MASCOT March/April 2002

ROADSTER NOTES -The Gearbox By-Pass

Bill Haverly

All Roadster and post-war Singer gearboxes have an unusual lubrication system. Unlike most contemporary units, they are not self-contained, and rely on a by-pass operation to retrieve oil when it emerges from the clutch shaft bearing in the front face. I was reminded of this when replacing the clutch on the SMX Roadster early last year, and anyone who has ever separated gearbox from clutch housing without first draining the box will know what I mean, as a hole in the front face will spew oil all over the floor! This 1/4" oil return hole "A" is the final stage in an external system for recycling the gearbox oil after it passes through the clutch shaft bearing. In the assembled unit it happens something like this:
Oil in the gearbox splash-feeds all the working parts, but because the clutch-shaft bearing is open at the front, oil passes through and drains onto a scroll just forward of the bearing lock nut. From here most of the oil is spun down into recess "B" in the clutch housing face and drains back through the return hole into the gearbox, and the rest feeds back into the bearing.

The reasons for using an exposed bearing are worth speculation. As designed, this bearing is in a well supported position straddling the front face, and gets the benefit of full-flow lubrication. On the other hand, if the casing had enclosed it at the front, the casting would have needed to be larger and heavier and then, being in a recess, the bearing would probably not have received such effective lubrication from within. Another aspect concerns oil seepage, the most common being between clutch housing and gearbox, where there is only a thin brown paper gasket to seal the joint. Tightening the holding studs is sometimes all that is needed, but more often the gasket needs replacing. This first becomes apparent when clean oil drips from the drain plug, appearing to be due to a leaking washer. Sometimes it is, but more often than not it will be a side effect of Singer's by-pass lubrication system.