I recently bought a set of Taiwanese pistons from British Only Austria for a customers A10.
There has been a bit of discussion about these pistons so I thought this time I would inspect them a bit closer and see how they compare with the genuine piston. This is the second set of these I have purchased, the first set are in my own plunger A10 but I've only done about 2,000 miles on that particular bike. So far I cant fault them.
Start with weight, the only genuine piston I have to compare is +.020" whereas the ones I have just bought are STD. The genuine one weighs 233 grams and the Taiwan one 222 grams so based on that I would say the weight is almost identical to a genuine BSA piston.
Next is grind, or "ovality". The Taiwan piston has .006" which is sufficient and identical to the BSA one.
The gudgeon pins are a nice push fit.
The rings are no good, once again they have too much end gap. I had the same problem with the first set I bought and once again will use Hepolite rings. I will e-mail BOA and let them know about this. The Taiwan rings supplied with the pistons have .027" end gaps, they should be around .012".
So other than the rings they seem quite good. They were ?130 plus the cost of a set of Hepolite rings which in my case I happen to have. They were the only ones I could find in flat top, 7.25-1 compression ratio.
Finding quality pistons is always a problem, JP's are expensive and depending on what you read can suffer from quality issues. GPM's as far as I know dont come in a flat top 7.25-1. With the exception of the rings these Taiwan ones seem ok.
I always put pistons in with clearance in the upper end of the specified range. The book recommends .003" to .005", I would never go under .004". This particular motor will have .0045" as the bores are good. The owner popped a hole in the top of the right side due to too much advance, too lean and rode it with it pinging like crazy.