KH500 Crankshaft Location

[Les Carter]

Just got the 1951 KH500 running and done about 100 miles and all seemed good after a complete engine overall. Until yesterday whilst out on the longest run yet about 20 miles non stop when it felt as though the engine was tightening up. I pulled the clutch and stopped. I checked the plugs, not running lean and kicked it over and it ran fine again doing another 10 miles no probs. All machining was done by T & L so I happy this is all good. Could it be the timing side bush expanding and nipping the crankshaft I wonder. This I set up on my milling machine table and used a dial indicator to shim to between 1 and 2 thou. I took great care when I did it and was happy at the time. I know that phosphor bronze expands a bit more than steel might this be the cause? Will it wear in with careful running in ? Any thoughts would be appreciated.

[Paul Slootheer]

Hello Les, I’am not shure about the expanding factor of phosphor bronze, but I wonder why you made use of phosphor bronze for a main bush? I’am sure there are many opinions about the use of this material for a main bush, but originaly main bushes where from soft white metal (Babbitt) with a steel or sometimes bronze cage/backing.

[Les Carter]

The bush is a standard white metal from Drags which was lined bored by T & L. I think body of the bush is phoshor bronze.

[Paul Slootheer]

Allright! Sorry I misunderstood, I thought the bush as a whole was made of bronze… Bronze on it’s own does expand more or less on a comparable way as alluminium, so it should hold very tight in the cases. your suspicion of nipping up will depend on the cleareance the bush is reamed to I guess. Unfortunalely the KH bush originaly is not secured with a peg. I experienced a simmilar problem on my KH but in the end is was caused by fuel starvation due to a over heating carburettor. When I fitted a thick insulator between the head and the carb the problem was gone.

[Roger Gwynn]

Phosphor bronze or similar is fine as a material for a timing side bush whether lined with white metal or not. The white metal increases the life of the bush when the engine is run with dirty oil or with dirt in the oil. If the oil is kept clean using modern cartridge filters etc the white metal is superfluous. Draganfly only continue with the white metal to avoid adverse customer reaction.

[Les Carter]

Got it wrong in my first post I set (or thought I did) the crankshaft clearance to between 2 & 4 thou. I have done some calculations and taking account of the difference in expansion rates between phosphor bronze and steel it seems (according to my reckoning which must not be taken as gospel) that with a 100 deg C increase the bush expands 1 thou more than the crankshaft location points. This probably explains why the factory specify the 2 to 4 thou clearance. I am stripping it down to check.

[Les Carter]

It was a bit tight so added 2 thou more shimming and was able to get a 2 thou feeler in between bush and large washer, I am happy its now with the factory recommendations.

[nevhunter]

A bit of end float doesn't lower the oil pressure. The FIT of the bearing controls that. Babbit lined bearings can run a tighter fit ( less running clearance) than bronze or Alutin does . That is because it has a lower co-efficient of friction and doesn't need as much oil flow to cool it. The thickness of the Babbit should be kept to a minimum to prevent it fatiguing and bits breaking out of it.. It can run on a softer shaft than bronze or alutin can.Being a soft metal they absorb dirt , but all plain bearings have a longer life with good filtration and there's the sludge in the oilways to be concerned about also. Nev