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Hi Charles, in a recent post, you mentioned the "front mainshaft snap ring syndrome" I have done a search for more info but havent come up with much. Would you elaborate, ie how to diagnose and repair it? My tranny is on the bench, if there is a mod needed, now would be the time.
Thanks,
Tim

powerwagontim wrote:Hi Charles, in a recent post, you mentioned the "front mainshaft snap ring syndrome" I have done a search for more info but havent come up with much. Would you elaborate, ie how to diagnose and repair it? My tranny is on the bench, if there is a mod needed, now would be the time.
Thanks,
Tim

The early transmission is notorious for the snap ring issue, there is only the one ring that holds everything on the shaft from 2nd gear forward. The most common way it happens is a result of someone using the incorrect thickness ring at build up. There are 4 rings to chose from, the correct choice is to use the thickest ring you can fit into the groove for an absolute no end play set up of a fully assembled main shaft group. Now be careful here, forcing a ring that is too thick into the groove can easily break splines also, common sense must kick in here. If a too thin ring is used, end play results causing the whole assembly to move back & forth on the shaft everytime gears are shifted. The stress of this will eventually fatigue either the ring itself or the groove in the shaft. Most often it will be the groove as it is cut into the splines rather than the full meat of the shaft. Once the splines begin to pop off, it's all down hill from there. The pressure against the ring will force the ring out of the groove, break the ring, or completely break out the groove. Once this happens, the main shaft grouping is then allowed to slide to & fro on the shaft until massive damage results. Gears start running into things as they can float out of position on the shaft. Early signs are jumping out of 3rd gear & harder than normal shifting. The vast majority of early series transmissions we tear down have this going on at one stage or another. You can tell by simply removing the top off the case. If you are able to slide the gears on the shaft too or fro more than a few thousandths, get suspicious. You will not be able to actually inspect the snap ring & groove condition until the main shaft is pulled out of the case. In the late stages if you keep driving with these issues unchecked, it will destroy most everything in the unit, I've seen a couple go so far that only the case itself was salvagable. Going to this point is rather foolish I think. It is very obvious that some serious issues are in play long before destroying everything.

The later series NP420 trans got a design change that virtually eliminated this problem. In that trans, 2nd gear has it's own snap ring taking much pressure off the end ring. This same design is used in the still later NP435 trans also, as it worked & held up very well. It is a wise move to change over to the NP420 transmission that was in all the M37B1 trucks. It will directly interchange as a complete unit, however the internals are very different in several areas. There are also improvements in the shifting tower.

Hi Charles,
Thanks for the information. Sounds like I will be taking a closer look at the tranny in my parts truck, a B1. At this point, all I have done to the early tranny is clean the case, so its not too late to abort.
Thanks,
Tim