Auxiliary Transmissions
by John de Marrais
Worcester, New York

Cletrac HG or OC3 crawlers and an auxiliary transmission.

Note: the following is offered free, so take it for what’s it’s worth. I'm sure someone can improve upon what I did.

The Cletrac HG or OC3 comes equipped with a Clark three-speed transaxle. The crawler was originally designed for row-crop farm work. Subsequently, it is not geared low enough for serious dirt moving, towing, or pushing, without slipping the clutch and/or lugging the engine. After installing an auxiliary transmission in my HG, it’s hard to imagine how I used it before the modification. It's a night-and-day difference. The little Cletrac crawler, in first gear and full throttle travels at 2.01 m.p.h. That's fast. With the three-speed auxiliary transmission installed and both transmissions in first gear, the same crawler travels at .64 m.p.h. It can push and tow with the engine almost at idle. Besides making the crawler more useful, the lower gearing will enhance the life of the engine and the clutch. Low gear is a problem in many older tractors; not just crawlers. This holds especially true when attempting to use certain types of PTO powered attachments, e.g. a snowblower or rotary tiller. It is sometimes impossible to keep the R.P.M. up to a high enough speed to operate the attachment, and at the same time, keep the ground speed slow enough to operate.

A look at some other crawlers and stock ground speeds in first gear and full throttle. An Oliver OC4 with the optional auxiliary marketed as the "Slo-Low" travels 1.5 m.p.h. in high range and .8 m.p.h. in low. John Deere MC travels .9 m.p.h. John Deere 420 travels .87 m.p.h. John Deere 1010 travels 1.4 m.p.h. Allis Chalmers H3 travels at 1.2 m.p.h. A few machines that use basically the same three speed Clark transaxle as the Cletrac HGs and OC3s as follows. Terratrac GT-30 travels 1.78 m.p.h. Case 310C travels at 1.74 m.p.h. The wheel tractors known as B.F. Avery or Minneapolis-Moline BF - are both basically Cletracs with wheels and use the same Hercules IXB3 engine and Clark three-speed transaxle and travel 2.42 m.p.h.

Specs. for the HG or OC3 with the Ford Model A 3 speed transmission installed as an auxiliary unit, ground speeds at full throttle are as follows:

With no aux.1st - 2.01 m.p.h., 2nd - 3.19 m.p.h., 3rd - 5.24 m.p.h., Rev. - 2.33 m.p.h.

With the Ford Model A three-speed transmission as an auxiliary transmission:

Range 1: 1st - .64 m.p.h., 2nd - 1.02 m.p.h., 3rd - 1.68 m.p.h., Rev. - .74 m.p.h.

Range 2: 1st - 1.08 m.p.h., 2nd - 1.72 m.p.h., 3rd - 2.83 m.p.h., Rev. - 1.26 m.p.h.

Range 3: 1st - 2.01 m.p.h., 2nd - 3.19 m.p.h., 3rd - 5.24 m.p.h., Rev. - 2.33 m.p.h.

Range Rev.: 1st - .53 rev. m.p.h., 2nd - .85 rev. m.p.h., 3rd - 1.4 rev. m.p.h., Rev. - .62 forward m.p.h.

Oliver offered, as an option for the OC3, a kit for installing an auxiliary transmission. It was made by Trasco, which I believe was a company in California. The kit came with a custom made clutch housing, a modified Ford Model A three-speed transmission and miscellaneous parts. Installation required using the original Cletrac 9" diameter clutch disk, and some of the original driveshaft parts - welded together. Once the modified driveshaft is assembled, there is no easy way to remove it without damage. The transmission's input shaft was either heavily modified, or remade to accept the Cletrac clutch disk and splines. The front ball-bearing retainer on the Ford transmission was eliminated and accommodation in the new clutch housing took its place.

I had never seen the Trasco kit when I began fabricating my aux. setup. If I had, I'm not sure it would have helped. I attempted to do the job with commonly available parts, wanted the finished unit to be easily serviceable, and avoided any high-tech machine work. To a degree, this was accomplished. My setup does NOT require complicated modification of the transmission. The driveshaft I fabricated uses stock Cletrac (Spicer) universal joints and is easily removable without damage.

