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Hi fellows,

I have another KZ1300 that came with a new in the box big bore kit. It also came with a complete gasket set with valve seals etc. This new bike is running but not very well and smokes oil like a train. Right now it seems to have an intermittent spark problem. It was still a steal at $1200 CAD for everything.

Cosmetically, the 79 is in better shape than my 81. My 81 most likely jumped its timing and smashed some valves so I've decided to put the the kit in the 79.

The kit comes with 64mm pistons. I need to get some details on how much the cylinders have to be bored and any other things I need to look at. I won't be doing this myself. I'll be paying someone else since I have no where near the ability or tools to do a job like this.

I read that this bike uses oval pistons. Is this true? What problems does this present with boring the cylinders?

Here are some pics of the new bike and the kit.

My two cents (for what it's worth)- you'll need a good machine shop to do this anyway. If you get them the parts (pistons, bores, etc.), they should know what to do. Just be sure they have as much stuff as they need, and know what you want to do toit.

I was unaware any of these bikes used oval pistons, big bore or no. Again, if the shop has the ppistons as well, they should be able to tell.

Back in the 70's and 80's all pistons were "oval" by a couple of thou. It gave the piston some room for expansion without causing a change in the fit between the piston skirt and the bore. The greater the contact between the skirt and the bore, the less wear on the piston. As an exaggeration think of a 2" pipe sitting inside of a 3" pipe. The contact between the 2 would only be a narrow spot. If you took a piston that was .005" smaller than the bore and sat it in the bore and shone a light down the side of the bore, you would see a narrow contact point between the piston and the bore. Ideally you would want to see at least a third of the piston contacting the cylinder wall both front and rear to displace the load across a greater area, but you still need to allow for piston expansion. Something has to give somewhere. So the piston is generally narrower across in the plane of the wristpin. Also the diameter at the top can be between .005"-.010" smaller above the ring lands than at the skirt. This also allows for expansion. The piston skirt will generally run at about 200 deg. F (oil temp.) and the crown can be as high as 1000 deg. F. There's a formula for figuring out the coefficient of linear expansion which would explain all this, but that's more for the engineers.
All the machine shop will do is fit the pistons so that the clearance front and rear is as spec'd in the service manual. Usually aftermarket pistons come with a size spec and fitment spec, since the originals may have been a forged piston of a certain alloy and the replacement piston may be a cast piston of a different alloy. Again, the coefficient of linear expansion may be slightly different between the 2 alloys and forged piston tend to expand less than cast pistons. It's a grain structure thing.
Just make sure you're working with a really good machine shop. Typically they would bore to .005" less than the piston measures and then precision hone to the correct clearance. That way they remove the boring tooling marks and finish hone with a really nice cross hatching to assist with breaking in the new piston rings. If they're not using a precision hone to finish off the job, walk away and find another shop.

I just got my 1300 running this summer after installing 64mm pistons. The main reason 64mm pistons are out there is because 64mm is a common ring size. Your pistons are most likely cast, but I'm just assuming they are an e-bay "big bore" kit. There should be a spec sheet with them that gives instruction fro fitting them. It is very critical that your machinist follows the piston manufacturer's instructions for recommended clearance and proper measurement of the piston to cylinder clearance for the reasons kawboy wrote about in the above post. Do not use the Kawasaki manual for this information, always use the piston manufacturer info when installing aftermarket pistons.

Having said that, there is really no big deal having 64mm pistons fit to you engine. That is about the practical limit for boring the stock sleeves so a future overbore is not possible. Make sure you pay as much attention to your valves as you do the pistons and cylinders, a good valve job and new seals are in your future as well.

Good luck and keep us posted!

I did a 64mm piston installation on mine. A local machinist punched out the pots to the correct size; gapping the rings was a little tedious and installing the block over the pistons required four hands and lots of cursing but nothing too bad.

I'm putting the engine back together as we speak so I can't comment yet on how it performs but I don't see any obvious reason it would fall on its face.

Thanks for that info fellas.

Tyler, now I understand what was meant about about oval pistons.

Now I need to find the instructions on the bore. I hope the info is inside the boxes I haven't opened