Z900 head gasket thickness?

[12:1]

Hi all, 1st post. After the stock head gasket thickness. Part no. 11061-1214. Cometic sell a 74mm at .018" (0.4572mm) or 74mm at 0.27" (0.6858mm)
I can't find the Zds spec but motors similar seem to be 0.70mm. Just need some confirmation so I don't  turn it into a handgranade. Cheers.

[Zed62]

Are you worried about the change in compression ratio?  Or the ability of the gasket to "hold"?  Will a .009" difference in thicknes increase the the CR enough to matter?  Let's check...In fact you can calculate it.
  OK, looks like the Z900 has 11.8:1 CR.
Per cylinder the displacement is 948/4 cylinders = 237 cc per cylinder.  The clearance volume is therefore 237/11.8 = 20.08cc.  73.4mm bore has area of 230.59mm^2.  A cylinder Ø73.4mm x .229mm (.009") high (difference in volume between gaskets) has a volume of .967 cc.  So, 20.08 cc - 0.967 cc = 19.12 cc.  Almost there...
237/19.12 = 12.4:1.  New compression ratio.

Definately more than I thought.  Is 12.4:1 too high?  Dunno.  Also query valve clearance.... / timing???  I would make sure to stay stock myself.  YMMV

I was comparing the two numbers you know for sure.  Gives an idea of the effect of changing thicknes of gasket.

[12:1]

Thanks for the detailed reply, wasn't expecting that, well done. I have ordered the OEM HG but I was hoping for a marginal gain of around 12-12.2:1 with a Cometic. Still have no idea if the stock is 0.7mm
I am transplanting a z900 motor into my z900rs. The only RS remnant of the motor being retained will be the head.
Lighter crank, high compression pistons, 2mm higher lift cams. The rods and journals on both motors are the same, valves are the same. I am not even going to clay the motor. The engine numbers are only 90 apart.
Head swap, shim adjustment, new head gasket, 12000rpm hear I come. Will hopefully peak at 128bhp, the high compression head gasket would get me to 130bhp. That's all I want, nothing more.
Disappointed with the RS high ratio 1st gear, 10000rpm red line and pitiful peak power. Having the revs maxed out in 1st at the apex of a hairpin is not fun and second gear sucks at 3500rpm.
I still want it to look like a teddy bear, just not be one.

[Zed62]

Wow, that is quite a list of high performance mods.  Good luck!  Also my math in the above has a mistake; I neglected to consider the clearance volume itself in the CR.  That will definately alter the results a bit.

[12:1]

Got the oem head gasket. Part no 11004-0769. Does all Z900B and RS's. Thickness is 0.645mm, measured in 4 places, 0.636-0.645mm

[12:1]

Originally Posted by 12:1

Got the oem head gasket. Part no 11004-0769. Does all Z900B and RS's. Thickness is 0.645mm, measured in 4 places, 0.636-0.645mm

Finally got my z900 motor in the RS and on the dyno.

Uncompressed head gasket thickness

Oem head gasket 0.64mm
Cometic 0.18" 0.53mm

Compressed head gasket thickness

Oem head gasket 0.57mm
Cometic 0.18" 0.46mm

Squish/quench/piston to head clearance went from 0.91mm to 0.80mm

Max power 130.13 bhp @ 10920rpm
Max torque 75.9 ft-lbs @ 7860rpm

Dyno tuner then put up a chart and compared it to an RS with ZX10 piston bbk, decked and ported head with a cometic head gasket.
It made

Max power 131.98 bhp @ 9960rpm
Max torque 80.50 ft-lbs @ 8000rpm

The motor, radiator and spare wheels ended up costing me less then $1000au after selling the parts off the salvaged Z900 I bought and I still have a complete stock RS motor sitting in my loungeroom. The tuner said the BBK  ZX10 piston RS modifications were in the $6500-$7000 range.
Glad I didn't  go down that route.
Aesthetically I prefer the look with no mock cooling fins and have yet to have any negative comments about it (that's about to change)
Essentially it was a plug and play swap, the only mod I needed to do was pack the z900 stator out when fitting the RS stator cover, I matched the RS airbox dividers to that of the Z900,  swapped to the same velocity stack configuration as the Z900 and also added two 42.5mm pipes to the front of the airbox, the vent sits on the top of the radiator and just above the frame steering stop. I fabricated it using the Z900 frame.

Advantages of the Z900 motor over stock RS:

Longer 1st gear. Input shaft is now 13 tooth, the RS is 12 tooth.
Lighter flywheel
Compression up from 10.8 to 12.2
Rpm limit up from 9800rpm to 11,200rpm
Power up from 96bhp to 130bhp
Torque up from 64 ft-lbs to 76 ft-lbs
No mock cooling fins.

Disadvantages

Stator is smaller as the RS has an extra 15amps for the heated grips.

How does she go? It's got enough for the street now so I am happy.

[porkchop]

12:1,
I was told by the Northeast USA Kawi Rep in 2017 when he brought a new Z900 to our ZRX rally for us to demo that the RS version was purposely detuned about 15HP "to be a more relaxed ride for the market it was going to draw".
Could never understand that logic but not my thing

Bought a Z new in 2019, started the mods soon after.