Thread approved

The service/workshop manual is a very good document that must be read and re-read several times. At least that's what I do.
Following is from the RX100 service manual. It is generic info hence applicable to all 2-stroke bikes.

Chapter 3
Engine Overhaul
Inspection and Repair

Cylinder

Inspect:
Cylinder wall
Wear/Scratches -> Rebore or replace.

Measure:
Cylinder bore "C"
Use Cylinder Bore Gauge.
Out of specification -> Rebore.

Piston

Inspect:
Piston wall
Wear/Scratches/Damages -> Repair or replace.

Measure:
Piston outside diameter "P"
Out of specification -> Replace.

Piston clearance
Out of specification -> Rebore cylinder or replace piston.

Thus reboring is a repair process, required if the engine had a bad seizure, or engine has poor compression.
If done correctly, it should result in equal to or close to stock performance. If not done correctly, it will result in poorer than stock performance.
A proper "big bore" should result in better than stock performance, but that merits a different discussion!

Reboring an engine is a specialist job, I won't write much about that coz I'm just another ignorant 2-stroke owner.
More experienced/knowledgeable members of the forum can add to that.

the bigger the bore the bigger the petrol bill :P

I am not too much expert like our senior members,but i can say, rebore is required when some(mm) gap becomes between piston and cylinder wall,due to seizure,low engine compression,sluggish pickup,when engine is started eating too much petrol and engine oil,etc.normally a well maintained stock cylinder and piston can last upto 1L kms or slightly more but a poor maintained rashly driven bike can require rebore at 60k kms only.please right me if i am wrong.

LOL................ Tell that to the big bore guys.

@ Rohit, spot on with your views.

Would like to add one more point to this (my personal observation and opinion - i may be wrong)

Inspect the piston.
Only the top of the piston should have carbon marks (black carbon deposits or black in colour due to the combustion.) If this carbon is seen below the piston rings then there is a definite failure of the piston ring which is due to compression leakage (usually happens when the explosion goes past the piston creating carbon below the ring) thereby needing a re-bore to perfectly fit a new size piston.

Please correct me if i am wrong.. this is entirely my observation and is not coming from a professional one.