Author Topic: Mains Bearing  (Read 1331 times)

Offline farmboyuk

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Mains Bearing
« on: 04.10. 2007 20:55 »
I've been informed me by a retired motorbike mechanic, that it sounds like my mains bearing is going. This is despite it starting 1st time and seemingly running ok. I've put any previous noise down to any drive chains, but he seemed to think that there was an underlying tone. I believe his advice, as it is noticeable at the top speed since as it can only manage around 60/65 mph. With winter coming, its a job I need/want to do, but was wondering if anyone could advise me as to what essential checks to do.
Also, I've talked to SRM engineering in England as they fit a replacement for the 'white bearing' but they mentioned that you could reduce noise on the engine by unscrewing a 'breather' nut? Does or has anyone heard about this?

Any advice is much appreciated


Offline a10gf

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Re: Mains Bearing
« Reply #1 on: 05.10. 2007 12:12 »
Some ideas, check the top end brass bearing and bottom end bearings by removing the cylinders and looking for any (more than normal or tolerable) up\down\sideways play at the pistonside and crankside.

About the timing side bearing \ bush,

To do a quick check remove the timing side cover and see if you can detect any play (more than next to nothing) in the crankshaft by forcefully moving the crank end up or sideways or whatever (the most important is the quality of the work done when fitting a new bush, needs reaming to exact tolerance with the correct tool and correct alignement to the left hand bearing to get it 100%).

Then the left hand bearing, same procedure, although it's better to split the engine to get to it to check it out and detect any inner race play.

Then you should check cranckshaft side play, which, if not "next to nothing", may need attention as well.

Not very scientific  ;) but a good basic way of getting some idea of the situation and find the further questions to ask to pinpoint the eventual problem. Comments and corrections welcome.


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Offline dpaddock

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Re: Mains Bearing
« Reply #2 on: 12.10. 2007 07:15 »
I can't comment re SRM's statement - it's too vague. Perhaps you should call them.

Check to see if the noise is because of excessive crank end-play. Do this by leaning the bike over both ways while the engine is idling. If the noise diminishes or goes away, it's end-play. I ran my DA10R for around 1500 miles with ~.020" play without any damage but the preferred clearance is .003", and this is a case split to install shims on the drive side mainshaft.
'57 Spitfire