SEEMINGLY UNDERSIZED CG125BRE BARREL / PISTON !?!

[piposan]

Hi folks, newbie to this forum so please go easy on me !,
Fairly savvy with bikes as I've been working on them for over 25 years !

Hoping someone can shed some light on this situation though as I'm struggling to find any info online.

I'm trying to find a new piston for my girlfriends CG125BRE 1997.
It had been bowing oil and after stripping down I've discovered that the barrel was scored, the piston is still intact yet has some scuffing on the base on one side, rings intact, slightly oval piston maybe due to heat ?

Thing is even after honing out the barrel to remove the scuff marks it seems the barrel still only measures 56.53 which begs the question what size was it prior to any of my honing ? theoretically it was most likely 56.50mm
So now finding a replacement piston is proving difficult, as far as I've discovered all standard CG125 pistons start from 56.50, this only leaves 0.03mm piston clearance which to me seems way to tight, can anyone clarify the max - min clearance ?

As a means to try and find out why the piston and barrel were damaged in the first place my thinking is that the previous owner had used a undersized piston, 56.20-56.25 on the skirt 55.82 topside, which would leave a decent 0.3mm clearance.
Yet the dimension I'm getting of the piston don't look to match any CG pistons I can find anywhere online as they all look to start from 56.50 (which is my barrel dimension!) so from which bike this piston was designed for I have no idea !?

So, could anyone advise ?

Thanks.

[gavcarter]

Pistons usually come in over sizes of 0.25mm 0.5mm and 1mm
You will struggle to find a piston that is (foe example) 0.13mm bigger.

Usually, you determine the bore size, compared to what its supposed to be, and work out which size would be suitable i.e if its worn by 0.15mm a 0.25mm overbore would work, if it worn by 0.3mm then a 0.5mm overbore must be done.

It is usually done by accurately machining the bore smooth, then precision honing to final fit.

[orac]

chinese replacemtn job done.

do ring gaps match that specified in the manual.

also, oval piston is unlikely, oval bore is more likely. which face was the scuffing, thrust face?

i quick look around give some idea of what you should be looking for

[piposan]

I think the point of my post has been overlooked slightly.

I am fully aware that oversized pistons are available at generally 0.25mm increments.

My query is that the piston I have seems "undersized" ! 56.25 - 56.15, It is without certain that it's also very very slightly oval on the skirt by 0.10mm.
MY GUESS IS THAT THE PREVIOUS PERSON USED A CHEAP REPLACEMENT PISTON !

Thanks for the reference by the way as this clarifies my question about the clearance value.
Although in the quote is states that the piston should be 56.45 - 56.48 yet all the pistons I can find online for the CG125 range from 56.50 upward, would this be a nominal value and they'd actually be more likely be within the 56.45 - 56.48 range ?

In terms of how the piston managed to become damaged on the 'thrusting' face in the first place, it's clear that the old piston must have been outside of the suggested clearance limit of 0.25mm and so was maybe slapping in the bore which damaged the skirt. On close inspection no other fault can be found, checked seal and cleaned valves.

Either way thanks for clarifying things, looks as if the 0.03mm clearance I have will do fine. :O)

[orac]

piston arent round, the exhaust side get hotter so are of a lesser diameter (giving slight oval shape). if they were perfectly circular they would seize in the bore in seconds.

this is part of the reason it not likely to be an ovalled piston, because they are oval-ish. I got some old barrells here, the piston that came out have a little wear of the thrust face but other wise perfect, the barrells on the other hand could be a mould for eggs. Heck even the ring arent that warn and still well withing tollerence.

my suggection, give the bore a good hone and stick a genuine replacment in there at stock size.

i take it the oil scraper isnt sticking either
____________________
Current rides - 2016 Triumph Street Triple Rx, 1994 Suzuki Bandit 400 VM, TGB 204 Classic 125cc
"with nothing left to lose, there is everything to gain. It's not the size of the dog in the fight, it's the size of the fight in the dog"

[331X2]

May I ask what equipment you're using to measure and has it been checked recently?