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Bottom end grumbling.

Although my m20 starts first kick, rides ok, the compression is great, and I seem to be able to do reasonable distances, there does seem to be a bit more noise coming from the crank case than other running bikes I have observed. So I think some bearing replacement is in order. I have probably asked these questions before, but I need some re-assurance that its not a hard job to replace then as I intend to take her to Normandy in June and I dont want a pile of parts still on my bench when that time comes.

Whats the best bearing set available and from where?
What other bits do I need to do a strip down and re-assemble (gaskets etc..)?
Is it worth changing out anything else when the motor is apart ?
Any specialist tools needed?
How long should it take?
Any other advice ?


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Re: Bottom end grumbling.

Richard I think 3 of the 4 main bearings are readily available, but we have our resident supplier and bearing expert Darren Wrudd on the forum to ask.

The replacement of these in the crankcase halves is quite a DIY job. But better to go through it when the time comes and you have the motor on the bench.......Stripping the big end for inspection is also quite possible if you have the correct socket/long bar/wedges/vice etc. But fitting a new one if needed is more of a professional exercise. Ron

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Re: Bottom end grumbling.

The bottom end 'rumble', if it's from the crankcase, could be either a main bearing or the big end...Both failures are relatively rare but I would say that a main bearing failure is the less likely of the two...(though obviously possible)

All the (4) main bearings are readily available...

The big end bearing is also readily available, though not a cheap item. Fitting a big end entails a total strip down of the crank shaft and can be classed as a professional job as the outer eye of the new big end has to be honed to size (using a correct hone) after fitting to the conrod. (and that requires a press)...Also reassembly and truing of the flywheel assembly is something that, at best, you will struggle with first time out. So, not something to take on if you are in a rush or are unsure of the procedure.. Any perceptible vertical play in the big end bearing will mean replacement but the only way to fully check it is to strip it out...

To remove and completely strip the engine you will need a magneto drive extractor, clutch centre extractor (to suit the clutch type fitted) and if you have a single spring clutch, a clutch spring compressor to replace it....

You'll also need piston ring compressors and a valve spring compressor if you want to check the valves/guides/seats...

A torqe wrench with a range of 0-50 ft/lbs is also worth having when replacing the cylinder head...

You'll need a full engine gasket set including a solid copper head gasket if you don't already have one fitted...

I would also take the opportunity to replace the original drive side bearing circlip with a modern one...Likewise with the magneto drive gear oil seal if you have an original fitted...

What else you may or may not need will depend on the engine components overall condition when you strip them out and how much you want to do whilst it's apart...

You'll also need the usual range of standard workshop tools...Ian

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Re: Bottom end grumbling.

If you are not familiar with rebuilding crankshafts,than you may better hand over the whole bottom end to a specialist.
If done right, it will last for a long time.
If not…

Re: Bottom end grumbling.

If replacing the main bearings, put each case half on a gas cooker (make sure wife is not home!) and heat until they fall out. Bashing the bearings out with a drift is not the way to do it. Outer drive side bearing does have a clip that needs to be removed.

If engine has been worked on by a bodger in the past, they may be already loose in the cases.

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