Farmall distributor gear specs

This is probably far-out but I wondered if anyone had info on the gear specs for the distributor drive gear that goes into all of the 4 cylinder letter series gas engines. I've seen some "engineering memos" describing gear materials and heat treatment and wondered if confirmation of this existed somewhere (?)

By measurements I think its a 32 DP (diametral pitch) 20 degree PA (pressure angle) American stub tooth form. 32T on the distributor gear and 16T on the drive pinion.

I'm aware that they can be bought from several sources. I have the equipment to hob spur gears and thought this would be an interesting project.

Just toured the John Deere plant in Waterloo Iowa, and they described how they heat treated and carburized and thermally quenched their gears, etc. after making them. I think there's a lot more to it than just cutting the metal.

I would be very interested to see how this project turns out. I am a amateur machinist trying to tool up to make a rotor gear for an E4A mag. I am currently having trouble selecting a hob because I don't understand the description in the catalog, "Range of Teeth Cut". for instance, the no.1 cutter is for " 135 to rack". Regarding your project, I would not be concerned with heat-treating the gears, unless you really want to. There is no turning resistance from the distributor gear, other than opening the points. I think a good piece of cold-rolled would be perfect.

Agreed selecting an alloy + heat treatment is going to also be part of the process.

I think an 8620 gear, carburized, would outlast all of us here in a distributor app....as you say its just turning the shaft and opening the contact point.

As far as making a gear there are several ways to go about it. Manually cutting a gear with a horizontal mill for example requires some way to index the gear (space the teeth) very precisely. A dividing head is typically the choice, a rotary table can index but its manufacture is not precise enough and a bad gear will result with one tooth that doesn't match the others (bad). Then you would choose a Number "X" gear cutter which is form-ground for a range of number of teeth. For my 32T gear I could use a 30T-40T cutter, or whatever the catalog has available.

Hobbing the gear actually generates the form, the gear blank is also geared (via machine setup) to the gear hob (that's a cutter that resembles a corncob) and the teeth are cut into the blank as both parts rotate together. There's more to it than that, there are workholding considerations and machining the blank so the ID and OD run concentrically are some of the more important aspects. A spur gear is fairly simple, a helical gear is more complex as there's another dimensional parameter for the angle of the spiral as if it were unwrapped on a paper. In the hobbing machine this is an additional phase shift between the rotation of the blank and cutter as the feed advances.

The point is not to bore you with esoterics, I have made spur gears before, just trying to zero in on the specs. Knowing what the OEM set out with is best, but in the absence of that there is the time-honored tradition of reverse-engineering.

fritz the cutter cuts only so many teath. you need the number that falls in to that range. your cutter that cuts 135 to a rack is one that will cut from 135 to a rack ( like a steering rack) you need one that will fall in to the range of theath that you want to cut. hope that this helps. Bob

I worked as a production scheduler in a gear machining department at Farmall for a year before I got
promoted back in 1976/'77. All the gears they made were forged 1040/1045 carbon steel and carburized. A
few gears were induction heat-treated. There was a row of Barber-Colman hobs that cut all the small pump
gears used at Farmall. All the blanks came from the West Pullman plant in West Pullman, Illinois, Chicago
suburb.

Your 8620 is WAY overkill for this distributor gear. I'd use 1040/1045 and Nitride the finished part.
FARMALL shaved most of their gears to perfect the tooth profile. The shaving cutter would be hard to find
now days. Mitsubishi CNC wire EDM machines have a gear cutting package in their programming, think I'd
wire cut the teeth if I had to make one. I'd make a few calls to tractor salvage yards and find a good
used one if it was me. I know the Thrill for you is watching the chips fly, but maybe making new 4th &
5th gear sliding gears for H's and M's makes more sense, and more chips.

The distributors and magneto were made at the West Pullman plant. Farmall was a LOT more self-sufficient
back in the 1930's to 1960. They poured most of their own castings, big and small, made almost all their
own engine parts, but I think West Pullman also made this shaft.

Good Luck! 37 years ago I could have walked up to Engineering on 2nd floor of the office building and
found the print and made you a copy. The engineers were used to seeing me make prints. But also a good
chance your local IH dealer had one on his shelf.

Heck yea that helps! I want to hear everything everybody has to say on this, no how elementary they think it might be. I already learned that a rotary table is the wrong way to go, and now I can see why. I'm still LOL over Dr. Evil's " The Thrill is Making the Chips Fly"... I never heard a more accurate description of a human endeavor