Motorcycle Repair: 1997 honda cr 125 engine crank ceased, honda cr 125, coolant leak

Question
recently i got a 1997 Honda cr 125r two-stroke liquid cooled dirt bike. So i was riding it and it stalled and it wouldnt start. two hours later it was a ceased up. i pulled the top end off and the piston and sleeve was fine. the crank wouldnt budge. the shop said i ran outta crank oil. i pulled the drain plug off and the oil was milky so engine coolant has leaked into the crank. I have no idea what to do? how would that leak into the crank? Would the engine be ok if i changed all internal bearings? or would collant still leak in? any help will be greatly appreciated thank you.

Answer
Elliot, 2 stroke crankshafts need oil for the main and rod bearings. The coolant flows through the cases and up the cylinder and cylinder head and is pumped around by the water pump.

The main bearings are on  both the flywheel side, where a seal leak can cause an air leak, leaning out the mixture being pumped into the crankcase and transferred to the cylinder through the ports, as well as on the transmission side, where a seal leak can draw trasmission oil into the engine causing plug fouling. Coolant can get into the cylinder itself, at the headgasket or can get pumped into the transmission past the waterpump seal, I imagine. Sounds like your transmission oil was contaminated with coolant from the water pump, unless there is a crack in the cases or some other kind of flaw.

If you change out all the trans and crank main bearings (basically a crankshaft overhaul), the transmission and clutch should be okay with a thorough cleaning in solvent and reoiling all the parts again. You must determine the actual cause of the coolant leak into the crankcase, however. Careful disassembly should reveal the failed part/seal that caused this contamination problem.

Go to www.bikebandit.com for images of your engine components in exploded views.

Bill Silver