Piston Rings

I have a Yamaha SR125 with the head off, and a complete set of new gaskets so i figured i may as well change the piston rings.

What new rings do i need, or is it a case of take the barrel off and measure what the cylinder has worn to (as another website implied)?

Cheers.

Measure a ring square in the cylinder and the gap between the ends of the ring will indicate the wear/ Rings required.

I haven’'t found any clear markings for over sizes yet

The gap will indicate total wear, cylinder wear and ring wear.

Measure the cylinder bore for taper and oval wear. This is done by measuring at various points for wear. First measure across the bores in line with the crankshaft at the top, bottom and centre of the piston ring travel where the piston rings contact the cylinder bores, then make the same three measurements at a right angle to the first set of measurements. Compare the two sets of measurements to determine taper and oval wear of the cylinder bore, then compare the results against the service manuals tolerances to determine which course of action to take.

If the measurements are within the service tolerance do nothing and refit the original rings and pistons as they were, assuming that the pistons and rings are serviceable (get yourself a Haynes or similar for inspection/measuring procedures and tolerances).

If the measurements are outside the service tolerance there are three courses of action depending on the extent of wear, the service manual tolerances will confirm what action to take either
Re-honing the cylinder bores and fitting new rings.Re-honing the cylinder bores and fitting new oversized rings 9not always recommended).
Re-boring the cylinder bores and fitting new over sized pistons & rings
Caution - do not fit new piston rings without re- honing the cylinder bores because the piston rings will never bed in.

All of the above &

you will be very lucky to find an oversize piston for the bike

your bike might not be able to have a rebore as the engine may not have been designed with any spare metal to grind of

the expense of the above could work out more then the value of the bike

specialist costs & gaskets so on and so forth

thanks people, thought it would be mostly a case of just putting new rings on, and as the bike is a heap anyway i may just get it working instead.

Do the gaskets , throw it back together , if it goes it’s good . Sr125 is not a bike to start over complicating things on .