VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1937-1945 1937 tranny problems

I am in need of shafts for 3 spd 1937 tranny
Idler shaft 4.433 #590831
Counter shaft 4.428 #590827

I also have a problem. Ok, several problems, but let's focus on the tranny.

Rebuilt the tranny using all NOS gears in 2006, only 300 miles on it. Counter shaft worn and bushings inside counter gear are loose. Haven't removed idler yet, but it feels like it has worn prematurely as well.

Also, end play between case and thrust washers is about .035 for counter gear, and .030 for idler gear. These gaps are nearly 2-3 times the amount found in a good used tranny, still assembled. What should these end gaps be, and are different size thrust washers available to reduce the end play gap?

I believe all of the gears are of the "hardened" style, indicated by a 5/32 hole as noted on an earlier VCCA chat site about 1939 gears. I note the 5/32 hole on the idler gear, cluster gear, and 2nd gear. Can't find markings on other gears, but based on the style of NOS GM boxes they came from, I assume them to be made after 1940.

Thanks for the help.

First, I got your PM and will check for the shafts later.
There is something seriously wrong if the bushings failed that fast. If the trans. is kept full of gear oil they should last 100,000 miles or more.
The thrust washers came is one thickness only. I can't find any info as to the amount of end play on the gears. Looked in all different year manuals. Are the washers new? Were the shafts new?

I do have a new 590831 rev idler shaft. It has no "flats". I have a new 3820058 shaft which replaced the 831 shaft in the '60's. It has the flats for holding gear oil....
I have a new 590827 cluster gear but no shaft.
Note that when the bushing type cluser shaft is used it must have the shaft with the flats. The bearing type shaft does not have the flats.
The cluster gear used a 1/16" thick thrust washer on each end. There again the washer for the bushing type cluster is different from the bearing type cluster.
The 1937-39 transmission did not use thrust washers on the ends of the reverse idler gear. The 1940 and up used a 1/32" thick washer on each end. The 1937-1939 had straight cut teeth on the 1st and reverse gear and no end thrust was exerted on the rev idler gear and no washers were required. The straight cut gear is whatt makes the 1st gear noisey on a 1937-39.

The first and reserve slider appears to be cause of problem, not hardened gears vs original gears as debated on VCCA chat site. Hope to detail events later with pics, but NORS 1st and reverse teeth not cut accurately and didn't spline well/deep enough with rest of GM NOS gears. Because the NORS gear teeth didn't seat deep enough, it forced excess pressure on idler and counter gears and shafts and wore them out prematurely, especially the counter gear/shaft. The counter shaft showed almost no wear on side of shaft opposite the point of the contact with 1st and reverse slider.

The NORS 1st and reverse slider was marked (I think) WT 220-12. Rest of gears were all NOS GM as I still had the empty boxes for them. Shafts were new too, but not sure if GM NOS or not.

Per WD Foster, he replies:
Very interesting information, I have a Auto Gear book printed in 1955 that lists the WT 220-12 as a replacement for both GM #590844 non-carbonized low/reverse gear & GM #590931 carbonized low/reverse gear. There seems to be nothing in print anywhere other than the 1938 Chevrolet Service Bulletin that says not to mix/match the gears.

The 1938 3/4 and 1 ton trucks were the first to use the "carburized" gears in the 3 speed transmission.
The carburizing is described as making the gears much stronger by heat treating them. It says it gives them a tough core, which is required for heavy duty truck service.
The 1938 Service News mentions the carburized gears should not be used with the passenger car and 1/2 ton truck series gears as the tooth contours are different and will not operate satisfactorialy. The gears were always listed under different part numbers in the book as well as a way to ID them. With that being said by 1950 the book no longer listed the two different sets of gears and the carburized part number gears were specified for all transmissions with no warning about mixing gear types.
The above is also true for 1941 and up 3 speeds as they had slightly different gears and part numbers. ?????

I would caution you that the 1938 Chevy manual does make a big point of the differences between the case hardened (truck) gears and the passenger gears (non case hardened) gears and indicates not to mix them. One might get lucky and mix them with no problem but I would not do it since I know a little about gear design (being an engineer myself).

The case (via flame) hardened gears were used on the trucks and are not interchangeable with the standard gears used in passenger cars. The case hardened gear is a stronger gear for truck application but has a slightly different involute gear tooth profile and will not mate with a non hardened gear tooth. These gears can be identified by the fact that they have a small hole drilled in the gear and the standard passenger gear does not have this hole. The hole is a 5/32 inch diameter hole drilled in the side of the first and second gear and the countershaft gear also has and extra hole (5/32 dia) drilled in the tube part of the counter shaft assembly,called the hub of the gear. Your shop manual may shows these differences. I have watched these parts on ebay over the last year and they do come up. Just watch out for the 5/32 hole. Good luck

The shape of each tooth, the so called tooth profile is slightly different on the truck gears and has a hardness Rockwell C sale reading (HRC) of 58 where as the gears for passenger cars have HRC of 45. The Thickness of the teeth on the truck gears is .630 inches (on 1st gear) where as the thickness of the passenger car gears on 1st gear is .655 to compensate for the lower strength. The Outside diameter of the gear is 4.275 for truck and 4.290 for passenger car. The root diameter for the truck is 3.520 where as the root diameter for the passenger car is 3.540. These differences also result in a slightly different involute profile of the mating surfaces of the gears and would not allow them to run together normally and could even result in interference conditions if severe conditions existed. Similar differences would be observed for second gear. At best the gears would wear more quickly but run together and could possibly make more noise than normal but still run together. I would not mix those gears on my car. I recently bought a GM Chevrolet parts gear for first gear and was shipped the truck gear by mistake (the seller did not know the difference) and returned it for the passenger gear which was shipped to my satisfaction. They were both Original GM Parts. There are also after market gears by Republic Gear. Both are for sale on ebay. Good luck.

It should also be noted that when gears do not have the proper involute profile they can either not run at all (interference) or run together but will not have a constant angular-velocity ratio. This results in a higher pressure angle on the tooth face and can result in increased wear of the softer of the two teeth that mate. But also the constant changing in angular ratio of the gear as it rotates will set up a vibration that could be heard audibly but even worse could increase the alternating component of stress on the teeth and result in early fatigue failure. I say "could", as the magnitude depends on how severe the differences in dimensions are. I am sorry I cannot translate the given differences above to actual differences in fatigue life. I would just avoid the situation myself by either running two passenger car gears together or two truck gears together but not mix them for mating. Different pairs could theoretically be used in the same transy, but even I would not do that just to keep my car all stock (unless I had no choice).

I have taken the hardness measurements myself and also have images of the teeth profiles to show the differences between the teeth mentioned above. I also measured the above dimensions myself since I had two gears side-by-side before returning the truck gear to the sender. The out-of-spec thrust values you mentioned will also play havoc with wear of bushings. I can send you the images of the teeth profiles if that will help you, just let me know.

Mike