0.020\" off

Hey guys.

I Finally received my 810 wiseco kit a few days ago :woohoo: that will be going into my GT 750(kz750-p1)

I have read on a gpz site that machining 20 thou off the head at the same time will boost performance quite a bit.

now i want to know how many people have actually done this minor mod and how they went with it?

im not quite sure i want to go ahead with it after just reading it on a website and have it pinging its head off i would prefer to hear some more real world experience, i know a high octane fuel will be needed but thats not a major concern.

A Reputable bike mechanic also told me to get the Jugs machined the 20thou instead of the head. Opinions???

Cheers

.020" is not going to make a large difference, it's only about a quarter point increase in compression (10.25:1 to 10.50:1 or so), but the GPz750 FAQ recommends it.
I know KoolAid Kid did it to his 83 GPz750 motor when he put the 810 kit in, but it's not running yet. When I do mine I'll probably mill it as far as it will go (.030"-.035"), but just like the 750 twins, I have spare heads so I can switch if it causes a problem.

Did the "Reputable bike mechanic" explain his reason for saying that you should take it off the cylinder block instead of the head? Both achieve the same increase in compression, both cost the same to mill (although you do have to remove the valves from the head), cutting .020" off the surface will not weaken either one noticably, so I'd say it's six of one and a half dozen of the other.

Be aware though that the o-rings sit on the top of the block, so they are going to be protruding an extra .020" if you decide to mill the block, and I don't know if that will cause a problem.

steell wrote:

.020" is not going to make a large difference, it's only about a quarter point increase in compression (10.25:1 to 10.50:1 or so), but the GPz750 FAQ recommends it.
I know KoolAid Kid did it to his 83 GPz750 motor when he put the 810 kit in, but it's not running yet. When I do mine I'll probably mill it as far as it will go (.030"-.035"), but just like the 750 twins, I have spare heads so I can switch if it causes a problem.

Did the "Reputable bike mechanic" explain his reason for saying that you should take it off the cylinder block instead of the head? Both achieve the same increase in compression, both cost the same to mill (although you do have to remove the valves from the head), cutting .020" off the surface will not weaken either one noticably, so I'd say it's six of one and a half dozen of the other.

Be aware though that the o-rings sit on the top of the block, so they are going to be protruding an extra .020" if you decide to mill the block, and I don't know if that will cause a problem.

i normally would take it off bottem of cylinder instead of top,but you have to remove sleeves to do it.

A couple of things you may want to take into consideration before you "cut" anything.
Milling the head or the cylinder bock will do the same thing, but you need to measure the piston depth @ Top Dead Center to see how much can be removed from the cylinder. You dont want the pistons sticking out of the cylinder at TDC.I'm not talking about the dome of the piston, the flat area above the ring lands. Depending on cylinder head gasket thickness you could have piston to head contact (NOT GOOD)
Also milling the head will retard cam timing , unless you use slotted sprockets and degree the cams back into spec.
Premium fuel will be a must. Remember these engines are air cooled, they get hot.

TwoCam wrote:

A couple of things you may want to take into consideration before you "cut" anything.
Milling the head or the cylinder bock will do the same thing, but you need to measure the piston depth @ Top Dead Center to see how much can be removed from the cylinder. You dont want the pistons sticking out of the cylinder at TDC.I'm not talking about the dome of the piston, the flat area above the ring lands. Depending on cylinder head gasket thickness you could have piston to head contact (NOT GOOD)
Also milling the head will retard cam timing , unless you use slotted sprockets and degree the cams back into spec.
Premium fuel will be a must. Remember these engines are air cooled, they get hot.

It's almost standard practice to mill .020" to .030" off the GPz750 head when installing a 810 kit, valves get nowhere near the pistons. The KZ750 head is the same, except for 1cc smaller combustion chambers on some years/models.

A Dremel will slot the cam sprockets, you don't need to enlarge the holes very far (.030 is more than enough).

And I would much rather mill the head than mess with removing the sleeves, especially if they have already been bored.

There are a couple of GPz750 FAQs that go into detail on this subject.

