I've got a Honda R8X that heat seizes when I'm riding it when it goes over 40mph.
The lad I got it off said he had it re-bored and that it was run in.
Will I need to re-bore this again or is there some thing that not been done right?
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It depends on where the engine is seizing
Since your scooter/motorcycle is a two stroke, if you are seizing due to a lack of oil in the gas at the crankshaft you are in big trouble. More than likely this is not the case. It is known to happen on rare occasions though. The main bearings of the crank basically begin to disintegrate due to oil starvation. There are typically ball bearings.
If you are seizing at the piston and rings there is a chance the engine will continue to run for you but quite a bit of life get's taken off the piston, rings and cylinder bore when this occurs. This has a higher likelihood of happening as most seizures are this type.
Troubleshoot You can test the compression of your engine with a compression tester.
You will need to lookup or ask a shop the compression in PSI that your engine requires. I would guess anywhere from 140psi to 170psi.
If your engine is below the number in psi the manufacturer recomends then you will need to follow the advice from @Paulster2 in the other post and redo the top end of your motorcycle by procuring an overbore piston and rings and getting the cylinder rebored to correct size for the new piston and rings.
Be sure you measure the current piston to ensure you get the next size up. Also validate you do not have gouges in your current bore that would require the NEXT size up piston and rings.
why is this happening
Typically seizing is from heat. If you have a liquid cooled vehicle then you need to make sure you have your coolant levels correct and all the air purged out of the cooling system. Ensure you are putting two stroke oil in your oil tank. Validate that the oil injection is actually working. Two strokes need oil mixed in with the gas to lubricate the crank bearings. The air fuel and oil go through the crankcase as a part of the combustion process.
Best of luck. Cheers!
I'll make a large assumption here, but it sounds like whoever rebuilt the engine (or bored it) didn't file fit the rings correctly upon install.
Since the engine was rebored, it would require a new piston and rings to fill the gap. I'm sure there are kits available to do this. If the rings are not file fit correctly, when they get hot enough, they will seize in the bore and lock the engine up tight. Once they cool down enough, they release (usually causing no real damage) and the engine can be run again until the same thing happens over again.
The only way to fix this situation is to pull the engine back apart and re-file the piston rings so they are correct. Unfortunately I don't believe there is any test you could do which would tell you if this is the problem without taking the engine apart.