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Re: Converting Model l 2 speed to swiftomatic

Some need to know information for those thinking about changing a two-speed two wheeler to a four speed two-wheeler.

In 1963 the Swiftamatic two speed differential was introduced, and at the same time Gravely changed axle shafts and axle housings from double bearing axle shafts to single bearing axle shafts. Installation of the Swiftamatic assembly requires that both axle housings and axel shafts be single bearing assemblies. Axle shaft bearing end play on the right side cannot be set if you only update the right axle housing on a pre-1963 tractor, there is no problem converting a later tractor.

Depending on the model of L or C, the conversion is somewhat involved or very involved. The Swiftamatic bronze worm gear works with an 8 lead worm. A straight L or C has an eight lead worm, a LI/CI has a six lead worm, and a LS/CS has a four lead worm. Changing a lead worm is a complete tear down of the chassis. You cannot mix different lead worms and worm gears.

Not all Swiftamatics are the same. The first units had five cog shifting components, later units have three cog. Three cog are more common and perhaps a better choice.

The first style of spider pin assembly used something called ‘thrust spacers’ on the four tips of the pin. The thrust spacers, shims, and bearings accurately located the gears on the spider pin. The precision cast powdered metal spacers or shoes rode on the inside surface of the bronze gear and wear became a problem. The powdered metal shoes would wear and so would the bronze worm gear, thus loosing important spacing. Some units self-destructed. The old style units cannot be rebuilt because you cannot find new parts. Some old units can be reconditioned somewhat with old parts that have less wear.

The sides of the very first Swiftamatics were held to the bronze gear with bolts and lock washers. Those bolts would loosen and cause major damage; two different types of metal locking tabs were introduced on later units, still later came bolts that could be safety wired.

The first Swiftamatic differentials used the same size carrier bearing on the left side as the single speed differential, at some later point that bearing size was changed to a smaller bearing. On two different Swiftamatic differentials you can find two different left bearings and that means there are two possible left axle housings.

In the summer of 1971, with tractor serial number J-65504, Gravely introduced the new style spider pin. The gears, spacers and bearings were held on the pin with clips and nothing touched the inside of the new style bronze gear. The I.D. of the bronze gear was opened to accommodate the new pin and a slot or slots cut in the side of the gear to allow pin installation. The bolts holding the sides to the bronze gear wear changed to allow aircraft style safety wiring, no more loose bolts. At some later point the bronze gears went to a machined casting, I believe the first were cut from solid billet stock. The number of slots in the sides increased to lighten the casting and lessen the cost.



Before you start the conversion, you need:

1. A Swiftamatic differential unit, preferably new style, plus thrust pin.

2. A right Swiftamatic axle housing and axle shaft with same shifting cogs as the differential clutch housing.

3. Shifting clutch with matching number of cogs, preferably three cog.

4. A single bearing left axle housing and axle with a bearing match for the left carrier bearing of the differential.

5. External shifting linkage.

6. Axle housing gaskets, possibly axle housing shims and axle bearing shims.

7. Axle oil seals if the old seals are leaking.

In the late 60’s and early 70’s zero tolerance machining was rare. The machining of the chassis castings, axle housings and the side component casting of the differential all had tolerances, hence the need for shims of different sizes on the assembly line. The bearings on the differential unit and axle shafts were shimmed to end play specs.

You cannot use Swiftamatic components from a 5000 or Pro series two wheeler in earlier tractors unless you change axle housings, axle shafts, and wheel hubs. You cannot use the shifting mechanism from Kohler powered units on L or C models.

An important step in the conversion is checking the end play on the differential carrier bearings and that is normally done with the advance casting and forward/reverse planetary components removed. With a long pry bar or large screw driver you check for just a slight ‘wiggle’ from side to side of the differential with both axle housings bolted in place. The side to side play of the differential is adjusted with shims behind the axle housings; remember new axle housing gaskets affect the end play dimension. The spec for the differential end play is .008 - .012”, but just a ‘little wiggle’ will work fine.

Axle shaft end play is adjusted with shims behind the bearing caps; again if axle cap gaskets are replaced, they may affect axle shaft end play. The spec is .008 – 012”. A worn thrust pin will alter axle shaft end play, if the ends of a thrust pin show obvious wear; it is time to replace it.

When installing or removing a Swiftamatic differential it is often necessary to loosen the hardware on the left axle housing, while working with the right axle housing removed.

When adjusting differential end play, that is adding or removing shims, it is easier with the right axle shaft removed, the differential will stay in place sitting on the left axle shaft. When checking end play, you only need three equally spaced bolts holding the axle housings in place, less hardware to remove/replace every time you make an adjustment in the number of shims. While shimming, you should try to keep the differential centered under the worm gear, use shims on one side or the other to help center. It is an ‘eyeball’ adjustment while studying the parts from front and center. Some chassis castings were so one sided that centering is not completely possible. Perhaps they were ‘Monday morning’ castings. I have found shim packs all on one side of the chassis and the differential still not dead center under the worm.

I chose not to cover changing an LI/CI to four speed in this document. It usually requires parts from several tractors. Most six lead worm gears come from the 400 series four wheelers, and it is a major job to get just the differential out of a 400 series four wheeler. You cannot use the thrust pin or shifting gear from a 400 series for such a conversion. Yes, the 400 series also had new style and old style spider pins and bronze gears, not a conversion for a novice.

Roger,
 
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Re: Converting Model l 2 speed to swiftomatic

Nice write up Roger. I have a pile of swifty parts that came with one of my machines. I will post a pic tonight so you can help me identify them ...
 

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Discussion Starter #3
If you are converting a older model L, you will also need to change the oil pickup tube to the newer flat style. Thanks for the tip Mike.

Roger,
 
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