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TOPIC: Z2 750 crank (Bossie the Kaw)

Z2 750 crank (Bossie the Kaw) 21 Sep 2015 03:45 #691138

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I'm getting 6.24 and 7.02 respectively, assuming the other numbers are right. This is best done by direct measurement of cc 's of oil in the combustion chamber and then summing the piston crown hump displacement.

CR = swept volume + TDC volume divided by TDC volume.

Am I wrong on this ?
1980 Gpz550 D1, 1981 GPz550 D1. 1982 GPz750R1. 1983 z1000R R2. all four aces

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Z2 750 crank (Bossie the Kaw) 21 Sep 2015 03:56 #691139

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Take the power wire of the Dyna and stick a comp gauge on it...... if your getting 100-110-120 psi wouldn't that show it ?
1978 KZ650 b-2
700cc Wiseco kit 10 to 1.
1980 KZ750 cam, ape springs, stock clutch/ Barnett springs.
Vance and Hines Header w/ comp baffle and Ape pods, Dyna S and green coils, copper wires.
29MM smooth bores W/ 17.5 pilots, 0-6s and 117.5 main
16/42 gearing X ring chain and alum rear JT sprocket....

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Z2 750 crank (Bossie the Kaw) 21 Sep 2015 04:03 #691141

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That would be unreliable due to the new rings and only 200 miles on it. I'm checking my valve adjustments this morning. I will do the wire test. I have some heavy gauge wire I can use. I can measure at the side where the piston is flat and then measure the center of the dome if that would help.
Steve
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Z2 750 crank (Bossie the Kaw) 21 Sep 2015 04:51 #691149

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I can measure at the side where the piston is flat and then measure the center of the dome if that would help.

Not sure it would.

A cc syringe and plate glass with hole in it over combustion chamber sealed with grease and filled with oil and no bubbles gives the combustion chamber volume, usually 1 cc within factory spec unless it has been skimmed or had a valve job.

Then, unless a perfectly flat top piston with no valve pockets, you calculate the piston dome displacment which comes from direct oil cc measurement.

You might be able to get a rough measure by tipping the bike on an angle, filling a bore through the plug hole with oil at TDC, noting the volume, then go to BDC and see how much you add, using the bottom of the spark plug thread as your fill mark.

Add the first two then divide by the first. Probably accurate within less than half a point of compression ratio I guess.

Usually the piston dome is above the deck height and subtracted from the combustion chamber volume and gasket width to give V(tdc) , in this case it will be much less, that's why we are making assumptions by back calculating from factory specs and some sort of direct measurement might be neccesary.
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Last edit: by Tyrell Corp.

Z2 750 crank (Bossie the Kaw) 21 Sep 2015 05:55 #691158

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If nothing else it would give me a stroke measurement. If it turns out to be 66mm, OK, it's a 903 crank. If anything else, it's a 750.
8mm would be a lot to miss. :unsure:
Steve
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Z2 750 crank (Bossie the Kaw) 21 Sep 2015 06:19 #691161

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I get it, you are measuring the stroke length with a wire.

The more I think about it, I think Tyler's numbers might be out , as V(tdc) is based on the 900 crank throw, shorter stroke = higher V(tdc) number.

Try the oil fill test , you got me interested now. Otherwise I get easily distracted on oil seal threads :woohoo:
1980 Gpz550 D1, 1981 GPz550 D1. 1982 GPz750R1. 1983 z1000R R2. all four aces

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Z2 750 crank (Bossie the Kaw) 21 Sep 2015 06:40 #691166

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Hind sight is 20-20...LOL....
it sure would have been nice if you had measured with a steel rule how far the piston drops from TDC in 180° !! HA ha
I would not be surprised to find out you do not have a 750 crank from what everyone is saying.
I think you "might" be able to drop a straight wire in a plug hole and mark it off against a top fin and rotate the engine to see how far it goes up or down.... etc.
A negative 8mm would be fairly easy to see if you can keep the wire straight up and down ....and always compared to the same fin and the same spot the top of the piston.
Seems like a goofy backyard mechanic way to do it, but what the heck... lol
8mm, that's .315" or 5/16" of an inch from a 903 at 66mm / 2.6" stroke.
So if you measured only 2.3" I guess it is a 750 crank.
Bruce
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2016 Triumph T120 Bonneville
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Z2 750 crank (Bossie the Kaw) 21 Sep 2015 06:46 #691169

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Tyrell Corp wrote: I'm getting 6.24 and 7.02 respectively, assuming the other numbers are right. This is best done by direct measurement of cc 's of oil in the combustion chamber and then summing the piston crown hump displacement.

CR = swept volume + TDC volume divided by TDC volume.

Am I wrong on this ?


Your correct.

CR= V(BDC) / V(TDC) where V(BDC) = V(TDC) + Cylinder displacement

If you take the stock displacement and CR you can calculate V(TDC) that includes chamber volume + Gasket+ deck height - piston dome

Then you can determine the additional piston dome volume for the 10.25 : 1 pistons

Again this is just estimation, the real measurements would be needed, but it should be in the ball park, and that doesn't look too good.
Someone familiar with these engines can comment about the normal head volume and so forth, I haven not had a 1015 apart.
In Swest's case its the reduction in cylinder displacement that really knocks the CR.

If this is the case with your engine Swest you may need to deck the block and plane the head if you want to run it that way.
If I knew what I was doing all the time life wouldn't be any fun.

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'81 KZ1300 A3 full restoration, custom big bore pistons, 1400cc 6 cylinder super bike
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Last edit: by Tyler.

Z2 750 crank (Bossie the Kaw) 21 Sep 2015 06:58 #691174

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Ok, I'll do my valves this evening. Here it is. My last reading is 54.50. I took multiple pics because the measurements changed slightly.
TDC

BDC


54.51mm


Old gasket




Another pic of depth.


