1502 Bigbore
This article has been written by Gert in Austrailia, he say's he doesn't mind getting emails if you need any help. He's contactable via gertroewer@hn.ozemail.com.au
Z1300 Kawasaki 1502cc big bore conversion.
Submitted by Gert Roewer Wallan Victoria Australia gertroewer@hn.ozemail.com.au
I have successfully done about half a dozen of these conversions. What you get is a top end that is not a lot better but a vast increase in mid range and bottom end torque. The breathing would need to be improved to make significant top end gains.
This is what you need;-
PISTONS. The early Z750 Kawasaki pistons are exactly the same as the z1300 except for bore size. Z750 is 66mm (against 62 for z1300) This will give an engine capacity of 1458cc. However I don’t use these. I use the 1 mm oversize Z750 piston which gives a cubic capacity of 1502cc. For some reason they also cost substantially less than standard pistons. If these pistons are not available anymore, Kawasaki released a 750 air cooled two valve bike not so many years ago that has a bore size of 66mm. The engine was apparently fundamentally the same as the old 750. More than likely the pistons from this would be suitable. Check for pin size (15mm) and deck height. In any case even if the pin size is say 16mm, the z1300 rods have enough meat in the small end to be able to be enlarged to this with no hassles.
SLEEVES. New oversize cylinder sleeves need to be fitted. I machine my own from centrifugally cast hollow cast iron bar stock from a firm called Flo-cast in Coburg Melbourne. I Don’t make the liner walls as thick as the originals because there is not much room. The outside of the sleeve has 5 different diameters including the flange at the top. The sleeves should be machined so that they are a little too “tall so that they protrude above the surface of the cylinder block. When assembled they can then be milled down true to the block. Also each sleeve flange has a flat machined onto it on both sides where it abuts the adjoining sleeve. This feature is redundant on sleeves No 1 and 6 on the outside only , even though the standard sleeves have a flat machined there.
MACHINING OF CYLINDER BLOCK. The z1300 engine uses a “wet sleeve” system which simply means that the engine coolant comes into direct contact with the outside of the sleeve. Each sleeve is sealed top and bottom by rubber “O” rings . At the top the sleeve itself is grooved and on the bottom the Cylinder block is grooved. The sleeves press out fuss free and after removing the bottom double “O” rings the block is ready for machining Machine the holes out to suit the new sleeves. I machine them to have .001 inch interference. Of course this process removes the bottom “O” ring grooves in the block and new ones need to be machined. Because of space constraints I machine the grooves to suit a smaller section “O” ring. I do not machine a new “O”: ring groove into the top of the sleeve because of the reduced wall thickness. I also make the flange on the new sleeves so that it fits into the original flange recess. There is still enough flange engagement to serve the purpose. To seal the top section of the sleeve I rely on a high quality silicone sealant such as 3Bond. Never had a leak.
MACHINING OF CRANKCASES. First all the studs have to be removed. Then the cases need to be machined to suit the larger sleeves. Invariably this process breaks into a couple of oil galleries that feed the top end of the motor. If this happens get the section TIG welded and then run a suitable long drill through the oil gallery to make sure it’s clear.
GASKETS. You cannot use a standard head gasket or base gasket. The base gasket is easy. A sharp pair of scissors and trim sufficient material from around the cylinder holes until correct. The head gasket has to be specially made. I am fortunate in that there is a chap here who does nothing but make special gaskets. Gave him a standard gasket, told him the bore size and he worked his magic. Kawasaki use a special patented silicone bead system around the water jacket galleries. He couldn’t reproduce this so the magic 3bond was called upon again successfully. I always get a pair made for each conversion theory being if ever you need one you don’t have to wait weeks for it to be made.
TUNING. When the engine has been carefully and meticulously reassembled, and started it won’t run right because of the extra capacity. Usually on the carb models the main jet size has to be decreased. Yes that’s right---smaller main jets. How much smaller is not always the same. Do it properly. Do it on a Dyno. Sometimes the needle positions have to be changed. Sometimes they don’t. The fuel injected models are a bit more of a problem. The system is not tuneable so we have to get tricky. I got reasonably satisfactory results by replacing the standard fuel pressure regulator with one that is adjustable and then adjusting the pressure with the bike on a dyno until the tuning was acceptable. It won’t be perfect but it gets around the problem cheaply. The ideal method of course is to use an after market programmable engine management system.
HOW WILL IT GO? My mate on his XS1100 (two up) and me on my z1502 (one up) Top gear roll ons at various speeds. I totally creamed him. He put his in 4thgear, I stayed in top. Still creamed him. He dropped back to third, I stayed in top and I still pulled slowly away from him. SATISFACTION.
