Mix rings/pinions on straight axle??

[Don L.]

Rebuilding posi-pumpkin for my 61. Anyone have info/experience on the allowable combinations of pinion/ring gears?? Specifically, can I swap out a 41 tooth ring gear(with a 9 tooth pinion)for a 37 tooth ring gear (to be used with the same 9 tooth pinion? I know the specs for 4 series/3series gears are different, but the two ring gears appear to be identical in thickness/diameter and otherwise equal(thus switchable?).

Based on measurements(I have not attempted a trial/error assembly yet), it appears the switch should work. However,the stampings on both gears have selected numbers which appear to indicate ring/pinion specs. On the 41 tooth gear: "9-41" is stamped after "GM 37200" and on the 37 tooth gear "10 37" is stamped after "GM 3961419". This suggests ratios intended of 4:56 (9/41), and 3:70 (10/37). Since the thickness of both rings is approx 1.05", I would think the 37 tooth gear could be substituted and used with the 9 tooth pinion. This 9/37 combo gives me a ratio of 4:11.........but will it work of do I have to find a companion 10 tooth pinion?

[Joe L.]

Re: Mix rings/pinions on straight axle??

Don------

As far as I know, you CANNOT "mix and match" ring and pinion gears. These gear sets are a factory-matched set. I am virtually certain that you cannot interchange the ring and pinion tooth combinations in order to produce a non-factory available ratio. In addition, even replacing one of the two gears (ring or pinion) with another IDENTICAL gear may cause problems. GM NEVER sold the ring and pinion gears seperately. When they were available, they were available ONLY as a factory-matched set.

As far as I know, aftermarket manufacturers (e.g. Richmond, US Gear, etc) only sell these gears as a matched set, too.

[Iron Duke NCRS #22045]

Re: Mix rings/pinions on straight axle??

My understanding is that ring and pinion gears are set up on a fixture at the plant and driven under load with an oil that has fine abrasives to essentially cut a tooth contact pattern. For this reason the ring and pinion is a matched set, and it is critical to get the pinion depth and backlash within spec so the teeth run in this predetermined engagement pattern. To do otherwise potentially reduces tooth life and/or can cause the gearset to be noisy.

Duke

[Jeff]

Duke is right, short life, big noise...

... if you do what you describe it will probably 'simulate operation' but unless you get very lucky, it will whine like hell although you may not ever notice the life expectancy being reduced by some percentage if you don't use the car much. R&P gearsets are extremely strong. If you think about it, every time you see a car wheelstand, the pinion is actually climbing the ring gear and supporting the weight of the car on the very small contact patch between the ring and pinion as well as absorbing the shock load that got it there, so whatever amount you do shorten its life by may not be noticeable. Besides, almost everything that runs in an oil bath lasts nearly forever although removing teeth from the pinion which is almost always the way steeper gears are achieved, weakens the pinion significantly.

Swapping gears from sets of the same ratio is bad enough but combining mismatched gears into non-factory ratios can result in mating gears of different pitch which is the same as trying to mate threads of different TPI or crossthreading. (There's a reason they make all those different ring gears with only a one-tooth difference between them.) That won't last long at all and will be very, very noisy while the torque of the engine and the multiplication of the trans make a valiant attempt to average the 2 pitches.

All in all, I'd say this is one of the worst ideas I've heard in a long time...

JP

[Ken Karsen]

Re: Duke is right, short life, big noise...

It Is A Bad Idea! Duke and Jeff are correct, it will be noisy as hell! It will defiinitetly work however, it won't work indefinietly. Tried this a few years back, I can still can remember trying to kick myself in the ass! The noise was awful, and everything had to be torn apart again. I thought it was a perfect match. It was eating itself, as it was trying to set. I won't do that again.

Karsen