My bike engine number is 3db-001690 yamaha 125 lc offroad.

It is a simple job to rebuild a dt125r as it is 8 or maybe 9 bolts holding the head on, drain the coolant into a container so you con use again, undo the 5 bolts holding the cylinder head on(after taking the spark plug and coolant pipes off) after this undo the cylinder bolts should be 5 bolts to undo(12mm socket or spanner)t take the cylinder off the undo the gudgeon pin holding the piston on, at this point all you should see on the engine is the engine block with a con rod coming out of it. After you have the engine dismantled measure the bore diameter and then go to B&Q or any hardware store and get a circular sanding head for a drill that is maybe 5mm-10mm bigger than the bore, put the sander inside the bore on full power and hold it there for atleast 1minute and 30seconds, this should get rid of all of the score marks on the bore do the same to the piston(go round all score marks to make it smooth), get som gasket paper(or a engine set for dt 125r but gasket paper is only £1.09 from most shops) blue tak it to the top of the bore them go aroud it with a hammer very lightly so u have a gasket to fit the head on both ends, use a line of gasket glue(big line about 5mm in depth) all the way around the head and assemble the engine, this will only be temporary untill you have to do agen but it is cheaper than spending £150-£190 on new bore and piston.

Wana no about my bike but only got engine number