Geoff Byman

On all my engine builds and on upgrade rebuilds I use threaded crank pin bushes. These are either 10BA bolt size to replace the 12BA bolts supplied with Slater’s wheels, or M2 bolts to replace those of the SanCheng RTR engines.

When these bushes are threaded onto the crankpin bolts, they need to be nipped up tight, whether the top hat rim is against the wheel or in the case of the leading axle (behind the valve gear), where the top hat rim is on the outside. Nipping these up tight can cause a little bit of a problem if using pliers or similar, as it can bruise the top hat rim, which subsequently has to be cleaned up when finally in place.

To eliminate this problem, two parallel flats need to be filed onto the rim to facilitate tightening with a spanner. To file the flats exactly parallel, together with filing to a known spanner size by eye is not the easiest job. On the rims of both 10BA and M2 bushes, I have found that a spanner size for 10BA nuts is ideal; the difference between 1 BA and M2 nut sizes is only 0.001in or so.

The simple answer is to thread the bush onto the relevant bolt size, with a nut nipped up against either end of the bush to stop it turning. It would be ideal to use steel nuts and bolts as the crankpin bushes are brass, then you won’t file the nut away as you file the rim to the nut face.

Gripping the bolt head in a vice, carefully file the top hat rim down to the flat of the nut. (See photo 1) Then remove the bolt from the vice, turn it through 180° and repeat. To make sure that you have turned it through 180°, use a slot- headed screw with the slot vertical in the vice, and then turn until the slot is vertical again. You now have parallel flats to accept a spanner to tighten the bush (See photo 2).