I read and hear bad things about these Italian pistons. 
Could someone tell me why I should not buy one 
John 
email (option): john.kater@chello.nl
Hi John...My main critcism of the Gandini (GPM) pistons is thier excessive weight in some cases...If piston weight exceeds the original by more than 1 1/2oz. the crank should really be rebalanced.
Strangely Gandini make pistons for the T140 Triumph and these don't suffer from the same problem...perhaps LF Harris, who sell them, tie Gandini more tightly to the original specification. Alternatively, Gandini may be using one, general purpose 'blank' for a number of different applications.
I have used the T140 pistons numerous times without any other problems, so I guess overall quality is OK...
Gandini are a major piston producer, providing pistons as 'original equipment' for a number of car manufacturers...I am always wary of stories about 'quality' as you can never be entirely sure there aren't other factors coming into play such as incorrect fitment, non adherence to running in procedures, incorrect engine set up (timing, carburation etc.) etc. etc. and people are much quicker to blame the piston manufacturer than admit thier own shortcomings..
Suffice to say you would imagine a major european piston manufacturer has got basics such as material specification right..
It is easy enough to get top quality pistions made which match or exceed the originals..the problem is whether owners are prepared to take on board the 2013 price structure that comes with the quality....Ian
email (option): ian@wright52.plus.com
It depends whether you like more vibration and where in the rev range it occurs..For example, if altering the balance factor moved the vibration right to the top of the rev range it wouldn't be very noticeable as that part of the range is rarely used....
I have had a number of cranks rebalanced and in my opinion the difference is quite noticeable.
If it wasn't important the manufacturers wouldn't have bothered to specify a balance factor or recommended rebalancing in the case of a discrepancy. You will find this recommendation in BSA literature.
If there is a 50g difference between a Heplex and a BSA piston then it also appears potential problems with aftermarket parts are not just a modern phenomenon..Ian
email (option): ian@wright52.plus.com
What is the original weight
It would be interesting to know, because I bought my BSA in 1968 and the piston is at least from 1962 and also .040" . John
email (option): john.kater@chello.nl
The weight of my Hepolite piston is 424 grams. 
John
email (option): john.kater@chello.nl
Interesting...So the Heplex piston is well within the 1 1/2oz (42grms. approx.) limit recommended whereas the Hepolite is 4grms outside it...Ian
email (option): Ian Wright
Ian,
What is the recommended weight of a piston for the M20 By the way, the 424 grams was the bare weight
John
email (option): john.kater@chello.nl
Hi John...There isn't a recommended weight as such. BSA would have produced pistons to their own manufacturing (weight) tolerance and therefore didn't need to quote weights, on the assumption one of these was being fitted. In the case of after market pistons, a comparison between the aftermarket one and the BSA piston it was replacing would have determined whether limits were surpassed...and of course BSA didn't recommend using anyone elses pistons!
With BSA out of the picture pistons now come from various sources and I would regard it sensible to use the BSA piston weight (as quoted by Kalle) as a datum.
The reason for that is that the weight of the complete piston assembly is taken into account, along with other factors, in calculating the out of balance masses prior to rebalancing..The crankshaft flywheels are 'balanced' so that the bobwieghts cancel out or oppose a percentage of the out of balance mass (the balance factor). This figure varies from engine to engine but in the case of most pre unit BSA singles is at or close to 58%..
It can be seen therefore, that if additional mass is added (or taken away) from the piston beyond the specified limits this will alter the balance factor percentage, put the assembly more out of balance and thus affect levels of vibration...
.Ian
email (option): ian@wright52.plus.com
Kalle, just found out, my Hepolite +40 piston with rings is 462 grams, close to standard
Gandini is about 40 grams heavier. So I will keep on looking for a +60 Hepolite. John
email (option): john.kater@chello.nl