Oversize pistons for an 88 Domi

I make that +60 thou.  67.5 - 66 = 1.5;  1.5/25.4 = 0.059".

The bore size should be the exact overbore.  I see RGM have +.060 pistons, so they should fit because they should bore to +.060" and the clearance is provided by the piston manufacturer.

What happened to your pistons if the bore is good?

If the bore does not work with + 60 pistons  i have heard that 99 or 650  pistons can be fitted  after a bore.

I hope that works out.  Do you have a proper bore gauge?  If the +60 is the result of a rebore it ought to be OK, but if it is the result of wear then all bets are off!

You can fit 650 std pistons to an 88 (68mm) BUT be careful if you try fitting 99 pistons as the skirts are too long and will hit the flywheel. Having said that, I think pistons supplied for 99s these days have the same skirt length as 650 pistons but still worth a check particulalry if you have found some NOS 99 pistons. 

Have you measured the bore, or some old pistons? Pistons are both tapered and oval.

Hi Gordon,  I have not tried that combination  myself,and  while I  would think you would need to check the  slot  in the bottom of the barrels,  I was thinking that the reduced throw  of the  88 crank in comparison to the the 99   would give the clearance  for the skirts , perhaps someone with  bits to hand  might like to  solve the puzzle. I bet Phil Knows .

The biggest problem that I have come across is the weight of new, replacement pistons. Any of the recommended versions are generally 50g heavier each than original Hepolites/Heplex/AE etc. This in turn may result in the need for a crankshaft rebalance or a teeth review at the next service.

You would  have to bore out to 68 mm  to use  std  600/650 pistons, but at least you would be able to use  decent old  SH  or old stock  pistons .  30 years ago  I bought some  dusty  unboxed bare +10  650 ss pistons  From Russels , rings and clips from Bratby  and used  the light 99 gudgion  pins . ( not good practise !) .Knife edged  and lightened the skirts ,and its gone like a bomb ever since.  Perhaps its a delayed action bomb !!. If the bore is good you may be able to use  lighter old stock + 60   88 pistons ,avoiding slotted skirt type  unless you are  an easy rider.

...because allegedly a worn 88 can be bored out to fit 99 pistons. Does anyone know the size of the exterior of an 88 cylinder compared with a 99? Surely a 68mm bore is as far as an 88 can go? 

And will there ever be new standard 88SS barrels?

I always assumed that the reason the 88 could be bored out to use a 99 piston is that they started off with the same  amount of "meat" in them.

'Twas many years ago but I had an 88 at + .060 and needing a rebore so rather than sleeve it had it bored to 68mm and fitted 650 std pistons. The wall thickness was ok. In fact, I suspect the 88 and 99 castings start off the same outer wall dimensions but the 99 just gets bored out more.

It worked a treat. 

The alternator 88 & 99 barrels were both machined from 22709 castings.  As were the first 650 barrels. But the latter had a modified front fin arrangement.

why an engine blow-up on a 99 is catastrophic as the thin barrel walls shatter when hit by a flailing con rod whereas the thicker walls on an 88 often survive.