The one catch to fabricating the aux. setup is, you need a Ford Model A car (not truck) transmission. As far as I can ascertain, it is the best choice for many reasons. It seems Oliver and Trasco felt the same way. That's not to say there aren't "one-time" finds out there for other parts that might work as well, or better. The Ford transmission is simple, rugged, compact, and replacement parts are available for it at a reasonable cost. That, thanks to all the Ford Model A car collectors. The original Cletrac powershaft coming out of the clutch housing, as well as the input shaft on the original Clark three-speed transaxle in the HG and OC3, is an odd size. 1 3/16" by 6. You will be hard pressed to find anything to fit it; I've tried. Coincidence, or serendipity, some vintage Ford equipment uses the same size splines including various Model T and Model A parts. This includes the output shaft on the subject Model A transmission. So... the original Cletrac yoke fits on it perfectly.

When I began searching for a Model A transmission, I didn't know what one looked like. After learning to recognize it, I began to find them all over. I do, however, live in a rural farming area where many Model A Fords were converted into farm equipment. Subsequently, there are still quite a few laying around, here and there. I found over a dozen this past summer. On the down side, I see them on Ebay at times for over $200. Supply and demand, I guess.

This marks the end of my first pontification regarding the auxiliary transmission. Next installment, I'll discuss how to recognize the Model A transmission, and the modifications it needs for use in the Cletrac. That is, unless someone has a better idea.

See Addendum Notes Below

Modified Cletrac clutch housing with steel transmission mounting plate welded to it. A separate diagram with specs. will be posted. The plate I welded to the bell housing was 3/8" steel plate, but it tends to warp slightly when welded. So, once welded and cooled, the assembly was finished milled - in order to make sure the plate was smooth and also to insure that its surface was perfectly parallel to the engine mounting end of the bell housing.

Front view of modified clutch housing. 7/8" diameter shaft made from cold-rolled steel with Ford Model A throwout-bearing fork. Steel bushings with 7/8" i.d. were welded into the housing for the new clutch release shaft to ride in. Bushings are available for a few bucks from many farm supply stores - they are sold as 3 point hitch adapter bushings. Notice that this Cletrac housing has the access hole on top; some don't. It's a nice feature and makes assembly easier. When final installation is done, there is one bolt mounting hole that gets blocked by the new clutch cross shaft (bolts that attach to engine). I drilled and tapped one new one to compensate.

Two Ford Model A three speed transmissions (for car, not truck). Left one is modified for use in Cletrac, right one is original. The optional aux. that Trasco used modified the input shaft on a Model A trans to use the Cletrac disk. Why, I don't know. I don't modify the input shaft and simply use a Model A disk. Modifications to the trans. are easy and low-tech with no machine shop work necessary. I do recommend removing the original ball bearings and installing modern sealed bearings. I have the specs. and part numbers.

I'm posting some photos of the Trasco aux. setup...

I'm posting some photos of the Trasco aux. setup...

I'm posting some photos of the Trasco aux. setup...

View from top with original clutch housing and driveshaft out to show modified clutch pedal mounting and linkage.

New Ford Model A clutch disk to be used in place of original Cletrac disks. Both are 9" diameter. Ford on left, Cletrac on right along with original Cletrac pressure plate. Note: new Ford disk is installed backwards, i.e. side marked "flywheel side" must face AWAY from flywheel.
Also, the HGs and OC3 came with two different versions of pressure plates. Image only shows one of them.
New Ford Model A disk, with dampening springs is available from many places that sell Ford repro. parts. Cost $40-$60. I don't advise buying one from NAPA. It won't be new, but will be an old relined disk and won't have the dampening springs.

View of modified Ford Model A transmission and fabricated driveshaft. This driveshaft uses the original Cletrac (Spicer) u-joints and comes apart sideways. Works very nice. Original Trasco aux. driveshaft setup was not removable without damage - or pulling engine.

Another view of installed aux. trans. with fuel tank out.