I'm now sure it IS a 750 crank. 54.51 - 1.40= 53.11.
Next is the compression test.
Steve
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Z2 750 crank (Bossie the Kaw) 21 Sep 2015 07:04 #691175

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Yup looks like you got it.
Now what?
Pull the engine crack it and replace the crank and rods?
Bruce
1977 KZ1000A1
2016 Triumph T120 Bonneville
Far North East Metro Denver Colorado

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Z2 750 crank (Bossie the Kaw) 21 Sep 2015 07:09 #691176

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Maybe I'll list it on eBay for $9000 OBO like that fool, buy a 903 crank for $200 and maybe buy a bigger kit. :lol:
Steve
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Z2 750 crank (Bossie the Kaw) 21 Sep 2015 07:44 #691181

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There is a flaw in the oil test CR check, that could be worked around if doing valve clearances - I'll leave you to work it.

over and out .
1980 Gpz550 D1, 1981 GPz550 D1. 1982 GPz750R1. 1983 z1000R R2. all four aces

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Z2 750 crank (Bossie the Kaw) 21 Sep 2015 09:38 #691188

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I'm satisfied it is a 750 crank. I also know I'm losing compression because of it. Too bad I didn't do this before I put in the new pistons. I could have done a CR test then and now. Oh well, I should be able to buy a 903 crank soon and list this one on eBay. I can't find any there so it should fetch some money. B)
Steve
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Z2 750 crank (Bossie the Kaw) 21 Sep 2015 10:02 #691195

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I wonder if the KZ1000LTD crank and rods (still in the open case I found for $25 earlier this summer) is worth a tinker's damn.
Probably rusted tight.

www.kzrider.com/media/kunena/attachments/4138/IMG_0958.jpg
Bruce
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Z2 750 crank (Bossie the Kaw) 21 Sep 2015 10:47 #691198

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I know this is all a moot point but for what it's worth...
I just measured my 1015cc piston and the top of the crown, furthest point going into the combustion chamber, is 30mm from the center of the pin.
The top land, which is the flat of the piston before the crown starts, and where I said it was just about flush with my over bored sleeve at TDC.... is 24mm from the center of the KL250 piston pin.
That old picture you had from some Z2 Guru....of a used Z2 750 piston, mentioned the crown top was 36mm from the center of the pin.... and the top land of the Z2 piston looks at least 2mm-4mm thicker then my 1015 piston, again, which measures about 30mm from the center of the pin to the top of the crown.
That is at least a 4mm-6mm difference by eyeballing it.

So, the distance from the center of the pin and the very top flat area of the first compression ring (not just the crown) is larger in the Z2 750 piston.
That means to me, regardless of what the stroke is, the KL250 1015 piston pin is higher in the casting, with respect the pin center and the top land flat, thus lowering the top of the piston and the flat of the top land into the cylinder at TDC ...at least 3mm-4mm then the same crank would do with the Z2 750 piston if it was the right bore for the 1015 cyl block you have.
I guess it is redundant but there is where you lost that 3mm to 4mm of piston top and deck height issue and now a lower CR and probably combustion chamber PSI too.
I wonder how much real horse power is lost to this though?
Bruce
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Z2 750 crank (Bossie the Kaw) 21 Sep 2015 12:07 #691214

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Thank you for taking the time to check it out.
I was mulling all this over myself. Being the stroke is 8mm shorter, it stands to reason the compression would suffer. The stock KZ 1000 cranks are heavier than the 903 so they would be of no use to me. As it stands now, the bike is running great and I should be able to buy a 903 crank soon. Although it is a lot of work splitting the cases, I believe it will be worth it. Z2 cranks just aren't available making them worth a lot of money. It would be wise of me to change it out before any damage can occur. B)
I'm glad I did it when I did. You saw the center roller and the missing guide. A broken cam chain was imminent and total destruction of the engine could have been the result. Dodged a bullet on that one. :whistle:
I went back to the old style manual tensioner. I may install a heavier spring too. In 40 years I never had one fail.
Steve
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Z2 750 crank (Bossie the Kaw) 21 Sep 2015 12:54 #691224

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As it stands now, the bike is running great

I guess that's the 'deal' with litre plus muscle bikes, always a load more than you need. :)

Good street cred chatting about race cams and high compression pistons, in the real world it is how it pulls on the road and about town with pump gas and a reliable lasting motor at FSM service schedules.

Re pm, That rare 750 crank, I think it may be scrap metal in USA but big money in other markets. If you list it online
make sure you have worldwide visibility. I've sold cars and bike parts all over the world, NOS 750 crank is $$$$$ if you can find one, also the super rare stuff you need to stick to your price and expect several months to sell to the right punter who really needs it.
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Z2 750 crank (Bossie the Kaw) 21 Sep 2015 13:02 #691227

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Thanks, the trouble is, these things are almost bullet proof. If one goes bad, I shudder to think what the rest of the motor looks like. :ohmy:
Steve
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Z2 750 crank (Bossie the Kaw) 21 Sep 2015 13:32 #691232

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Just read thru this and was thinking since its all miss matched you could just put kz1000J pistons in it to make up some space.
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Z2 750 crank (Bossie the Kaw) 21 Sep 2015 14:21 #691241

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the trouble is, these things are almost bullet proof.

...that is the problem, anyone want a good in service spec 550 /750 crank and rods - yous for a bottle of red. Kawasaki were too clever in their engineering , these ££££ forged/pressed cranks now you almost can't give away.

Customer complaint: Dear kawasaki, please make our cranks more crappy to give the small guys a chance of maiking some dough here 30 years later to fund our restoration projects :laugh: .
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