I have not heard of anyone milling the block on the GPz750 motors, seems like everyone mills the head.

Post edited by: steell, at: 2006/08/27 15:23

steell wrote:

Be aware though that the o-rings sit on the top of the block, so they are going to be protruding an extra .020" if you decide to mill the block, and I don't know if that will cause a problem.

K steell, I'm getting a little disturbed here, on disassembling my top end there were no o-rings that I could see (82 CSR 650), some came in my gasket set, 4 large o-rings came in the set but I havn't the faintest idea where they go or fit, they don't seem to belong anywhere that I can tell...

Are o-rings used on all KZ 650s or...?

nfswift wrote:

steell wrote:

Be aware though that the o-rings sit on the top of the block, so they are going to be protruding an extra .020" if you decide to mill the block, and I don't know if that will cause a problem.

K steell, I'm getting a little disturbed here, on disassembling my top end there were no o-rings that I could see (82 CSR 650), some came in my gasket set, 4 large o-rings came in the set but I havn't the faintest idea where they go or fit, they don't seem to belong anywhere that I can tell...

Are o-rings used on all KZ 650s or...?

Look at the bottom of the two rearmost outside head bolts where they enter the block, should be an o-ring around each of those two because that's how oil gets to the top end. There are also two o-rings at the bottom of the cylinder block for the two oil passages, oval ones IIRC. If it's a top end gasket set then I would guess that the 4 large o-rings go between the intake manifolds and the head.

Post edited by: steell, at: 2006/08/27 16:27

steell wrote:

TwoCam wrote:

A couple of things you may want to take into consideration before you "cut" anything.
Milling the head or the cylinder bock will do the same thing, but you need to measure the piston depth @ Top Dead Center to see how much can be removed from the cylinder. You dont want the pistons sticking out of the cylinder at TDC.I'm not talking about the dome of the piston, the flat area above the ring lands. Depending on cylinder head gasket thickness you could have piston to head contact (NOT GOOD)
Also milling the head will retard cam timing , unless you use slotted sprockets and degree the cams back into spec.
Premium fuel will be a must. Remember these engines are air cooled, they get hot.

It's almost standard practice to mill .020" to .030" off the GPz750 head when installing a 810 kit, valves get nowhere near the pistons. The KZ750 head is the same, except for 1cc smaller combustion chambers on some years/models.

A Dremel will slot the cam sprockets, you don't need to enlarge the holes very far (.030 is more than enough).

And I would much rather mill the head than mess with removing the sleeves, especially if they have already been bored.

There are a couple of GPz750 FAQs that go into detail on this subject.

I have not heard of anyone milling the block on the GPz750 motors, seems like everyone mills the head.<br><br>Post edited by: steell, at: 2006/08/27 15:23

i guess i should of been more specific,the reason we would deck the cylinder is if the pistons are already down in the whole it brings them up closer to top of cylinder on tdc.if you really want to get every last bit out of motor you can deck the head also,this is just the way i was taught to do it many years ago on 900-1000 motors anyway.

Yea, I took care of those, but I mean four o-rings larger in diameter than the cylinder bore!

Well i just dropped the barrels (jugs) off to the machinist and he said he prefers to machine the head. Mainly because going from aluminium to cast steel when machining the tops of the barrels causes the cutter to chatter because of the different hardness of the two materials. he did say something about doing the barrels in certain cases to bring the piston closer to the top of the bore.

Hopefully i will have them back at the end of the week.

nfswift, those 0-rings go around the bottom of the cylinder sleeves and seat against the case.

Wireman, I don't know much of anything about the 900 or larger motors, I am just a 750 twin and four cylinder guy

Jay_G wrote:

Well i just dropped the barrels (jugs) off to the machinist and he said he prefers to machine the head. Mainly because going from aluminium to cast steel when machining the tops of the barrels causes the cutter to chatter because of the different hardness of the two materials. he did say something about doing the barrels in certain cases to bring the piston closer to the top of the bore.

Hopefully i will have them back at the end of the week.

This will not happen if you have the correct equipment for doing the job.

Jay