Re bore spec A10 650

[Guy Wilson]

I'm finally getting around to fixing my 'small engine rattle' which amounts to a rebore, replacement crank, pistons  etc...

I'm having a spare set of barrels sleeved and I was going to bore the sleeves to fit pistons that I have which appear to be +40 thou
The diameter of the +40 pistons are 71.2 mm. (my micrometer is manual) (i think it should be 71.16 mm on paper)
The original barrels were standard bore at 70mm, the piston diameter appears to 69.4mm.  so a difference of .6mm (again it could be .5mm)
Would it be fare to assume the bore size is the piston size + .5mm/.6mm (or +20 thousandth of an inch) larger than the piston?

Guy

[KiwiGF]

Sorry for asking the obvious, but are you sure you can bore the sleeves to +040" ? (+1.0mm)

That would be pretty marginal on the sleeves I have on my bike, the sleeves would end up pretty thin. With sleeves of 1.6mm wall, they would be only 1.1mm thick. I have heard (but have no personal experience) that down to a 1mm wall thickness is ok but going that thin does carry some risk.

I know you would have considered it but fitting standard bore Pistons would give you at least one rebore, and whilst you may be thinking you don't need a rebore option bad things like a cracked ring can happen  before a rebore is needed due to wear

In answer to your question, around 004" (0.1mm) clearance is commonly used for an A10 but I would read other threads on this and also follow the guidance of the piston maker and machinist. I used a bigger clearance, others have used less. Pistons are not round so measuring the clearance needs to done in a particular way.

Pistons also usually have a smaller diameter above the top ring.

[Guy Wilson]

Thanks Kiwi, I had the same thought about the wall thickness and its something  i was to  check at the machine shop before we start the work.  The original barrels were standard and although the skirts were damaged a bit, I did consider sleeving the original barrel, but opted to use the spare barrels as they were undamaged . I got confused looking at replacement Hepolite pistons that were quoting the bore size + .02 '' etc against me measuring the piston diameter.  I've misplaced my manual and didn't have the specs to hand.
the sensible thing is to fit standard pistons and cut the sleeves to fit.. I may go that way still if I have someone coming out for the UK soon. The original pistons had flat / slightly con-caved  crowns and the engine was very smooth with almost no vibration. The spares I have are domed and may be a completely ride.. I'm riding a triumph around at the moment. it desperately needs a rebuild and the crank balancing...if only have more time...
Guy

[KiwiGF]

Hi Guy, the concave Pistons would probably be the originally fitted 6.5:1 compression ratio Pistons but I've not seen any new ones of those for sale (not that I've really looked), so called flat top Pistons are around 7:1 and seem accepted to be good compromise between power and torque, domed Pistons would probably be 8:1 or more and would change the engine characteristics quite a lot compared to concave.

I'm not sure what the freight and import duty would be in Kenya but In the past I've shipped new Pistons from uk to New Zealand using Draganfly and also British only Austria and it was not too expensive as I guess they do not weigh that much. Oddly I bought Australian made Pistons from Draganfly in the uk, as it was cheaper than than buying the same Pistons from our cousins in Aussie!

If the skirts are damaged I'd would aim to keep max sleeve thickness and strength? Depending on the damage (pics would help here, we love pics  ).

I've got flat tops in my GF and a 356 (super rocket or late golden flash) cam and the engine is still docile and torquey at low revs, but then again I kicked over an A10 recently that had never had the engine rebuilt and it was really easy to kick over compared to mine  maybe it was low on compression due to wear though.........

[Guy Wilson]

its was totally original and really smooth and gentle ride before it stopped.  I'd prefer to get it some way back to how it was, especially as the triumph is everything the BSA isn't.  The standard flat top piston would be the way to go. I'll see if can find some in the UK. Dragonfly are good and I've bought there before. Feked are also reliable.
Barrels below. I may have already posted these...
Thanks!
Guy