[Rick Parkington]

Ha ha, that would be a real shame John! See what Chris says; I think one of their options is to hard chrome the mainshaft and grind it to fit a trued up flywheel bore, which makes sense. Once the mainshafts are square to the flywheels you're nearly there! Hope it works out.
Cheers R 

[singleminded]

Thanks Rick..I had phoned Alpha today and said I needed to talk to someone, Chris won't be back in till Monday. I too worried that If I just sent the crank off then a big end might just be slapped in and returned, hence the need for a recommendation..Maughns will make me an oversize pin but only if they are forced to.
another restorer I've spoken to, I must ring him again, was worried whether he could bore the flywheels accurately enough for a perfect job.He has my respect for being straight with me.
I stripped the flywheels today on the advice from Maughns to check if the holes are oval, they seem to be pretty reasonable. the keyed side is better than the drive side. I also found that the main trouble with the conrod not being central in the flywheels is the bigend outer race is loose in the conrod and the one of the thrust plates is cracked.
The big end itself is in great condition,sadly.
I'm sure i will track someone down to do the job, if I don't then I suppose it is all going back under the bench ..John

[Rick Parkington]

Hi John, 
Thought it was worth commenting, since this is a road I am travelling at present!
I currently have two cranks under repair, one is a 1929 500 the other a '35. The '29 drive side flywheel was completely free on the crankpin when I stripped the engine, the later crank had been 'got at' by someone in the past and needed the big end bores re-machining true and an oversize pin made.
In both cases, the drive side main-shaft was out of line with the flywheel. If you spin the crankshaft half between lathe centres the flywheel oscillates slightly, you can measure this on the flywheel rim with a dial gauge. On one crank, with the gauge zeroed at the big end, I found + 5 thou at the opposite point (bottom centre).
I blame this on Sunbeam's long shaft and heavy flywheels but it is not unusual on any big single; if the main bearings are worn or loose in their housings they don't hold the mainshaft securely enough to prevent the fit of the mainshaft taking a battering in the flywheel every time the piston rises and falls. This leaves the crankpin in full charge of keeping the flywheels parallel, causing it to fret until the wheel bore ceases to be a proper fit. A parabolic witness mark on a crankpin is a good indicator that this has happened.
In the 'good old days' this was generally ignored as not worth worrying about, big ends were renewed with little attention paid to the fit so long as it all bolted up tight. But if the crankpin and mainshaft are a poor fit in the flywheels the fretting can increase until either the big end comes loose (as in my '29 job) or the crank pin breaks.
I had taken my cranks to Alpha Bearings but unfortunately director Max Nightingale died very suddenly a couple of months back.
Formerly part of neighbour Chris (Autocycle) William's business, Chris has now taken over again and is working steadily through outstanding jobs - mine included. He is still taking on jobs - but selectively, owing to the fact that he sold the company in the first place because he is working toward retirement, so it is possible he has had a gutfull of wonky Marston cranks and doesn't want to do it but it's probably worth giving him a call 01384 253030.
I have heard (and experienced) good and bad with Alpha over the years. I learned much of what's written above from talking to Max Nightingale. He wouldn't confirm it but I suspect that since in the old days Alpha were effectively a pattern parts manufacturer, the same applied as today - they were producing a cheap alternative to genuine spares with all the quality issues that involves, especially in the 70s when they were making big ends for long obsolete bikes of very little value.
Max did say one of the greatest headaches was people (myself included) who wanted a fit-yourself big end, which they then fitted into worn flywheels, blaming the pin when it eventually snapped like a carrot. 
I'm sorry this is not a very good answer to your actual question, John! I thought it was worth enlarging a bit on the situation for the benefit of other readers.
I think there are several people who could make a one off pin - Conway Motors/Godden Engineering down here in Kent for example but most make crankpins for specific machines and doing a one off is less popular. I always went to Alpha because cranks were their speciality and for all his faults - mainly over-optimistic delivery/costing predictions! - Max really understood the job and had the kit to deal with the problem as a whole.
Of course it's arguable that the mileage most of these bikes do now, it isn't essential to go so deeply into it and an old-school repair would be cheaper and probably last well enough; I guess it's up to the individual.
Hope this is some help and good luck with it!
Cheers Rick         

[singleminded]

Hi everyone..
I'm considering building a M90 engine I have had since 1971 into a frame and cycle parts that I have accumulated over a number of years.
I've found on close inspection that the interference fit on the drive side flywheel/bigend pin has gone.
This may mean that the flywheels need boring and an oversize bigend making..Has anyone got a recommendation for someone to do the work in the UK..John