Piston to barrels sizes

[mm man]

Hi
I had a barrel rebored and a meteor piston supplied with it around 20years ago it has never been fitted and stored dry and is still as good as the day it was done.
I fitted it a few weeks ago never ran well when you could finally get it to start ! so I stripped it down after around 10 miles the piston looks like it's done 20,000 miles ? ( blow pass the rings )
I measured the bore with a bore gauge and it reads 57.8 all over !
The piston is stamped 57.8
The piston ring gaps are around 20" thou
The piston skirt in bore is around 6" thou
It shows signs of piston slap
On a 4th oversized what actually is 57.8 the barrel or the piston ????
Have I just got a badly made piston or has my barrel been over bored ?

Thanks for any help it's much appreciated
Ps no I don't take any measurements before running

[Fast n Furious]

Too much clearance there.
The piston / bore clearance should be nearer to 3 thou for an iron barrel and ring gap around 10thou
I don't know about Meteor pistons today but they used to be garbage years ago.

[Adam_Winstone]

Looks like it was over-bored, which is why many dealers will not work to stated measurements and require the piston when carrying out a rebore.

However, this reminds me of a rebore done for a pal before Milan 97, which immediately had slap. When this was queried with the UK Lambretta dealer that had done the rebore, they stated that they did them all like that as they couldn't rely on 'scooterboys' to run in a barrel and piston. Said pal then had a rebore done by another dealer to the next size up and the piston slap noise was no longer there, with the motor being run in before Milan and not giving further issue. It sounds as though you need to look for an oversize and another rebore.

Adam