Technical Tips

Classic BSA &Triumph Motorcycles


BSA All Twins/Triumph 500-350 Twins pre '68

As owners of these machines you may be familiar with fact that your bike has a full plain bush on the timing side of the crankcase. The bushes have a tendency to wear and sometimes quickly especially if vibration is harsh due to poor crank balance. Good oiling as talked about previously is important for a decent life. The best performance I have seen from a bush motor was an A65 ridden easily and serviced regularly. It had 50,000 miles on it. The worst performance was 1500 miles on a newly rebuilt motor that was flogged hard. If you have an A65 or A10 BSA, or a 350/500 Triumph '57 to '68, then read on.

A bush can give good service if set up right and tight, and serviced every 1000 miles and fitted with an oil filter. By fitting a gauge you can keep an eye on the crankshaft condition and by fitting packing to the oil pressure relief valve you can raise the oil pressure to 70lbs or so thereby keeping a good film of oil between the crank journal and the bush.

The problem is keeping that film. When captured, oil in a hydraulic line can't be crushed, so if you could capture the oil in the gap between the bush and crank the crank journal and bush should last a lot longer. Cold start ups are important, so don't rev the crap out of it until you see pressure on the gauge. The bigger the gap the less chance of keeping oil in the gap and the less available oil to transfer to the big end/rod bearings. When oil becomes scarce at the rod bearings due to losses at the bush, the rod shells wear quickly then more pressure is lost, a vicious circle. Until 1972, BSA did not fit steel rod caps like most other British makers. This helped with ovality in the conrod bigend housing. This ovality also caused oil losses. When rebuilding aluminium capped rods have them checked and or resized, and fit new bolts and nuts. I like to have the crank main ground to .008/.018 /.028 etc. Then when I fit the corresponding .010 /.020 etc bush, ie, heat the case and drop in the bush, I am able to line the hone bush with a modified Sunnen hone to give me .0005 oil clearance. This setup spins easily with no perceptible movement, but be careful running it in as plenty of heat cycles are required. I have done it plenty of times so take no notice of what your expert mate thinks. After 10/12 standing heat cycles, plenty of half mile heat cycles will be needed, gradually building up. A similar method is to set the timing side case up in a mill and bore to the required clearance. Before setting the end float to 2 to 3 thousandths, whack the crank on each end with a heavy soft mallet after warming the cases. This seats the bush and roller bearings. Just fitting a .010 bush on a .010 grind usually ends up with a .002 or bigger oil clearance. Not a good starting point. Change the oil 3 to 4 times in the first 1000 miles, the more the better. 1972 also saw a cast iron body oil pump for BSA which also helped keep oil up to the crank. The standard rubbish aluminium pump loses oil out of the joint faces when operating.