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Transmission for Big Twin 4-Speed

Transmission for Big Twin 4-Speed

The 4-speed transmission on Big Twins

The 4-speed gearbox introduced in 1936 together with the OHV-V engines is a so-called constant gear mesh transmission. Two shifter clutches, one on the main shaft (3rd/4th gear) and one on the countershaft shaft (1st/2nd gear) combine the respective gear combination. No more complicated to manufacture combination gears for the counter shaft, 3rd and 4th gear are loosely mounted on the main shaft, 1st and 2nd gear on the auxiliary shaft. And for the first time, the speedometer drive sits in the gearbox like in a car. If you change the wheels, you no longer have to bother with the speedometer drive and the shaft. The main shaft is supported in the 4th gear and carries the clutch on the far left end. The secondary chain sprocket mounts on the outside of the 4th gear sprocket spline, between the clutch and the gearbox housing. The highest gear, i.e. 4th, is 1:1 direct drive, ie. the clutch, main shaft, gearwheel and chain sprocket rotate at the same speed in fourth gear.

What do I need to pay attention to in this gearbox?

The oil level is very important, it should be 650 - 700 ml. A bronze bushing is pressed into the gearwheel of the 4th gear of the main shaft to guide the main shaft. If it is manufactured correctly, it has a lubrication groove, but it is one of the first parts to suffer from insufficient or poor, dirty oil. This bushing also suffers whenever one of the two chains (primary drive or rear wheel) is too tight. In addition, the main shaft carries the clutch at its left end, which weighs several kilograms and is hardly ever finely balanced. Every time you take off or change load, a jolt goes through the entire drivetrain. It is not for nothing that this 4th gear from Carlo's is available as an accessory with needle bearings on the main shaft.

The transmission breather is a hollow bolt with a cross-hole in the head, and it is originally installed in a rather unfavorable position so that splash water can penetrate. Therefore, keep to the 5000-mile oil change intervals for the gearbox, even if it seems unnecessary. Who wants a water-oil emulsion in the gearbox instead of lubricant? But we wouldn't be W&W if we didn't have a solution. It's called the PanAm transmission vent screw.

Six gears in my Shovelhead Chopper? What would I need that for?

It has already been said elsewhere that the more gears the bike has, the better the connection when shifting up and the smaller rpm differences when shifting down. For the 4-speed models from Knucklehead to Shovelhead, there is a solution in the form of the 6-in-4 gearbox from BAKER. Six gears in a housing that fits into the 4-speed frames, complete with kicker.

Have any questions?

Our service team will be glad to help out: Mondays - Thursdays 08:00-17:00 CET, Fridays 08:00-16:00 CET, Phone: +49 / 931 250 61 16, eMail: service@wwag.com