Most WD parts lists and workshop manuals for 4-strokes state that the maximum oversize for rebores and available parts is +040...........so you can go +20 and then +40 before a new barrel is needed (so they say)...........

The two stroke Flea and ML however quote +015 and +030 as the maximum.........why the difference with these ?

Hepolite, respected and well established, listed oversize pistons up to +080 in ceratin cases...........well above the WD standard of +040...........

I'm no engineer and know we are talking fractions here, but some expert opinions would be good to address this anomaly and to answer the question as the why +040 as the WD maximum when you could clearly go larger, so to speak according to Mr Hepolite.......

It could be the manufacturers considered that two rebores covered the (intended) working life of the machine and they preferred that you go and buy another rather than carry out endless overhauls to keep it running...

It could also be that Hepolite and others simply saw the opportunity inherent in providing for that overhaul strategy by going beyond the manufacturers chosen limit....

Most cylinders have the capacity to take a larger piston...I do it habitually...

The 720cc M20 for example involves a bore diameter increase of 8.5mm over standard and even this can be accommodated by the original cylinder casting..

In many cases therefore not only can a larger than recommended 'overbore' be accommodated but a cylinder liner as well if necessary to retain original capacity and the use of 'standard' specification pistons...Ian

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

Thanks Ian.........I'm also wondering if the increased availability of a wide range of oversize pistons (eg - Model C Enfield) were to cater for those (many) users of ex-WD machines post-war in frugal times......if you bought an ex-WD mount from a dealer it may have been already running on a plus 40, and whilst not cheap a rebore and piston when necessary to the next oversize rather than a new barrel or sleeve and rebore may have been an attractive option for many to keep a bike on the road.....maybe Hepolite and others were simply catering for this austere market ...?

Good point Steve..and very possible I would have thought....Ian

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

Pre-war Norton parts lists don't seem to include oversizes, but they must have been available. Early WD lists are in .010" increments to .030". By 1940, +.040" was listed and the M&I Manual states that beyond .040", the cylinder should be lined, but that liners were not to be rebored and any subsequent replacement was to involve a new liner.

The liner OD for the 79mm Norton is 82 mm Apparently some early cylinders are quite thin close to the flange, but later ones have more meat and can even be run at 82mm bore, giving 528cc instead of 490cc. Presumably a piston this large would require a crank re-balance ?

I've got a new Norton cylinder over at the farm Rik...........perfect condition too.......no liner fitted.........I THINK (from memory) that the bare bore measures around 82mm but can't be certain without checking..........

Keeping it for a rainy day, my own 16H currently being on +20 and I've stashed away a STD and +40 piston for the future.............mind you, at the current rate of riding and progress I will be around 80-odd before I ever use it.........

Not looking forward to removing all the hard cosmolene preservative though.......

The standard 16H bore is 79mm, so if your new barrel is 82mm it's either one that's had a liner removed or a big 4 barrel

email (option): horror@blueyonder.co.uk

I think it's a 16H Horror (without liner that would take up the 83mm slack)......plus is of the pre-38 design to with exposed valves.............