Engine Rebuild

Thanks guys, this is awesome.

I don't know about the different jugs as of yet, since it might be a bit costly, but I'm definitely going to be looking at all of your suggestions. How crucial is a port/polish when boring up a size?

Heres a pic of how ratty my bike is looking these days. This was a year ago, but she hasn't moved from that spot since I tuned her up and took her about 60km or so to shake loose the cobwebs.

She is in need of some LOVE

Oh yes. I got love to give her, but not until september, I'll be out of the country till then. But I'm going to have parts awaiting my return so that in a week or so after I get back she'll be good as new.

about how much is needed to be milled?
a few thousandths or a few millimeters?
didnt really want to get into head work...
i have a good 650 head and a good 750 head.
which would be better smaller valves smaller combustion chamber...or bigger valves bigger bigger chamber.
i figure on the 650 head....
Ed

77_650B1 wrote:

Thanks guys, this is awesome.

I don't know about the different jugs as of yet, since it might be a bit costly, but I'm definitely going to be looking at all of your suggestions. How crucial is a port/polish when boring up a size?

If you mean going up +1 in piston size, that's only an additional .020" diameter so the stock intake and exhause as is will work perfectly. Port and polish will give it a bit more, not mandatory.

EddieBoy wrote:

about how much is needed to be milled?
a few thousandths or a few millimeters?
didnt really want to get into head work...
i have a good 650 head and a good 750 head.
which would be better smaller valves smaller combustion chamber...or bigger valves bigger bigger chamber.
i figure on the 650 head....
Ed

Others may differ, but my opinion is that the additional flow from the
750 head more than outweighs the minimal loss in compression.
Especially if you mill .030" off the surface of the 750 head.

so steel, you would go with the 750 head? milling .030-035 instead of
.020 off of the 650 head? what do you think the compression would be
w/ the 750 head milled at .035

vach wrote:

so steel, you would go with the 750 head? milling .030-035 instead of
.020 off of the 650 head? what do you think the compression would be
w/ the 750 head milled at .035

I took some measurements and did the calcs a while back on a 810 using the 750 head, and IIRC it was 10.75:1 vs 10.25:1, cutting .030" off the head. With the hemispherical combustion chambers of the 750 heads and GPz750 cams, I don't think I'd want a static compression over 10.75:1, and that's using Premium gas. I did a compression test on the GPz750 motor in GpzInfested's KZ650SR before we pulled it from the donor bike, and it had 170-180 psi with stock bore and stock 9.5:1 CR. IIRC, the static compression pressure limit for the hemi chamber is 200-225 psi on pump gas. I think the 810 at 10.75:1 and GPz750 cams is bumping that limit, and KZ750 may push it over (depending on overlap).

You don't get a large power increase out of only bumping compression up, I think it was only 1-2 hp going from 10.25:1 to 10.75:1.