one apart. ( well, other than to peer into one and say "yep, looks like transmission parts in there.") #2- if it were shifter alone-say if the issue was caused by a shift stop being out of whack or something like that...wouldn't I have the same problem into first? or if it were strictly the shift arm (on the case) into 4th? give me a few ideas to work with, please fellas! this is gonna drive me to drinking....
We have a M21 in the Modified. It always grinds going into 3rd if changing down. No problems if shifting up. Make sure you have full clutch travel.
Weak/missing springs on the syncro hub. Broken or missing "dogs" between the syncro hub and the slider. Possible worn detent or weak spring in the side cover, but I doubt that is the cause.
I rebuilt one far too many years ago. Seems Shovelhead does hit the likely first cause issues. The detents aren't under the load that the syncros have. Re-reading your description, sounds like a worn 3rd syncro.
Lux, It could be the shifter..... The "proper" method to adjust the shifter is.... Back off the stop bolts ( WAY OFF ) Remove all the rods from the shift arms on the trans. With the shifter in neutral, install a 1/4" rod thru the body of the shifter, ALL the way thru the body (Both sides ). Make sure the shifter arms are in neutral, jiggle them back and forth so they are detented. Proceed to adjust the rods, 1 at a time so that they slip freely into the shifter arms.. Get picky here..this is the most important part. ^^^^^^ When you have the 1/2 and 3/4 rods all in place....Reverse as well... Remove the 1/4" pin. Shift the trans into 1st gear..Slowly tighten up the stop bolt untill it just barely touches the arm in the shifter body. Shift the trans into 2nd, do the same to the other stop bolt. Test drive... If it still grinds/pops out etc... Then you have a synchro issue.. If you do end up repairing this trans..Do yourself a big favor ..Replace the synchro "slider keys" ..Often overlooked, and new ones make a world of difference ! Dave
I have a M-21 in a chevelle I build a few years back. The trans was a fresh rebuild so I didnt believe I would have any problems. But when we took it out for the first ride I threw it in gear and took off, about a mile later it popped out of gear (1st). Didnt think too much of it at the time, but it continued to do it for the rest of the ride! I did nothing to adjust the linkages and continued to drive the car for about a month or so. I noticed that as time went on it would pop out less and less. I finaly broke down one day and messed with the linkages a bit. After that it worked perfect. I truly feel that it just took some time for everything to work together. All the gears were not orignal to eachother, and it just took alittle time for them to wear together. I cant say for sure if this is your problem, but I wouldnt go tearing it back apart just yet. Mike
>>>>>,Back in the '60's & '70-'s ,,i ran Muncie M-20,,M-21 ,& M-22's in everything ,,rebuilt alot of mine as well as friends ,,SO ,,a Big DITTO for Dave Lewis' remarks ,,,he's Right On da Money Here!!!
Sounds like a slider to me. I am assuming that you said "fresh and clean",,meant a rebuilt box with new synchros "brass",,correct? If you did everything right ,,it should be the slider that is causing you the trouble. Good luck. Tommy
the trans is "fresh". I adjusted the shifter exactly as dave said- which is basically how the hurst website says to do it, and how I have always done it... any ideas on a good "how to rebuild a muncie" book? and if so, what are some things I could do while in there to toughen it up a bit? it is behind a STOUT 455 olds. clutch is good- I had to build my own z bar and corresponding linkage. it is all heim end now. centerforce flywheel, clutch and hat, throwout bearing, pilot bearing from mondello, yadda yadda yadda. all inside a lakewood bellhousing. and it'sadjusted right and centered with the crankshaft. the shifter is a super speedway style- also with straight rods and heim ends. leave it to the one part of the car I didn't build myself to fuck up...ha!
Had that same problem along with, when at a stop with clutch pedal down had a bad hum. Had a bad pilot bearing (the one thats in the engines crankshaft)
>>>>>,I've seen the brass syncro rings bad outa da box ,,depending on the mfgr ,,pull the cover and look real close to the third gear syncro ring ,,any missing,,chipped teeth ?? i've ran smooth syncros in my drag M-21's to make a home made crash box ,,anything other than full throttle , i had to double clutch it to keep the gears from grinding ,,,try drivin' it down the road and going into 3rd gear ,put it in neutral (double clutch It) rev it slightly ,,push the clutch back in a shift it to 3rd gear ,,if it don't grind ,,,bad 3rd gear syncro ,,if it still grinds ,,problem sumwhere else ,,,
I've never seen a book on the Muncies except for the shop manuals from GM. They are pretty obvious when you get it apart,blocker rings worn or teeth ground off, broken dogs, excessive slop in the syncro hub. The only thing that the service manual does not address is the case. They are a POS in my opinion. Inspect for cracks around the ears to the bellhousing, and for cracks in the case at the input bearing area. The countershaft area normally does not crack, but years of abuse can cause the counterbore to elongate. This leads to oil leaks, the countershaft "rocking" in the case forward or backward, or stretching the case beyond use. I tore one up once so bad that when I sat the case on the workbench with the input gear facing downwards,the countershaft fell out of the front of the case onto the floor. They are not supposed to be that loose. And, if you pull a lot of hole shots, and your car is reasonably heavy or real sticky tires, you might check the output shaft for twisting. The splines WILL twist on the mainshaft, ask me how I know about that, in fact I still have one from my heyday of the early 70's in the garage somewhere as a memento of my younger days.......