I'm posting some photos of the Trasco aux. setup that Oliver offered as an option. It uses a Ford Model A 3 speed transmission with the input shaft modified to accept the stock Cletrac disk. Kit required using the original stock u-joint yokes, and welding them together. They were not easily removable. Kit came with a nice custom bell-housing. I didn't get my hands on this until long after I had already designed and installed my own setup. If I had found it earlier, it would have saved me some research time.

I'm posting some photos of the Trasco aux. setup...

I'm posting some photos of the Trasco aux. setup...

Original Cletrac HG/OC3 mount on left, modified for aux. setup on right. Modified version bolts to fabricated adapter plate on the Model A trans.

Original Cletrac HG/OC3 mount on left, modified for aux. setup on right. Modified version bolts to fabricated adapter plate on the Model A trans.

Throwout carrier. Hopefully self-evident. Not a Ford or Cletrac part.

The plate I bolted to the Model A transmission is 1/2" thick. I think using the plate is necessary for a number or reasons. It provides a place to bolt the fuel tank mount to. Also, the four bolts that are used in mounting the Model A transmission, when used originally in the Ford car, come through the inside the bellhousing. So, the transmission cannot be removed by itself since these mounting bolts cannot be accessed unless the engine is out. I wanted the transmission to be easily removable, so with the adapter plate, it is bolted to the trans with the bolts pointing towards the trans, and then, four bolts pointing the other direction mount it to the modified bell housing. This way, you can access and/or remove four bolts at any time and easily remove the trans. (with plate still attached to it).

I'm posting some photos of the Trasco aux. setup...

I'm posting some photos of the Trasco aux. setup...

I'm posting some photos of the Trasco aux. setup...

I'm posting some photos of the Trasco aux. setup...

New part numbers for Aux. trans.

If anyone is using information I've posted about constructing an aux trans. setup. I found a good part number for the throwout bearing and carrier I used. Mine came off a Cockshutt farm tractor, but it is the same one used in the 1947-48 Dodge Powerwagon. The entire assembly - the carrier with the bearing is a NAPA N1313 for around $70. Just the thrust bearing itself is: N1087 (at NAPA) or 2062(BCA) or CT52A-1 (Nissan-Ford).

I also found it for sale at a vintage Dodge Powerwagon parts place for $35.
Bearing only CC581499 $25
Sleeve CC581500 $10
Bearing Assembly CC581498 $35
Pull Back Spring CC573318 $5 Site is at:

For putting sealed bearings in the Ford Model A trans:
New sealed output bearing is box # SKF 6306-RSJ or on the bearing - MRC 306SZ The old unsealed output bearing was New Departure Endee 3306

New sealed input bearing - # on box: CR 6208-RSJ actual number on new bearing: SKF 6208-RS1 (or R51). Old unsealed bearing was: New Departure Endee 3208

Other Auxiliary Transmission Links

Click picture for full sized image

Original image (not mine) and can't remember where I got it. I digitally installed the Ford Model A 3 speed to show how it all fits.

Model A transmission car setup.

Model A transmission apart.

Trasco transmission.

HG and OC-3 driveline.

I don't have a degree in mechanical drawing, so don't complain to me. What do you want for free?

Original driveline setup.

This image is two Ford Model A 3 speed transmissions, side by side, rear view. Left one is modified and has original Cletrac Spicer u-joint spider installed. Trans. on right is original and unmodified.

Diagram of clutch actuating arm that will have the same travel and pedal pressure as original Cletrac HG or OC3.

View of aux. Model A trans. installed with modified fuel tank mount. I left the tank off for photographic purposes. Shift lever was heated and bent to clear tank, and top was threaded to accept original Cletrac shift knob.

Close up of fabricated driveshaft using original Cletrac/Spicer u-joints and yokes. Comes apart sideways. I added grease fittings.

Hopefully, self explanatory.

I'm posting some photos of the Trasco aux. setup...

I'm posting some photos of the Trasco aux. setup...

I'm posting some photos of the Trasco aux. setup...

Ken Cornell's HG with Ford A trans.

Ken Cornell's HG with Ford A trans.

Ken Cornell's HG with Ford A trans.

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Blake Malkamaki
21092 North Norrisville Road
Conneautville, Pa. 16406
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Revised: Saturday, December 15, 2012