My cylinder got gouged by the piston. Can I bore

Whenever a motor is taken apart so that you can install piston rings, it is rather common to replace many other parts at that time.
Commonly, all of the replaceable bearings ( crank & cam) are changed.
Also commonly, the engine block is bored so that new pistons can be fitted, because of wear on the cylinder walls.
Commonly, an inside micrometer will tell the technician how much to bore the block.
The block is usually bored from .010 up to .040.
If the block is so bad that a .040 will not be big enough, the block is scrapped out.
Then, "oversized" pistons and rings are fitted to the new size hole.
You would measure the size of the bore to tell which size rings to buy.
God bless your efforts.

most piston /ring kits come in .010" , .020",.030",.040" oversize
most engine re-conditioners use cnc borers set to an over size and bore to that
after honing to get the correct cross hatch pattern in the bore , that is it
the ring gap is what is important and it should be around .003" per ever 1" of bore
so for a 3" bore the gap should be around .010"
experience has proved that if the cylinder is bored to say .020" oversize then when the gap for .020" oversize rings is checked ,it generally comes in around .015 and even as large as .020"
max specs for ring gaps is .025" which is for a worn cylinder
just honing a bore is a waste of effort as bores wear in a taper ( larger at the top than the bottom and this taper increases the wear of the new rings and can lead to broken rings
the ridge at the top of the bore has to be removed to allow the piston to come out easily and to prevent piston failure when the new rings are fitted
have a talk with engine reconditioning shops and then make an informed decision
remember that if the cylinder can be separated from the crankcase , you can buy standard or oversize piston/ring/cylinder sets as assemblies so there is no need to bore out or hone.

New valves, valve guides, valve guide seals; all fine but probably not needed. When the piston was changed, did you run a cylinder hone in the bore to check for distorted cylinder walls? Was the old piston seized up at any time? Were there scratches or grooves cut into the cylinder wall from being seized? Did the new piston get new rings? Chances are you will need to have the cylinder bored then install a new 1st oversize piston and rings. Please rate my answer.

The bore has to be measured at a number of places to see if it is out of round 0.01mm or tapered 0.05mm
You will need to take it to an engineering shop that can do the boring, they can measure the piston and bore to determine what oversize it will need.
with only light scratches probably 1 oversize 0.25mm, if it has been bored before the oversize is stamped on the top of the piston. 0.25, 0.50, 0.75
A rough measurement can be done with a feeler gauge, between the piston skirt and bore.
piston clearance for a 125 is about 0.055-0.060mm with a limit of 0.1mm

Yes it can be bored, .050 over is fairly common on this bike. Before boring the cylinders, the new pistons should always be matched to the old ones. You always find problems with aftermarket parts. Have the cylinders bored to match the new pistons. Don't bore the cylinders and expect the new pistons to be a perfect match. I hope this helps.

Blue smoke is usually a cracked piston ring or worn out oil ring.
The only time the valves should be suspect is if the valves were recently lapped or replaced.
It's possible in those cases that the valve seals might leak oil into the cylinder.
Most often though, you will be looking at the piston rings, and the cylinder itself might need to be honed or bored.
If the cylinders need to be bored because of gouging due to scraping of the cracked or shattered rings, then you will need an oversize piston and rings to fit the new bore.

May just need to adjust valves. Not sure about your oversized rings, if needed oversize should have been bored out for new pistons.

in adition to the above coment, which i must say is spot on, the posability of a used barel and piston if a new piston is not available could be worth considering, other things you may wish to check on that particular engine is camshaft seizure, in a uk bike it runs directly in the machined surface of the cylinder head, oil failure due to blocked oil filter has severe consequenses, located on the l/h side of the engine under a 22 or 24mm spaner head cap just below the gear change lever, this is a washable filter retained by a cap and spring, correct replacment is inperitive. are you located in UK? HOPE THIS HELPS YOU

Should be able to bore 50 to 60 thouthands without any trouble. Parts should be available thru Wiseco.

yes, your cylinders can be bored. The factory offers pistons in 0.25 and 0.5mm oversized.