One thing that can be problematic about cars, and classics in particular, is that they eventually wear out. Every part of them is subject to wear and tear, but unlike humans, old cars and parts can be rebuilt quite easily and often inexpensively. The gearbox in our subject '72 Corvette was making an excessive amount of noise—even for an M22—shifted like a box of rocks, and sometimes it would stick in Fourth gear. It was time to give the Muncie four-speed a rebuild.
The Muncie four-speed is as legendary as the cars that it was put into, with the M20, M21, and Rock Crusher M22 models all part of the hot rod vernacular. From the OEM factories to racetracks to concours restorations, the Muncie four-speeds remain desirable transmissions to have. Even in this era of five- and six-speed conversion swaps, Muncie four-speeds (especially M22s) command top dollar today. We've seen Rock Crushers for sale at swap meets for up to $2,000. Fortunately, they are extremely reasonable to rebuild should yours not be shifting at its best.
In addition to the inexpensive rebuild cost, another great part about refurbishing a Muncie is that you really don't need any special tools. Sure, a hydraulic press is nice and so is a pair of snap ring pliers, but the job can be done without those if you choose, making it a great DIY project. Equipping your four-speed with fresh bearing and synchronizers will improve the driveability of the vehicle, and that will surely provide a better experience behind the wheel.
While we would have loved to show you each and every step in the rebuild process, there are just too many to show within the confines of this article, so we're going to cover the key parts and show you what to look for during disassembly and reassembly. There are a lot of great resources on the Muncie transmission, but we particularly like Paul Cangialosi's How To Build and Modify High-Performance Manual Transmissions books from CarTech books. It covers the Muncie and T-10, as well as a number of other transmissions.
Aside from the obvious issues with our subject Muncie box, it was not the original transmission to the car, so a full inspection was warranted. This gearbox was in a '72 LT-1 Corvette (some of you may remember Project Homewrecker from a few years back), but by 1972, you could only get the M22 with the LT-1 if it had the special ZR-1 road racing option, which this car did not. We had Super Chevy's Mark Lundquist run the numbers and it appears our Rock Crusher was from April 1970.
After ordering a Motive Gear rebuild kit (PN T297RAHDMK), a new Hurst shifter (PN 3917992), and a shifter installation kit (PN HUU-3738611) from Summit Racing, we brought the Vette to AntiVenom in Seffner, Florida, where owner Greg Lovell performed a basic rebuild to get our classic sports car up and running quickly. Check out the captions to see what it takes to get your Muncie back in shape.
01. The Motive Gear rebuild kit (PN T297RAHDMK) retails for $120 from Summit Racing and is an inclusive kit that provides everything you need (short of gear oil) to rebuild your Muncie transmission.
02. To ensure smooth, precise, and short shifts, we upgraded from the stock shifter (it was in dire need of a rebuild, too) to this Hurst unit (PN 3917992) that sells for $263 through Summit Racing. Pictured here is the optional (we recommend it) Hurst installation kit (PN HUU-3738611) that will add an additional $162 to the package.
03. Removal of the Muncie transmission from the C3 Corvette begins with the disconnection of the shifter rods from the case. This is followed by the driveshaft removal and the draining of the transmission fluid. It’s greasy enough down here without getting a gear oil shower.
04. According to Greg Lovell of AntiVenom, sometimes you can unbolt the shifter from the factory mount, unbolt the transmission, and then slide it back to get it out. In our case, we needed to disconnect the clutch linkage and unbolt the bellhousing to extract the transmission from the tunnel.
05. If your Muncie is anything like ours, a thorough cleaning is the first step in the rebuild process. Ours had years of accumulated oil and dirt. We recommend using some sort of product that will soak in, as our parts washer struggled to get the transmission clean.
06. Under the cover of slime we found this marking on the tailshaft. We knew it was an M22, and it would seem that someone noted it for storage, or perhaps while it was for sale at a swap meet. You’ll always want to run the numbers and check the components inside before believing anything that is written outside, though.
07. With the Muncie finally clean, Lovell begins the disassembly with the removal of the side cover. Along with the side cover will come the forward-gear shift forks.
08. The front bearing retainer is next. Be sure to inspect this part before reusing it or rebuilding the transmission, as you may need to order a new one.
09. The tailshaft housing is next. Remove the perimeter bolts that secure it to the main case. Before it will come off, though, you’ll need to remove the reverse selector shaft.
10. To extract the reverse shifter shaft, you’ll need to drive out the roll pin that secures it to the tailshaft housing. Then slide out the shifter shaft and remove the tailshaft housing.
11. With the tailshaft off, remove the reverse idler gear, thrust washer, and countershaft from the back of the midplate. Now you can now separate the midplate from the main case and then remove the output shaft and upper gearset.
12. Now that the main gearset is out of the way, empty the case of any remaining components. Here you can see the 1-inch-diameter countershaft, which was a later model upgrade to the earlier 7/8-inch shafts.
