Hey folks, I have a 1972 Skylark Custom convertible with 3 speed manual transmission on the column. The linkage keeps getting messed up.....you are driving down the road and can't get it out of second gear. I bought an M20 from ebay that came out of a 68 cutlass with no history the car. The tranny shifts into all gears on the bench and removed the side cover and the gears all look good to me (I think). I don't know much about manual transmissions but I hear the term synchros and make sure you use the right oil..... By looking inside the transmission, does this have synchros and what do they do? Also, what oil do you recommend I use? Oh and by these numbers.. anyone know if this is truly a '68 M20? Muncie main case number 3925660 Patent number 3088336 Side cover number 3884685 Thanks
Synchronizers are what allow the the transmission to be shifted from gear to gear with the driver having to synchronize the transmission(double clutch). Yes all the Muncies are synchronized. You will need a quality GL-4 gear oil. Brad Penn has the perfect stuff for the Muncies. That is what I use. DO NOT USE GL-5 oil. http://www.summitracing.com/parts/bpo-023-7729/overview/
Be careful about going by just casting numbers to determine what model Muncie you have. There is alot of conflicting info concerning casting numbers on the internet. You can usually fairly safely determine the year range, but which series (M20,21,22) can be sketchy going by just casting number. Plus, unless you know the history of the trans from the beginning, gear sets couldve been switched out over the course of 40+ years. The best thing to do is determine your gear ratios. Thats the most concrete and useful info you can have when figuring out what you want to do to get the trans to really jive with the rest of your setup. An easy one is to put it ion first and mark the input and output shafts. M20's had a 2.52 first gear compared to the M21's 2.20. You can spin the input until the output makes 1 revolution and if the input spun exactly 2 and a half times its an M20 gear set. I can tell just by your pic its either a 20 or 21 as the angle of the gears teeth is too steep to be a 22.
It appears you have a M20.1" pin 2.52 ratio.Two grooves on the cluster.The Input will also have two grooves.The "660"case was a 68-69 case.You should have a "584"extension housing but it could have a "429" Housing which would be incorrect for your 72.The gear set looks decent ,engagment teeth look a little worn but the sliders are the originals.Rebuilds are generally done to get new bearings and syncros in place.I would run this used as is except for a extention bushing and seal is highly recommended.Also check your Driveshaft yoke for wear when changing out.your backdrive and steering column are also going to be an issue.Rear gear ratios need to be addressed too.Pm me for help with that part of the swap.Gear lube should be a good regular(not synthetic)80-140 Vis.Bill
I went from a three on the column to a 4 speed on the floor with my 64 Special and it was like a whole different car. It really woke the car up. Get yourself a good shifter like a Hurst competition plus. The trans should go right in since it's the same length, mount location and drive shaft yoke. The hardest part is cutting that hole in the floor especially is you have a nice rust free car, you'll hate cutting it.
wow..good info guys... thanks 1) the input spline has 2 grooves... when in neutral (both shift levers in the center) I spin the input shaft about 2 and a1/2 times the rear shaft spins once. 2) When I grab the input shaft it has a little play in it (1/16 to 1/8 inch) .. is that normal? 3) the rear tail housing has 3857584 GM on it so I think it I correct 4) I found this picture in this forum... I think it shows the linkage for the reverse lockout. My 3 speed has something similar utilizing a steering column linkage to a similar "U" type plate on the 3 speed linkage ... do I use the original linkage rods that came with the Hurst shifter and buy a reverse lockout kit from The Parts Place? https://www.thepartsplaceinc.com/ part #MT4942Z 5) Does anyone know who re-chromes shifter handles reasonably? (driver quality) 6) since this is just a weekend warrior hobby car to cruise to local shows with the kids..... what will I notice if I don't change the rear end gears.. (I had a stock saginaw 3 speed) thanks all and sorry for all the questions but do truly appreciate everyone's help!
