i have a1979 xs400.do i need a rebore. standard bore 69mm. mine is 69.94

David Houston

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i have trouble with low compression mine is 120psi should be about 165psi i tried a wet test with small amount of 2 stroke oil in each cylinder, refitted the spark plugs (did not fit the HT leads) the wet test brought the pressure up to 135 psi on each cylinder. stripped head off to make sure the valves and seat were ok, all valves and seats were good no leaks. measured cylinder bores 69.97mm on one clinder and 69.90mm on the other , should be 69mm.do i have to get a rebore then fit oversized rings or would fitting a new set of standard rings help.
i appeciate any recomendations.
cheers dave
 
See your other thread, if those numbers are real (likely not) you don't even have oversize to go to as you are bigger than those too.
 
Factory Over sizes for these run .25, .50, .75 and 1.00mm So you would need the 1.00 pistons and rings. There is a German site that has them up to I think 1.50 but are very expensive. Make sure your bore size is accurate before you look for pistons and rings.
 
I would not do that until the EXACT cylinder wear dimensions are better known as in rock solid. I'd be knowing what piston was last in there too, any oversize already used should be marked on top of piston if you scrape the carbon off it.

If that one number ending at .97 is real then you can only be looking for a 1.25 over piston as the 4th oversize 1.00 will be almost impossible to bore correctly for, it requiring an utter master of the boring art. You are talking only .0012" to put a new hole in and pretty much impossible. Split in half for both sides that's even worse and the reality of it says don't do it. Anybody saying he can is full of crap, you'll get a junk cylinder back.
 
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