Quick Question regarding piston/jug tollerences

HI

Im based in the UK and am about to start a 1992 KZ1000P build from a part bike I have obtained. I dont have any history other then I have 2 top ends, 1 has been trashed by what looks like a broken valve and one looks good, I don't know what year the jugs are from. I don't have a complete set of pistons, what I do have have 50 imprinted on them, so I am guessing they are 0.50 over. This is my first build so this is probably a stupid question.

I have had the good jugs measured and the garage have told me they are all 69.88mm are round with no wear, and only need a light hone, From the manual 69.4-69.412 is standard and 69.9-69.912 is 0.5mm overbore, have these been overbored by 0.5mm? Can I just buy a set of 0.5mm over pistons and rings and expect them to fit?

I just want confirmation before I start spending money, the project will go up on here once I start

Ian

If they were bored .5 over they should be at the minimum tolerance.
If I were you I would have it bored the next available piston size. That
being stock replacement at .5, .75 or 1. over or performance aftermarket.
It would insure 4 fresh piston and ring sets.

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+1 When they bored mine, they actually numbered the pistons and bored each cylinder to match them exactly. A new set of +0.50 pots might work OK, but going to +1.0 and boring is safer if you plan to keep the bike and ride it.

On mine, Kawi factory oversize were available only in +0.50 and +1.0 but aftermarket were available in every step of +0.25 up to 1.00 over.

Im going to get an engineer I know to get some digital calipers on the pistons I do have(3 out of the 4) and see what size they are. They do fit and are a very close match. The garage where doing some work on a car engine head, and its not quite what I expected so I'm not completely comfortable with what they have told me about the measurements, especially being 0.02mm under the minimum size.

The pistons have a lot of buildup on the top but they are not marked up at all. They are Art pistons, are they stock ones?

Im building the bike for myself so unless I'm 100% confident they are right Ill fork out for a 1mm rebore and new pistons all around.

I just want to make sure I'm not spending money I don't need to. This is my first bike build, and the car Im currently working on is my first major mechanical work!

Ian

Yes ART are factory pistons for many motorcycles of the era.

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New Old Stock KZ650 piston:

I have had the good jugs measured and the garage have told me they are all 69.88mm are round with no wear, and only need a light hone, From the manual 69.4-69.412 is standard and 69.9-69.912 is 0.5mm overbore

When I had mine bored, they bored to match the pistons they were installing (not the manual spec). I think that .02mm is less than one thousandth of an inch. I believe the bores you have should work with any set of 0.50 over pistons, but measuring the pistons is a good idea too.

bountyhunter wrote:

I have had the good jugs measured and the garage have told me they are all 69.88mm are round with no wear, and only need a light hone, From the manual 69.4-69.412 is standard and 69.9-69.912 is 0.5mm overbore

When I had mine bored, they bored to match the pistons they were installing (not the manual spec). I think that .02mm is less than one thousandth of an inch. I believe the bores you have should work with any set of 0.50 over pistons, but measuring the pistons is a good idea too.

That is kinda what I am hoping, if I can get another 0.5mm over piston to replace the broken one and just have them honed(assuming the new piston is identical to the others I have) it will save me quite a bit of cash. Then I just need rings, gaskets, cam end caps, bolt set, engine painting, ignition....



Ia n

The measurement has come back as 69.8 for the pistons, Im going to risk buying a ART 0.50 over piston and measure it, if that is 69.8 then I'm good to go. Otherwise I will have to go with a 1mm over bore

Thanks for you help!

Ian