New on here and to old bikes (old anything). Been looking through threads to pick up on things; sorry if I end up asking dumb/novice questions – but that is because I’m a dumb novice :-) But better to ask than foul things up. I can live with the raised eyebrows if you can live with the q’s!

I already have the bike (M20) and have started stripping the engine as it was very noisy compared to others I have listened to. I had checked the tappet gaps and they were not the problem. I find the valve guides have a lot of side play in them so that has to be one source. Is it likely the valve stems will have worn too, the heads look used but ok? Is there a publication that gives tolerances for the mechanical parts e.g. limits on stem thickness and so on for the bike? I have the basic owner/strip down manual.

There seems no perceptible play at all in either the gudgeon pin or big end (tested by hand movement only), but the piston skirt does seem quite scored to my untrained eye particularly the back/rear of it – or is this to be expected?. So, I am ruling out piston slap as a mechanical noise source. But there may be evidence of oil coming up the bore or valves into the combustion area, either linked to the scored piston or worn guides or both. The only mark I could see on the piston top seems to be 83.

Given no big end movement what is the feeling on whether I should strip it down totally to check the main bearings etc?

Thanks for any advice.

Hi Rog..and welcome to the forum (and the strange world of M20 ownership)...

Firstly, mail me off forum for some dimensional information...

Regarding your engine if there is visible wear on the valve stems they will need replacement..This can be confirmed by measurement if needed...
Good quality replacements are available from G&S Valves...

The guides will have to be replaced..If they are slack on the stems and you don't know how long they have been in there, it's the sensible thing to do...

You can get replacements from C&D Autos...

If the piston is badly scuffed it should be replaced...The clearance between the thrust faces of the piston (the front and back of the piston below the rings) and the bore is the key dimension...
The barrel itself will probably not be worn evenly. You should take it to a reboring shop for them to measure it accurately at various points to determine exactly what you have before deciding whether a rebore is needed (and the next size piston up)...What bore size is it on currently?...You should find the size marked on top of the piston if you clean it up carefully...

An original BSA piston will probably have a part number which indicates the size...An aftermarket piston will be marked STD. for a standard bore and, for example, +.020" or +1/2mm for a first rebore size...

If the big end feels good, the crank turns smoothly without noise and the engine has been running recently you don't HAVE to strip it down...If it has been laid up for a long time it is wiser to check it out more thoroughly..

One thing you should do in either case is to remove the sump plate and drop the oil pump out of it...This may well require some attention if it is too tight...It should turn freely by hand when you turn the driving tang that projects from the pump body...Ian

email (option): ian@wright52.plus.com

Hi Ian

Thanks for the very helpful reply. I will be in touch via email.

Cheers

Rog(er) Strachan

Next question. I stripped the engine but did not spot that some of the bearings are 'one sided', don't know the right term, the inner can only come out one way. I am guessing this is to take thrust outwards from the centre and the 'closed' side should be towards the outside of the engine on both sides? I've searched the site and cant see anything in my manuals etc to find this, could someone confirm one way or the other? Thanks. Rog.

That's right, fit the roller bearings with the 'lip' on the outside, or furthest from the crank...

Fit the outers and when you fit the inners make sure the number codes on the bearing inner are facing the same way as the number codes on the outer...Ian

email (option): ian@wright52.plus.com

OK gents, learning a lot as I go along but next question.

Having read on here about suppliers and Russells having genuine NOS BSA stuff I bought some bits from there. None of it came in wrappings that suggested old BSA or WD, I am off target here or is it likely these are modern repro?

I got a new piston from them thinking it was a GPM, it came as a Gandini so I'm guessing that is the 'G'. It has an extra ring below the pin, what I've read on here suggests leaving it off - is that correct? It also came with standard circlips, but another thread I've read suggests I should only use wire circlips. Which would be best, wire or OE?

Use the clips that came with the piston...Clips shouldn't be changed from the type the piston was supplied with..

Gandini is the name for GPM as you correctly surmise...

I would run with the extra ring..Some M21 pistons had an extra ring and I ran one for years without any problems...

Russells supply mainly original parts which don't always come in the original packaging..but they do have some replica stuff...What was it you bought?..Ian

email (option): ian@wright52.plus.com

Ian, I got an oil pump spindle, a clutch chain wheel and centre and new valves, as G&S were out of stock. I had enquired about NOS BSA bits and the quote came back with only the piston identified as a repro item. But all the bits look new manufactured, even to my untrained eye. Which is ok if you know that is what you are getting - but I'd asked about NOS. So I'm thinking Russells might be off my list of suppliers! But maybe I was just unrealistic?

Hi Rog

Les at Russell Motors is a God in my opinion.

He is in his 70's now and a really good source for parts (NOS when he has the part, but good quality stuff if not).

I phone him in the morning and I usually get the part the next day.

Best of luck getting the bike back together.

Regards

Pat

email (option): sacombsashtrees@hotmail.com