[iansoady]

Another vote for keeping what you've got if possible before I shoot off. With a bike that's unlikely to see a lot of use (unless you plan on covering thousands of miles) you'll find that you're running it in for years. Mild  scoring should hone out.

[VicYouel]

Either the Hepolite part 3897 or 3238 look as though they are the right ones. both with 7:1 compression ration. Diameters are roughly70 mm. Slightly spherical to fit a 70mm bore actually. Oher dimensions are:

3238 Compression height 1 5/8  length 2 11/16 gudgeon pin 3/4 dia, 2 3/8 length,  two 2mm compression rings and a 2 mm oil ring

3897 is the same except it has 4  2mm rings.

Dimensions in inches and I think the compression height is the distance between the centre of the pin to the top of the piston dome.

You will probably fine the number embossed on the inside of the piston but definitely on the yellow box.

Good hunting

PS May be worth contacting Bantam John who has a large stock and John Harding who both advertise in OBM. In any case get one in stock if you intend to use the bike. May cost £100 but more like 40 or 50 at Shepton.

PPS G&S valve guide part number is 3012G used all the  waybetween 1925 and 1936. If you find an original it may be worth club laying in a stock.

[wessex_man]

I agree with Vic If it's not that bad leave it alone. Otherwise a gentle hone will probably sort you out. There are a few which will offer a re-bore service but you will have to supply the piston. Like for like would be best even if it takes a lot of autojumbles to find one. However it is not often possible (i.e. not be Sunbeam originals). As long the gudgen pin height is the the same and you end up with more or less the right compression. You may have to opt for that option.

Re valve guides you really need a friend with a lathe.  Norton 16H guides can be modified to suit use the old one as a reference. Alternatively one can be made from cast iron. If the guide is shot then the you probably need a new valve(s)? Again you can modify a more readily available one. i.e. you can shorten valve lengths re-turn collet grooves and turn down oversize valve heads and re-seat them. The more you can do yourself the less costly it is.

[Andy Cubin]

Thanks Vic,

its a 1933 model 8 and the liner and piston are a desirable rather than a necessity.

The exhaust valve guide however, is a goner.

Andy

[VicYouel]

MMMMM!  how gentle is the scoring..... it may hone out and may be no more than few thou oversize. That would be a good thing as I have always found a couple of thou oversize prevents seizure with modern fuels....... don't ask me how i know that.  The rings may be ok or you could buy oversize but gap them a few thou widerwider than recommended. If it did not smoke then I would wait until you find the right piston and ride it.

With these higher than recommended clearances you should expect another 30,000 miles with modern (not synthetic) oils.

Meanwhile I would continue searching  for an original piston; there is a supplier from the Swindon area who attends the spring and autumn aoutjumbles run by the VMCC in the large hall who always has a stock of original  Sunbeam hepolites and rings (can also supply rings to order) .......DJ Turner of 29 Highworth Road, Stratton St Margaret SN3 4QL (mislaid his tel number).  This is not to be confused w ith the rather posher and more xpensive show run by Morton's  next month at the same location.

What year is your model 8? I can send you piston details and part number from my old Hepolite catalogue. There are 3 different 70mm  pistons for 1930 to 1933 with different compression ratios.

I thoroughly recommend http://www.hthoward.co.uk/  for the bore/hone etc and they can no doubt get hold of a sleeve although there is a specialist listed in obm.

If it was a 9 or 9A I have collected a lot of spare pistons and barrels over the years..... just in case. So they will be out there because I am not the only chap with vintage bike OCD!

Vic

[Andy Cubin]

Upon stripping the engine I have found that the cylinder has been re-bored at least twice, probably 3 times and there is gentle scoring on the cylinder walls.

The piston has been replaced to match (+0.060") so, whilst she may run for a while, now that I am in this state of undress, I am considering a cylinder lining and new piston.

Anyone recommend a good supplier who can do this work for me?

Cheers

Andy