Pinion Gear Compatibility

[Cartravel]

I have two ring gears, 34 and 37, and a 9 tooth pinion gear. Is there a way to tell which ring gear goes with the pinion or does it not make a difference?

[Tom Endy]

It does make a difference.

There may be a way to tell if they are originals. There should be a three-digit number stamped on the end of the pinion gear. Look on the back side of the ring gear for a matching number. The number will be handwritten with an electric pencil and will be difficult to see. You will have to bead blast it or wire wheel it and hold it to the light at an angle to read it.

I have seen this hundreds of times and am convinced Henry had a worker pull a pinion gear from one stock bin and pull a ring gear from another and write the corresponding number on the ring gear, for whatever reason. maybe his way of matching them...after they have been on the road for 500 miles.

Tom Endy

[bobv]

34/9 =3.78 ratio 37/9 =4.11 ratio. Both ring and pinion should however be replaced as a set not mixed.

[Cartravel]

Tom, I am following your excellent article for this build. Thanks. I've got the number from the pinion and will look more closely at the ring gears.

Do you think that Ford would have machined each R&P set differently? I can't imagine they would have gone to that expense. Perhaps they did some sort of lapping in or running in process. If that is the case, I don't see why the set would have to be matched. After a few miles they will be run in. On the other hand if the pinion gear profile is completely different for 37:9 vs 34:9 then I would not want to mix them. On the Model T, Ford made 10 and 11 tooth pinions that could be run against the same 40 tooth ring gear.

[Bob Bidonde]

These slides may help you. Note that there are 2 gear sets with a 9-tooth pinion. These pinions are not interchangeable.

[Kurt in NJ]

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lay the pinion gear on the ring gear, the correct ratio will mesh, but that doesn’t mean that they are matching , as Tom said, look for any matching numbers etched on the gears

[Cartravel]

Ok, here are some photos. The pinion has 9-7-4. It is difficult to read the 37 gear, but I can make out a 5 and 8, so it does not match. The 34 gear is clean as a whistle. So what you guys are telling me is to throw all three in the trash. If I could confirm the pinion was once matched to a 34 ring gear, I would use it, but no one has given a way to identify what type of ring gear it was originally matched to. I suspect there must be many R&P gears out there that got separated from their original mate. Do you guys just throw them out?

[Synchro909]

Originally Posted by bobv

34/9 =3.78 ratio 37/9 =4.11 ratio. Both ring and pinion should however be replaced as a set not mixed.

So goes conventional wisdom but recently, a friend was building up a model A from pretty much nothing and needed the rear end gears. He asked me if I knew where he could get some. I did - in my back shed! For years, I had thrown any half decent gears and bearings in a 25 litre (about 6 1/2 US gallons) drum of oil "just in case". I ended up with oil up to my elbow by the time I fished out all he needed and I helped him put it together after making sure he understood that it would probably be noisy. Once it was installed, it ran as quietly as any I have heard. I almost regret selling them.

[Tom Endy]

Originally Posted by Cartravel

Ok, here are some photos. The pinion has 9-7-4. It is difficult to read the 37 gear, but I can make out a 5 and 8, so it does not match. The 34 gear is clean as a whistle. So what you guys are telling me is to throw all three in the trash. If I could confirm the pinion was once matched to a 34 ring gear, I would use it, but no one has given a way to identify what type of ring gear it was originally matched to. I suspect there must be many R&P gears out there that got separated from their original mate. Do you guys just throw them out?

Over the years I have taken a number of rear ends apart that had mis-matched R&P numbers. I suspect after a few thousand miles they had lapped themselves in and became matched. In those cases, I went ahead and re-used the R&P set and the customer reported the overhauled rear end worked fine.

The nine-tooth pinion gear used in the 378 R&P set is shaped different than the nine-tooth gear used in the 411 R&P set. These you would not want to mix.

There is probably a way to tell the difference, but I do not have that knowledge.

Tom Endy

[Cartravel]

Thanks. That’s not what I wanted to hear, but it’s what I need to know. I think I might try a trial build and see if they seem to run together ok.

[nkaminar]

You could try meshing the gears together by hand and observing the fit. You could try using some heavy grease to look at the pattern of the mesh. It should be almost full across the gears and not just at the tops or bottoms. There is a slight crown so it will not be completely across unless the gears have worn in.

The best advice remains to buy a new matched set of gears. It all depends on what you are going to do with the car. Local ice cream trips, tours, car shows, or a trip across the US.

[Cartravel]

Originally Posted by nkaminar

You could try meshing the gears together by hand and observing the fit.

Yeah, that is what I plan to do. Ford has illustrations on what the mesh contact should look like.

[Bob Bidonde]

The term matched relative to a gear set applies to a run-in of the gears as set by their original manufacturer. Once used in a vehicle, the value of a matched-set for practical purposes goes away due to wear. However, one could argue the wear pattern of a gear set in service still makes the set unique.
I have found that the precision of the Model A's differential gears allows mixing of used gears of the same ratio without consequences. One of my Model A's has 50,000 miles on a mixed 3.78:1 ring & pinion and it runs without any issues. My other Model "A" also has a mixed set of 3.78:1 gears, but with only about 5000 miles on them and no issues.
I suspect that there are many many Model A's running successfully with mixed differential gears.

[rotorwrench]

Tooth sizes have to match. More teeth means narrower teeth on the same base circle diameters. The OEM parts were generally machine lapped to match and then numbered as a matched set.

[nkaminar]

Even with the high precision available today, ring and pinion gears are matched and shipped from the manufacturer as a set.

[Cartravel]

I found someone that compared the pinions for the two sets. The pinion gear is smaller for the 37:9 set. You can determine which pinion you've got by measuring the diameter at the big end. It is about 2.5" for a 37:9 pinion and 2.7" for a 34:9 set.