13. As the front bearing retainer was removed earlier in the process, we now have access to the front bearing. Snap ring pliers or something similar makes removing the retaining spring much easier here, but you can do it with a pair of screwdrivers in a pinch. With the snap ring out, you can tap the bearing out of the case. In some instances, it may fall out.
14. At the midplate bearing, there is another snap ring holding it in place. After removing this bearing, we found the bearing to be quite noisy and likely the cause of the excessive noise coming from the gearbox.
15. With the middle bearing snap ring released, you can remove the midplate and start to disassemble the main shaft. The Motive Gear kit includes new synchronizers as well as every snap ring to make sure everything goes back together nice and snug.
16. With the amount of debris found at the bottom of the case, there’s no doubt this transmission was due for a rebuild. Be sure to clean the main, case, midplate, tailshaft housing, and front bearing retainer if these components are to be reused. There are a number of companies now that offer replacement options for these components, some of which are much stronger than the originals. Most of these are designed to work with your original Muncie components as well.
17. Likely a source of some of the debris in the case and some of the oil coating on the outside is this worn tailshaft bushing. The Motive gear includes a new one, as well as a new seal for the end.
18. The factory shifter was the cause of our transmission sticking in Fourth gear. For the uninitiated, the shift rods fit into plastic bushings in the levers, and these bushings can and often do wear out over time, causing poor shift engagement. This can lead to poor operation inside the transmission and subsequent damage if not remedied. We solved this by installing a new Hurst shifter, along with the recommended installation kit that includes new rods, levers, bushings, and clips.
19. As we mentioned earlier, the front bearing retainer should always be inspected for wear. Here you can see a groove has been worn into the retainer, and just on one side of it. We had to order a replacement, and stock ones are available for around $50.
20. Once you have cleaned the case free of grease and grime, you’ll want to clean up the gasket mating surfaces and then give everything another quick cleaning. New gaskets are included in the Motive Gear kit.
21. An easy way to check out synchronizer wear is that the gear will sit down on the synchronizer if wear is an issue. You can more easily see the difference if you swap out the old and new ones, but don’t bother reusing an old one just because it looks good. You’re in it this far and the new ones are included in the kit. A shortcut here can lead to a long way back to another rebuild.
22. The front and main bearings in the Motive Gear kit are upgraded ones that feature additional bearings to better distribute the torque load your high-performance engine will put to it.
23. There are two sets of new needle bearings in the rebuild kit. The thicker ones are for the back of the input shaft and the thinner set is for the countershaft. Use a quality, high-temperature grease to hold them in place, and be sure not to leave any out.
24. New strut keys are included in the rebuild kit, and these here are being fitted to the 3-4 slider assembly. You can use a dab of grease to hold them in place, or you can try your hand at holding them in place as you assemble the unit. Either way, don’t forget them.
25. The front input shaft and bearing are held in place by a new snap ring. You can also see that the front bearing nut has also been put on. You might think this nut goes on with the flange facing the bearing, but that would be incorrect. The flange faces the retainer and provides an airtight seal. Installing it backwards will lead to a leak and the nut will grind into the retainer.
26. As part of the effort to make this rebuilt transmission a leak-free unit, new external shifter shaft seals are included. If you detect excessive wear and/or movement between the shifter shafts and the cover, you may need to spring for a new side cover. These holes in the side cover can become elongated from wear over time, which can lead to poor shifting quality. Aftermarket covers that utilize needle bearings to support the shifter shafts are available, and provide for a smooth and precise action.
27. With the transmission assembled, we moved on to the Hurst shifter installation. Since the Corvette was not equipped with a Hurst shifter, we used a tap to chase the threads in the shifter mounting holes on the transmission case to be sure they were free of any debris. Then, the shifter was mounted using the supplied hardware. As the Hurst unit is replete with handsome, classic looks, it also features a center biasing that keeps the handle centered when in Neutral. The H-pattern is also tightened and the throws are shorter.
28. The Hurst installation kit is a no-brainer, and ensures that the precise Hurst action works to the best of its ability. New rods, bushings, and clips are included for a trouble-free, bolt-in installation. Keep the white alignment pin in the shifter while you install and adjust the shift rods. Be sure you have the rod orientation correct for each gear, otherwise your First gear will be Second, and your Third could be Fourth. It’s easy enough to switch them around under the car should you get them backwards, though.
29. Adjustment of the shifter stops is next, and it is fairly straightforward. Move the shifter into First gear and then turn the bolt in until it touches the handle. Then back the bolt off half a turn and tighten the nut. Do the same for the rear stop with the shifter in Second gear. Double check that you have positive gear engagement in all gears and you’re all set.
30. The rebuilt Muncie M22 was ready to be put through the paces once more, this time with improved feel and effort. And the classic good looks of the Hurst shifter inside will garner looks at the car show in addition to giving you a better handle in the gear changes. Back on the road behind Homewrecker’s 383, the Muncie was an absolute joy to row through the gears. You could throw effortless powershifts at high rpm and missing a shift was nigh impossible.