Forum

[garyl]

Hello, my 1940 WD/C has developed an engine knock and I would be grateful for any suggestions as to what it might be. Thinking it was a bearing problem I’ve had the main bearings, big end and small end replaced and have recently got the bike running again but the knocking is still there. The bike starts and runs ok but after a few minutes running normally the knocking suddenly starts – even at tick over. Has anybody got any ideas what it might be or can suggest anywhere in West Berks that I might be able to take it for someone to have a listen?

[Gwilly]

Hi Garyl, This sort of thing is part and parcel to the mechanical/engineering trade where all moving parts have a potential to malfunction, either due to poor design, wear or metallurgic failure.. Diagnostics being the first step to solution and repair, as a doctor uses a stethoscope to listen to a patients chest, then so can a mechanic listen to the various areas of the engine in operation to track down the likely source of the noise..
Try using a metal rod or long screwdriver pressed against an ear and work from the top end to the sump. left side and right till you find the most likely source. could be worn rocker knock or piston slap, crank bearing, primary side chain or clutch drum, all have the potential.. You really need to define the area… Sorry can't help with West Berks but one of the lads from those parts will know i'm sure.. cheers gwilly.

[Norm]

Piston slap?

[Alan R]

Hi there, GARYL------------- while we wait for some more answers I thought this might cheer you up ??------------------- http://www.youtube.com/watch?v=Z5z5HXX2HS4 ---------------

[garyl]

Thanks chaps.
I had wondered about piston slap but I thought that is worse when the engine is cold and improves as the engine warms up. Am I wrong? Mine is fine until it's warm and then starts knocking. I've tried using the old screwdriver against the ear trick but still can't pinpoint the problem. I'll have to invest in a stethoscope.
Alan R - very good! I remember it well from when I was a sweet sixteen year old.
Anybody else got any ideas?

[simon]

If its a top end sort of knock then I would suggest checking the rockers as I had issues with mine getting sticky when hot. Is the knocking regular, at engine speed, low or high pitched? Also is it provoked by the throttle or more pronounced on the overrun?
S.

[Norm]

Garyl, did you check bore to piston clearance, if not you may have to do it, running out of ideas if the bottom end has just been rebuilt

[garyl]

Thanks again chaps. Simon, it's a side valve so no rockers to worry about. I can't hear it knocking while I'm on the road but when it's on the stand it doesn't seem to get louder when I accelerate the engine or on deceleration - just faster or slower. Norm, foolishly, I didn't check the piston clearance when I put it back together after the bottom end rebuild. I've ordered a stethoscope now and that should be with me in a couple of days - if I can't pinpoint the knock elsewhere with that I'll have the barrel off and check the piston clearance.

[simon]

Ah of course a side valve . One thought, does it have a split skirt piston and if so has it been assembled with the split to the front?

[Beezabryan]

Start with the cheap & simple things like check all engine, gearbox and exhaust fixings are tight and that none of them are missing.