Alex,
The fact that the oil is leaking out the timing cover joint and from around the magneto is actually a good sign. It proves that the oil is at least getting that far. You know the old adage that when your British bike stops leaking oil, it is because it ran out.
However the fact that the oil is getting into the engine, does not mean that it is getting to where it belongs. First the leak is probably nothing more than a bad timing cover joint gasket, or lack of one. The oil is leaking out rather high. Normally the level in the timing cover is enough to submerge the lower run of the gear train. There is a 1/4 inch hole that communicates and drains off the excess into the crank chamber that sets the oil level.
Inside the timing chest cover is the oil pump. It is possible that on the delivery side the joint face gasket has failed, allowing the oil to hemorrhage out into the timing chest, rather than being delivered to the crankshaft. Oil fed through the crankshaft and escaping from the big end bearings is what lubricates the cylinder bores. Since the sump is scavenged, you do not have any remedial splash lubrication. The oil pump has, or should have, a primitive pressure relief valve. If this is stuck open, spring rotted, or what ever; oil returns from the pressure side back to the delivery side without ever leaving the pump. There is a spring loaded quill that feeds the oil into the end of the crankshaft. If the spring is weak or omitted, the oil may just leak out into the timing chest rather than enter the crankshaft. Or if the internal galleries in the crankshaft are plugged with sludge the oil may not enter. However if the crankshaft were not getting any oil, you would be seizing the connecting rod bearings and not the pistons. And if you seized the rod bearings the engine would not be freeing up when it cools down. (Also due to the scarcity of crankshafts you will have converted you motorcycle into a static museum piece!)
So assuming the oil is getting into the crankcase chamber via the rod bearings, the problem maybe the pistons were fitted with the improper clearance. I have had pistons that have been made of unsuitable grades of aluminum that had excessive rates of thermal expansion. I had one set of 1950s aftermarket NOS pistons fitted to my '34 that kept seizing. I kept easing down the tight spots till the pistons rattled and it still kept seizing! A different and more reputable brand of piston was the solution.
But first I would just try increasing the oil delivery, a lot. Is the engine burning oil? If not, increase the oil rate till it does smoke like a navy destroyer hiding for cover. The number of turns on the sight glass needle valve is meaningless; as are the numbers on the dial. What counts are the number of drips per minute. The faster and harder you use the engine the more these should be increased, but since the oil pump and valve are not throttle controlled, you have to compromise. But it is better to start out will too much oil and foul a few plugs, than to have too little and, well, seize the pistons. It is hard to say how many drips per minute to start with, a new tight engine needs more than one bedded in. Again, start off with the engine smoking and cut it back in increments. If the anti-fouling pockets are still intact the engine ought to tolerate a over abundance of oil, though your neighbors and the environment might not!
-Doug
-Doug
