Just like Henry Ford's first automobile, final drive on the Harley-Davidson Servi-car is a simple chain-drive mechanism. A chain attaches the transmission output gear to the rear drive sprocket and both rear wheels rotate at the same speed. This final-drive system was cheap to manufacture and very durable.
Differences In Servi-car Rear Axles
Early Servi-car axles (1934 to 1936) have a big center, with a single mechanical drum brake inside the housing. This brake has two shoes and is internal-expanding on a fixed pivot. A parking brake is mounted inside the rear axle housing.
In 1937, the Servi's brake system was upgraded to a drum brake on each rear wheel. Somewhere in the 1940s, the axle halves were welded together instead of riveted.
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The 1941 to 1950 rear axles are slightly different than 1951-1972 units. They will fit up to 1972, but not be "correct". Last-year Servi-cars (1973) were disc-brake equipped and had six-stud wheels.
Rear Axle Truss Rods
The Servi-car axle has one front and one lower truss rod to keep the axle from bending. There are four coupler nuts which have right-hand threads on one end and left-hand threads on the other end. To remove them, tighten (shorten) the shorter side of the rod, which creates slack on the other side.
The right front truss rod coupler is "smooth". This is for clearance and goes toward the center by the stoplight switch. This coupler has internal lock washers. The other three couplers use external lock washers.
Unlike automotive rear differentials, Servi-car axles do not require gear oil. They are lubed with approximately four ounces of wheel bearing grease.
Servi-car Axle Rebuild
To split the axle open, tap out the axle housing bolts and studs. There are two bolts which measure 4-1/2" in length. The bolts for the nuts face the shorter (right) axle half. Two of the three studs measure 6" and the third measures 5".
The left axle is longer and measures 26-1/4". The right (shorter) axle measures 20". Both fit 1941-73 Servi-cars. In the event your axles are damaged, they are still available in the aftermarket.
I didn't need to separate my axles, but if you do, there are nine bolts and nuts which need to be removed. A spring wire lock ring holds them in. This allows disassembly of the differential.
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Note that there are small "C" clips at the end of the axles at the differential gears in the center section. These are referred to as axle lock half rings, or retaining clips. Two are used per axle.
Inspect all parts for pits, grooves, and gauges. Replace any parts if there is any doubt to their condition. After inspection, clean all parts with solvent and blow dry them with compressed air. All bearings, if not needing replacement, should be cleaned and repacked with automotive wheel bearing grease.
Before the truss rods were reinstalled, they were grinded, filed, sanded smooth, primed and painted. Remember to install the coupler, nut, and lock washer in the correct order, or you'll be taking them off again!
Slowly and evenly, loosen the low-end truss rod fittings, then tighten the high-end fittings until the rods are parallel with the axle. Do not over tighten them, as it may prematurely wear axle parts.
Before reinstalling the axle to the frame, check tightness of all bolts and install grease fittings. The axle housing takes about four ounces of grease.
Servi-car Speedometer Gear
There are two different speedometer gears for Servi-car axles. Note how many teeth the speedometer gear has. Gears were the same from 1938 to mid 1962. If you need to replace the gear, measure the distance from the gear face to the end of the axle. The new gear needs to be pressed on. Grease the speedometer gear before installing in the rear axle.
If you're replacing the axle grease fittings, you may not find them at the local parts store. They are 1/4" fine-thread pitch with a head size of 5/16".
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Servi-car Rear Wheels
All year Servi-car rear wheels are hard to find and high in price. Early Servi-Cars (1937-1950) have 16" x 3" spoke rear wheels. These early rear wheels are a taper-fit with a key to the axle, and the hubs are specific to them. Aftermarket spoke wheels are available for these early axles and retail for about $1,000 each.
From 1951 to 1972, Harley Servi-cars had five-bolt automotive-style steel rims, which are 16" x 3.5". Last-year models (1973) have six-lug rear wheels and only fit the six-lug axle.
For my 1961 Servi-car restoration, I chose Dunlop D402 wide whitewalls. Tire size is MT90B16, which fit the 16 x 3.5 rear wheels. They require 5.10 x 16 center-valve tubes. Don't forget to install rim strips before mounting tires.