BigBore Kits

My 7 is a 93 R model with stock stroke. I have tried various big bore kits with some success, but the cylinder walls are really to thin for the 2mm kit. Though that is what I’m currently running in one of my motors. Its a Wiseco kit they never advertised to the public part r793, that they no longer make. Its identical to the 13.1 superbike piston they sell in 71mm size for the 91-95 motors. Though its rated at 13.1, the most compression ratio I could get was 11.9 to 1 using all the techniques available.

Compression ratio is the volume of dead space above the piston at TOP DEAD CENTER versus the dead space above the piston at Bottom dead center.

The ZX7 for example uses valves that have dimples in the face of the valve. You can buy aftermarket kit valves that are flat faced which reduces Combustion chamber volume some. The 93 chamber measure out at 10cc with stock valves and 8.5cc with kit style flat face valves. There is almost no metal to mill off the head before you cut into the valve seats so that isn’t a option. The other options are thinner head gaskets and pistons with higher domes and setting of deck height. I set my deck height at -5 thousands, which means at the top of the stroke the edge of the piston is 5 thousand of a inch down in the hole from coming all the way up. Then the only thing keeping the piston from hitting the head is the compressed thickness of the head gasket, about 30 thousand of a inch, which effectively gives you 35 thousand piston to head clearance. Not a lot. I have run it as tight as 30 thousand and could see where the markings on the top of the pistons was kissing the head. To tight!!!. But the compressed thickness of the head gasket has to be included in your comp. ratio figures along with the deck height and the piston dome value. That has to be measured with the piston installed in the bare cylinders. A good engine building book will explain the exact procedures. I grew up in the auto racing world with a father who built Nascar engines, and that’s the exact way they do it. Now last year Rusty wallace was busted for exceeding the Nascar mandated 14.0 to 1 ratio with a spec of 14.09 to 1. Their argument, a valid one was that 14.09 to 1 and 14.0 1 is the same thing. So Nascar eventually agreed and reinstated points and the following day changed the rule to read 14.000 to 1. Did that extra almost .1 tenth help. He was untouchable that day so it sure didn’t hurt. I thought they were being real creative. Horsepower is made from airflow through the motor and compression and as one engine builder once told me, Engine dynamics are the same for everybody!!! The oems lie about CRs because it’s a good selling point. Never been to Michigan or know any shops there. The ZX9 having a longer stroke has a taller block so the head sits up higher which is why the engine mounts don’t line up on the top. I have seen one of the aftermarket kits for this and was quite impressed and felt it was worth the 450 dollars.

One good thing about the 94-97 ZX9 motor is all the aftermarket ZX7 91-95 race valves cams springs retainers all fit this head!!! I still think the 93-94R model ZX7 was the best, I never liked the shorter stroke motor., apparently neither did Muzzy as he built all his big raptors off the stroke dimensions of this motor with a 4mm overbore or a 2 mm overbore if your talking the newer motor. Its just how you look at it. The 96-00 with a 2mm overbore and a 91-95 crank stroke, or a 91-95 motor with a 4mm overbore is the same exact thing.

Ken Waters

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