I have sent off two barrels for re-boring. I wasn't sure how badly damaged they were so I sent them to the shop to have them checked out. When they got back to me they said the barrels were both at about 82.6mm and that they thought they needed to bore them to around 83 or 83.5 mm. I need to get a piston to them to measure for the bore size.

Some questions:

I believe I read in another post that the clearance between piston and cylinder should be .0035-.0050" is this correct?

Am I correct in thinking I should get a piston that is +.060 (because I think that .040 might be cutting it too close on the damage...?)?

If that is correct, can anyone tell me if they have any experience with italian made pistons like these: http://www.ebay.com/itm/66-1190-60-BSA-M20-PISTON-COMPLETE-60-OVERSIZE-83-5mm-GANDINI-ITALIAN-MADE-/271112976908?pt=UK_Motorcycle_Parts&hash=item3f1f97ba0c

Or, alternately does anyone have a +.060 piston for sale (plus rings and/or gudgeon if possible)?

Also, a couple fins broke in shipping, i seem to remember someone saying they could be braised or aluminum welded or something?

Thanks so much, as always!
Nick

email (option): nicktog@mail.com

Hi Nick, the Italian pistons should really be avoided as they do cause problems. The fins can be brazed back on, or if they are missing you can build them up very slowly using a mig welder but they have to be just a very short run and then leave it until it is absolutely cold, then do another run and leave it again until it is cold, if you try to rush it, it will crack as it cools. Make sure you clean the area throughly with a sanding disc or something before you start.

email (option): davmax@ntlworld.com

Hi Nick..It's impossible to assess the size piston you need without seeing the damage in the barrel...The best solution is an 'original' piston which at +.060" means Hepolite/Heplex etc. (BSA pistons only went to +.040" o.size)
The main problem with the Italian pistons is the weight..They are substantially heavier than standard for some reason, to the extent that really the crank will need rebalancing if they are used. Piston clearance is correct at the figures you quoted...
A customer of mine recently fitted one to a B33 and he successfully lightened the piston in the area under the head and around the gudgeon pin bosses.
After fitting this modified piston the bike is running well...A lot of hassle though and the M20 piston may not be the same from that standpoint.
Australian JS pistons suffer from the same problem I hear, though I have no experience of them myself.
Either way it is worth enquiring about the weight and comparing that to your old piston. The weight difference should not exceed 1 1/2 oz.(42.5 grm)...
Russell Motors may have an original type piston and Vale Onslow and Bantam John are worth a try as well...Ian

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

Hi,
if the man from the workshop says the bore is 82,6 doesn't that mean a slightly worn first oversize (+0.20 = 82,5 mm) ?

hi nick,russell motors are listing pistons complete with rings, pin and circlips
at £60.00 + the vat (vear and tear).
the clearance should be .004" to.006" so it sounds as already stated that you are on a first bore at +020" if there is no scoring a quick hone may be all that is required.to go straight to +060" sounds a bit drastic as that is your limit
then its a re-sleeve and a rebore and a new piston.
why are you having two barrels done? then you need two pistons etc or is one for a spare in a lot of years time?
i would speak to your reborer and see what he thinks,and certainly only go to plus .040 on this occasion if it will clean at that.
that leaves you another rebore another day.there wont be any more barrels
cheers rick

email (option): richardholt@rocketmail.com

Gents, as always, thank you so much for your responses, like others, I couldn't own/operate my M20 without your help.

Dave: Thanks for the info on the fins, I will give that try. It's too bad about the italian pistons, they look nice. The problems you describe... I assume vibration mostly, any other problems? (Just curious, I'm not considering one of these).

Ian: Again, just curious in case I ever have to go this route, but how did that customer lighten the piston? Unless there is some elegant way of doing it, I assume he used a drill press and took a little off each half trying to keep it balanced? I have written to Vale Onslow and am waiting for a response, do you have a contact for Bantam John?

Michael and Rick: yes one cylinder is scored, and that's where the confusion comes in, because the shop owner (very patient to deal with me--this is his first sidevalve) thinks a .040 over *might* do it, but I was thinking I would just go to 060 to be safe. And you both make a good point, why take that life off one of the cylinders by over-boring it? You changed my mind. I had thought to myself that I would make myself a spare and re-use the piston if it ever came to that... but with better .040's out there still, it's better to buy another piston than follow my original plan. I'm doing both at the same time just to have a spare ready, and not have to do it again later...

So my plan, though it's turned a little bigger than I'd hoped, is I'll order one .040, get it to the shop, and try to bore the better cylinder to that... If it goes, great, and we'll try to do the other the same size... If it doesn't then we'll try that piston in rougher one, and i'll start the hunt for a hepolite 060 for the other... if not i will be really desperate for a hepolite 060... if anyone sees one around, please let me know.

Thanks again and if you have any other good advice for me, let me know!
NT

email (option): Nicktog@gmail.com

Hi Dave,
your advise about repairing fins; is that for aluminium of steel heads, or both ?

The advice is for cast iron, for ali I use tig, you can also do cast iron with a tig using a dissimilar metal such as nickel or stainlesss steel, but most people only have access to arc or mig. You can build up the broken fin with mig or arc using a mild steel rod/wire, but you literally have to do one "blob" of weld at a time and let it thoroughly cool before continuing, if you rush it it tends to break open around the edge of the weld, especially if welding a concave weld. Altenately you can braze a broken fin back on.

email (option): davmax@ntlworld.com

Hi I posted on the parts forum as well, sorry about the double post, but getting a little desperate here.

Both of my ebay cylinders were trash... they would need to be bored to 080 over.

Looking for a cylinder if anyone could help out... I can't bear the thought of no riding this summer.

I have the stock piston that's currently in, and one that is 040 over. Looking to get a usable cylinder... even if it needs some work. I live in the US, I'm not looking to get gouged, but I would be willing to pay a decent amount to get that to me if anyone can help.

Thanks for looking.
NT

email (option): nicktogATgmailDOTcom

Just get one sleeved to suit your existing piston? I've successfully welded ordinary mild steel plate for fins by brazing and shaping plenty of times. I've heard of L.A sleeving in your part of the woods.(Is it sleeve or sleave)? Mind blank!

Thanks Douglas,
I should have mentioned that I'm trying to work that angle too, I just can't remember if mine are the sleeve type or not...
I see that burtonbikebits had M20 sleeves. Does anyone know of another supplier?
Thanks!
nt

email (option): nicktogATgmailDOTcom

email (option): teladelujo@ msn.com

HI John, Thanks.

It does look like they have them in stock (P/N 66-88 right?). But then when I try to add it to cart, it says out of stock.

I don't see them available at De-Groot, fleabay, draganfly or burtonsbikebits... can anyone help?

Thanks again,
nt

email (option): nicktogATgmailDOTcom

Try googling FW Thornton in Shrewsbury, they may have them.

email (option): davmax@ntlworld.com

Russell Motors had them as well fairly recently, so they are worth a call...Ian

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

Thanks, guys!
I'm on it.
nt

email (option): nicktogATgmailDOTcom

email (option): teladelujo@ msn.com