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Pre-War Flywheel Clutch Throw Out Bearings

by Doug Kephart

The captive ball thrust bearing used on the pre-war flywheel clutch seem to do in the hardened flat surfaces they are expected to run against. At least later on they turned groves in the race to match the radius of the bearing balls (see pic below). But it is the earlier ones that just have flat surfaces, which are the subject here.

I have used in a couple of cases bronze Oilite™ (oil impregnated sintered bronze) thrust washers with good results. As there are no off the shelf sizes exactly as desired, I used a stock size of Ø3" x Ø1.5" x 3/16" thick and modified this. The 3/16" thickness is correct; all that was needed was to turn the outer and inner diameter to the required size.

Douglas motorcycle pre-war flywheel clutch throw out bearing

Though it did not really apply in my case, as the thrust bearing was the right thickness, there is one important point to keep in mind about turning oil impregnated sintered bronze. That is, use a very sharp tool. Otherwise the tool may skid and smear the metal aside and seal off the oil impregnated pores. This is particularly prone to happen on the light finish cut when the tool is reluctant to bite into the metal. The outer and inner diameters are not thrust surfaces, so need not the same care.

Douglas motorcycle pre-war flywheel clutch throw out bearings
Later style with proper grooved tracks machined for the balls to run in.

On later flywheels with the Ø2" hubs, Torrington makes a needle thrust bearing #NTA 3244 that drops right in. This can only be used where the hard thrust surfaces on the clutch sleeve and the clutch throw out cam are in good condition, and have not been grooved by the original ball thrust bearing. This bearing is thinner than the original, and the cam ring rollers may need to be repositioned if possible to compensate.

The arm of the clutch throw out cam, as always, needs to lift the clutch when the cable is at right angles to the arm for maximum effect (as so with brake arms.) The Torrington needle thrust race mentioned costs even less than the Oilite™ thrust washer. Personally I prefer the bronze thrust washer; it is more tolerant of imperfections of the thrust faces. Also it is has slightly more friction than the needle trust bearing, which is not a bad thing as it acts as a clutch brake. Useful if your clutch does not release completely.

© 2002 D. Kephart



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