Discounting a maladjusted clutch or operator malfunction, gear grinding can be caused by worn entry bevels on the teeth of syncro sliders, worn, cracked, or broken brass blocker rings (the usual is a radial break at the clutch key notch), worn or more commonly chipped or broken clutch keys, broken or missing clutch key springs. Worn or damaged syncro teeth on the gear itself is the usual culprit when the transmission jumps out of gear, especially on overrun, but sometimes it can be a combination of wear on the syncro slider, gear syncro teeth, and/or interlock plates. Before you pull the transmission, you can check the interlock simply enough by dropping the shift linkage rods off and pulling the side plate. Hold the shift forks in place with your fingers to keep them from falling out of the side plate and interlock mechanism while working the arms though the various gear positions. Look for any obvious hangups or misalignments caused by burred or worn interlock plates. Look also for broken or mispositioned springs causing the interlock plates to not engage the shifter forks properly. Good time to check the shifter forks for signs of wear or damage also. With the side cover out of the way, you can slide the fourth gear syncro slider fully forward to engage four gear. This exposes the third gear syncro teeth on the brass blocker ring and the gear itself. Look these over to see if there is obvious wear (rounded over teeth without straight edged bevels, burrs, or just "chewed up" looking). Finding heavy wear or damage here is a pretty good indication you've found the reason for the transmission "jumping" out of gear. If you're not sure, check the teeth for wear by comparing them to the teeth on new parts (or detailed photos of new parts). I also look for damaged or worn clutch keys, but often times this is hard to see until teardown, if they are not actually missing. Fix any problems with the interlock, and if you don't see any obvious problem with the gears or syncros, temporarily replace (old gasket, snug but not torqued bolts) the side cover and linkage, then take the car out for a test spin. Maybe you'll get lucky and not have to pull the transmission for disassembly, just remember to retighten everything. If you find you need to pull the transmission down, this is one of the simpler transmissions to work on, as is R & R, but do yourself a favor by getting your hands on a factory GM shop manual for just about any car that used a Muncie. They all show the disassembly/reassembly process in detail. Using a little improvisation, you can get by without most of the tools shown in the factory manual, but buying the correct type of snap ring pliers (flat blade jaws, sprung shut, as per the picture in that same factory manual) will save a lot of grief. Getting the tailshaft housing off will take some dexterity and fooling around (more so when reassembling). If you can manage that and keep track of the 117 or so loose needle bearings in the counter gear along with the 10 larger ones on the mainshaft nose that sit loose in a cage inside the main drive gear (GM parlance for the input shaft), blowing apart and rebuilding one of these is pretty straight forward. I always figure I'm going to be replacing even slightly questionable brass blocker rings, clutch keys, and clutch key springs when I pull one of these down. Buying a "small parts kit" (most used to contain new countergear needle bearings among the other bits, but the bearings might separate depending on the supplier) is a good idea, along with replacing front and rear main bearings if they were noisy, or don't turn smoothly. While I've always been inclined to reuse undamaged gaskets, now with current parts so expensive, I'm not above dressing the entry bevels on the syncro sliders with a die grinder and lightly deburring the syncro teeth on the gears themselves in an effort to extend their life. Doing this sometimes you win, and sometimes you just get a little extra practice with R & R and assembly before you bite the bullet and buy new gears and/or syncros, but for me it's worth the try. I got pretty good at screwing these things together when I was street racing an O/T Camaro years ago, and like riding a bicycle, you might get a little rusty buy you never really loose the knack.
Check if you have the side cover from the late M-21 they call it the 306 design . It has stronger detents to keep it from coming out of gear.
tons of good info here! I should mention this thing is rebuilt and has around 5 miles on it. it kicks out of 3rd so hard that it will hurt ya if You don't have a real good grip on the shifter- and this is at normal driving speeds. I haven't even gotten close to really letting it have some power. damnit. I just wanna do some burnouts.
Someones idea of rebuilt is purely speculative.If you assume that all of the gears are replaced on a stick trans,you are mistaken.To replace all of the gears on a Muncie would place the cost of the unit out of reach.Most unknowing rebuilders replace bearings,gaskets ,and blocking rings and call it a day.The brass rings have no bearing on weather a trans stays in gear.Your problem is the small teeth on 3rd gear,as well as the slider teeth that mesh on those teeth.Compare the teeth on 1st gearwith the worn teeth on second gear and you should see what I am refering to.If you have any questions PM or call Dan 507-263-7411 or 612-490-1967
Sounds like the entire gear pack has too much front to back clearence in it. We have had to make shim kits for these before. You have got to keep those little tiny ass teeth on the gear locked into the slider or KA-POW it's popping out with a ton of pressure. The side cover is a fixed position part that holds the shifting forks so when you let the entire assembly move for and aft the shift forks can't keep the slider engaged to those tiny ass teeth on the gear itself. When you analize the entire system it's amazing it ever works with much horespower >>>>.
Excess end play on 3rd gear or the small teeth the synchro collar engage on the gear are worn. Probably a combination of both. The side faces of the short teeth should be straight and parallel for at least 2/3 of their length. Wear on these and the mating teeth on the collar with too much end play on the gear = instant blow out of gear.
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