A Kawasaki Z1, KZ900 or KZ1000 air cooled four cylinder motor is one of the most versatile motorcycle motors ever built. That is why it is still with us today. It is the small block Chevrolet of motorcycles. A stock cylinder block can be resleeved and bored up to 1261cc or 78mm. That is an increase of 358cc over the original 903cc motor or a total of 40% increase in displacement. Once you step up to an aftermarket block, the displacement can be increased to 1500cc or an increase of 66%. All of that is without changing the stroke.

I have been building these motors and performing machine work on them since I bought my first Z1 in the summer of 1974. I have over forty four years of experience and almost all of it has been devoted to the air cooled four cylinder motor. The question is frequently posed to me about what is the optimum size of motor to build, and what is the cost of each option. We will start at the lowest cost option and work our way up.
A stock 903cc cylinder (Z1 or KZ900) has 66mm pistons. These blocks have thick cylinder walls and can be bored up to 71mm using all of the stock components. This yields a displacement of 1045cc.

If you would like to leave the cylinder so that it can be bored again, to extend it’s life, then you have the option to go to 70mm or 1015cc. A stock 1015cc cylinder (KZ1000 up to 1980) or 998cc cylinder (KZ1000 1981 and later) does not come stock with as thick cylinder walls. These cylinders can be bored up to 72mm. This yields a displacement of 1075cc.  Some manufacturers advertise that a KZ1000 cylinder can be bored to 73mm or 1105cc. I have had bad luck with this in the past and would not recommend it to anyone.

Depending on the type of pistons you select and the type of motor you have, the piston kit for 1015cc, 1045cc or 1075cc will cost from $520.00 to $560.00. The piston kit comes complete with rings, wrist pins,circlips or Teflon buttons, head gasket and base gasket. Boring costs $175.00. So your total will be $695.00 to $735.00.

If the motor is to be used on the street, any of the above options has my strong recommendation. All can be operated under the standard conditions that a stock motor can be operated under. The increase in displacement and compression will cause the motor to run hotter when there is not a continuous flow of air around the cylinder. However, as long as the motor is not allowed to sit motionless for extended periods of time, no damage will result. Next comes the option of resleeving. Once sleeves have been installed in the motor, the size can go from 1105cc to 1261cc. In order to get the oversized sleeves to sit into the upper crankcase, it will have to be bored also. In order to bore the upper case, the bottom end will have to be disassembled and the cylinder studs will have to be removed. However, removing the cylinder studs is not that big of a deal, since with the increase in displacement the prudent thing to do is replace them with heavy duty versions. So, if you choose this option, what size motor should you build? There is an old Pit Stop Performance saying that goes “there is no modification you can make to a small motor that you can not make to a big motor”. The simple translation is that your money is best spent on displacement first. So, if you are going this way, my recommendation is to build a 1197cc motor. The cost of all piston kits is at $520.00 to $560.00. Sleeves cost $259.00. The machine work is $420.00 and includes removing the old sleeves, boring the block for the new sleeves to an interference fit, heating the block and installing the new sleeves, seating the sleeves on a 15 ton press, resurfacing the top of the cylinder, boring and honing the new sleeves to match the piston kit. This comes to a total of $1199.00 to $1239.00 and $100.00 more if you want copper o’rings installed in the top of the sleeves. It is another $80.00 to bore the upper case and $45.00 to remove the cylinder studs.  I would only recommend this option for a race motor. Or, at least for a street fighter that is only ridden sparingly on the street to get to the location of the race. The reason for this is that the motor will get hot in a hurry. The large displacement guarantees it.  If the motor is operated on the street as a daily rider, you must keep it moving to keep it cool. The final option is a full out racing cylinder. An MTC big block, and piston kit ready to go costs $1460 to $1515. Available displacement is from 1327cc to 1500cc.  You already know my recommendation. Prices quoted above are not an offer to perform the work at a particular price. They are presented as examples of how to compare your options.