Pinion 9 tooth

[roddyb34]

Having read the various threads regarding ratios 3.78 and 4.1 ,,I am lead to believe that although the pinion tooth count for both ratios is 9 teeth there is a variation in tooth angle specifically to the 37 tooth and 34 tooth application and should not be mixed ,,,,how do I determine the correct “tooth angle” ,,I have a a 37 tooth crown wheel now in the coupe and discovered I have a 34 tooth c/w in my collection I wish I had realised I had prior to assembling my rear ,,,I was hoping to do a simple swap with minimal disassembling ,,,the diff was built from several units so not having prior knowledge about this it went together OK and has performed well for a few years so far ,,I was contemplating a Volvo Laycock overdrive conversion and have gathered the parts to do it but the front shaft modification had me stumped,,,I am thinking the ratio change to 3.78 would be sufficient to run my coupe at a comfortable rate of speed and performance to suit my needs without a lot more work ,,,thanks from Rod ,,,

[Joe K]

Like you, I have often thought of a possible "match" between the 3.78 parts and 4.12 parts.

What brought this to me was the arrival of a rear axle MINUS the pinion and torque tube - yunno, the classic "trailer" conversion where the hole in the punkin is blanked off.

Meanwhile, I have that removed small truck 4.12 axle sitting around either waiting for a new pair of gears OR adaptation to an overdrive (many prefer an overdrive paired against the 4.12 - it seems to offer a more "normal" gear pacing for some drivers/terrains.)

I am suspicious that the two gear sets are entirely different animals.

Think of it this way: either a 3.78 or 4.12 use EXACTLY the same housing - either can be interchanged with a complete unit of the other. While the diametral pitch/tooth size of the pinion MIGHT be the same, to achieve the proper tooth ratio, the diametral pitch (diameter) of the crown MUST increase. If increased, it won't fit in the housing. A new housing would be required in addition to the new gear.

Another way to look at it: In order for everything to fit in the housing, the larger tooth count crown requires a smaller pinion. A smaller pinion is compensated for in the construction of the crown with an "offset" between the crown flange mount and the gear teeth/action. You use the same housing, the same differential cage, but the gears are "different" to bring in the additional three teeth.

So I think the answer to your question and mine is NO. You can't "mix & match" pinion/crown parts without getting into other issues.

Alas. For me it may be the opportunity to try out a 3.26.

Joe K

[daveymc29]

I took a 3:78 out of my coupe and installed a 4:11, used the matching pinions and crown wheels and didn't had any problems with the differential housing. I have also put Laycocks in several A's that I have owned over the years. I got mine from Rich little and they were a straight forward exchange, not problems wit the shapts on either end. I gave him my drive shaft and the torque tube and got them back ready to install. I think they are a wonderful choice, the withstood the mad driving of my Austin Healey days with the A/H having about 150 HP. Never broke one.

[Y-Blockhead]

Gear Vendors make kits for vintage car, including Model A's. They use the "J" model Laycock, I believe.

I have a Gear Vendor behind my 318" Y-Block/C-4 in my '55 Ranchwagon, basically gives me a 6 speed automajic... works awesome.

[Joe K]

I took a 3:78 out of my coupe and installed a 4:11, used the matching pinions and crown wheels and didn't had any problems with the differential housing.

I think the original query was "can you mix & match."

Whether you put in a 3.26, a 3.54, a 3.78 (or the unusual early variant which is a similar but not exact ratio) or 4.12, all will fit as MATCHED pairs into the Model A Punkin.

Glad you had success in the adaptability of a 4.12 to an overdrive.

Curiously, my unmolested 4.12 axle has a torque tube which tapers "continuously" from universal housing to punkin. All the other 3.78 axles had the 2/3rd straight tube 1/3 taper torque tube more commonly seen. I have considered that Ford might have used the different torque tube patterns as a way to distinguish axle variants for assembly - but so far no mention of this in the judging standards.

My truck was an assembly performed at the Somerville, MA production facility March 1929.

Joe K

[rotorwrench]

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The ring gears have to have different tooth counts to get to the ratio needed. Starting with the tall ratio gear sets, the tooth count on the rings are the lower count numbers which means there are specific tooth spacing distances and width of tooth sizes to fit that ratio on a ring of a set diameter to fit the carrier. As ratio numbers get progressively larger, the distance between teeth have to get shorter and teeth have to get narrower to fit the same diameter ring gear. All of this also affects the pinion gear. Even though they may have the same number of teeth as a larger or smaller ratio, the width of the teeth change to suit the width of the ring gear they are matched to fit. They are all matched sets in that each pinion and ring of a specific ratio are fit together to insure proper tooth contact and that set is kept together.

If folks fit a different old pinion to an old ring then it will either get a good contact pattern with acceptable backlash or it won't. If they fit well together with a good contact pattern and backlash is within specs without shimming things too much then consider yourself lucky. This is a rare situation indeed. I prefer to use matched sets to begin with in order to save myself time and the grief of starting all over again if it doesn't work.