MASCOT May - June 2001

Roadster Notes - Starter Ring Gear Replacement

Bill Haverly

When it comes to engine repairs, a lot of work can be done by a competent DIY mechanic without resort to the engineer. However, other aspects of reconditioning like rebores, crank grinding and replacing valve seats, require specialist equipment and are best left to the professionals. It's the grey area between these two extremes which concerns me here, and recent replacement of the ring gear on SM1500 engine in my SMX typifies the sort of job that can be done at home without incurring the expense of an engine specialist. Valve guide replacement is another example.

The manuals usually advise removal of the old ring gear by splitting with hammer and chisel, but a more civilized approach is to heat up the ring with a blow torch or calor gas flame for about 5 minutes, then drift off evenly with hammer and soft metal punch. What follows next could sound more like (some) bizarre cooking instructions than a practical application of the laws of expansion and contraction, and is best done in an unoccupied kitchen! Set the oven to gas mark 6 and place the ring gear directly on the top shelf. Do not use a baking tray. Pop the flywheel into the deep freezer and set the oven timer for 15-20 minutes. In the mean time, get an old table or work surface close to hand together with some large grips and heavy duty gloves or oven mitts. When the timer goes off, place the chilled flywheel engine-side up on the table and drop the expanded starter ring on with the engagement face downwards. It should fall straight on to the shoulder, but if it jams, remove it with a copper hammer and try again. Re-heat if there is further sticking and make sure the ring is fully home on it's seating. This method can also be used to reposition the ring on a flywheel when wear has become excessive around the top dead centre position - Roadster ring gears are not yet quite as rare as the Dodo, but it will not be long before repairs like this are common place