Piston Gap Clearance

Hey,

I've got a 1977 KZ750 Twin that is getting a +1mm rebore. I was wondering what the recommended piston cylinder clearance should be.

Thanks,
Jon

I don't have my manual handy, but the standard formula is .001" per inch of cylinder bore.

Since you are doing this, surely you have a manual handy to look it up?

I think it's .0015-.002" on each side, .003-.004" total, but I'd look in the manual to make sure.

that sounds pretty loose Steel. i would think .001" to .0015" total would be better.when they say per inch of bore they are usualy talking about ring end gap.(.003" per inch.of bore)
My 1135cc has .0015" after many miles and my 1327cc has .0025" after many miles.
the manual will say somewhere.

Ok, I don't have the 750 twin manual handy, but I do have 1000J manual and 750 four manuals handy.

100J piston to cylinder clearance .043mm - .070mm, that's a little over .001" - .00175"

KZ750 .040mm - .067 mm, that equals .001"- .00154"

Notice that the 1000 has more cyl to piston clearance than the 750? That's because the 1000's have bigger bore, and since the 750 twin has an even bigger bore, I'll stand by my original figures.
But the manual has the final say

Oh yeah, piston ring end gap.

KZ750 .008" - .027", that's almost .003" per inch of bore, but I sure wouldn't run them that close (.008" ) especially in an air cooled motor, I'd use a minimum of .020".

By the way there have been some tests of various ring gaps on a dyno, there were no detectable differences in hp between .015" and .040" ring end gap.


The guy that's racing the 750 twin runs .0005" piston to cylinder clearance, but he is running coated pistons in Nikasil coated aluminum sleeves, not iron sleeves

jtgraha2 wrote:

Hey,

I've got a 1977 KZ750 Twin that is getting a +1mm rebore. I was wondering what the recommended piston cylinder clearance should be.

Thanks,
Jon

This is your lucky day, I have the factory manual for the 750 twin.

piston cylinder clearance: 0.043 - 0.070mm (standard)

Ther is no min/max or service limit spec

server lockup duplication

steell wrote:

Ok, I don't have the 750 twin manual handy, but I do have 1000J manual and 750 four manuals handy.

100J piston to cylinder clearance .043mm - .070mm, that's a little over .001" - .00175"

Unless the flu has made my brain goofy, I thought that .043 - .070mm would be about .0016 - .0028" ?

I recall you always multiply by 4 and scoot the decimal point.

bountyhunter wrote:

steell wrote:

Ok, I don't have the 750 twin manual handy, but I do have 1000J manual and 750 four manuals handy.

100J piston to cylinder clearance .043mm - .070mm, that's a little over .001" - .00175"

Unless the flu has made my brain goofy, I thought that .043 - .070mm would be about .0016 - .0028" ?

I recall you always multiply by 4 and scoot the decimal point.

Well, I said "a little over" :lol:

I was talking to my sister on the phone, helping her with a computer problem at the same time I was typing the post, guess I'm just getting to old to multi task

jtgraha2 wrote:

Hey,

I've got a 1977 KZ750 Twin that is getting a +1mm rebore. I was wondering what the recommended piston cylinder clearance should be.

Thanks,
Jon

BTW, the specified clearance is at the widest part of the piston which is the bottom of the skirt. The piston tapers to the top and you will likely see about .007" piston to cylinder gap at the very top if the clearance is to spec at the bottom.

Just metioned it because I had a cow when I checked the clearance of my freshly bored cylinder at the TOP instead of the bottom of the piston.....:laugh:

thanks once again for the help...can't wait to get to put the new engine together.