Kz750 Cylinders - How big can you go?

You need liner even if you want to nikasil plate because the liners need to drop down into the top case. My old man burnt up many a 2 stroke motor when he raced the 250cc clase (micro sprints, maybe?) when they went to nikasil plating he never needed a re-bore (just use acid to clean out the burnt piston bits, new piston and rings and back on the track for the feature).

Injected wrote:

I think the physical limit on the bore spacing is 73mm and with a stock stoke that = 904cc.

I am running JE 70mm = 831cc, but those are custom and I believe they are in the $1200 range now... prices for that stuff shot up in the last couple of years.

Recently I had a block outfitted for that 70mm setup and it cost $500 to get liners machined, installed, bore/honed. The majority of the cost is in material removal.

Custom liners will cost somewhere around $500 unless you can scrounge 900/1000 liners but they will need machining to prep them ($200)

Its more cost effective to stay with 810cc, I just happened to have the 830 kit sitting around and my buddy said to replace those was over $500, so I used them... ended up costing around $300 more than just getting a bore job done.

If you were into racing I could see spending the $2K, but for a street motor I don't think it makes much sense unless you have the stuff already.

That big bore also requires some cylinder head work, a set of cams, and some bigger carbs to work properly... so there is another $1K minimum... and that's not including the tranny and clutch work that might be required also.

At that price point you could buy a used 900 with a 1075 kit and call it a day !

Thanks for chiming in! I believe it was one of your posts that got the wheels turning for me.

What had to be done for the 900/1000 liners to be able to be installed in the block? And at the price for pistons...I understand why most people are just going with the 810cc kit.

Lorcan wrote:

My 904 block has 17mm pins, custom rods, 73mm high compression Z1000 pistons and is designed to run on methanol.

The 880cc kit was sold by Mike Chestnut of HPU. To date, the 810cc kit is more reliable and faster

And that puts the seal and stamp on it.

Do you have any pics of the 904 block that I could add to my cycle porn?

Thanks for the input, Lorcan!

I forgot to mention the top case does need to be opened up to accept the larger OD liners.

The 900/1000 liners need the upper flange turned down slightly so the 1-2 or 3-4 don't hit when they get pressed in. You could put a flat on them but it is easier to do on the lathe. Because the 900 has a longer stroke the bottoms need trimming to the 650 length to clear the con rods.

The bore limit for 900 liners is 70mm which ends up giving 1.5mm liner thickness after boring (a little bit more than a 810 kit in a 750 liner) The bore limit on the 1000 liner is 73mm - not sure what the wall left is on that one, but it has to be close to 1.5mm (note that the 1000 liner will not fit in the stock 650 block without breaking out of the casting = big block)

I am not sure how thin the casting is around the liner - can't measure it with any accuracy because of the cooling fins, but it's thin for sure... beyond what Kawasaki designed for that motor.

Most of the work is in boring the block... the 900 liner flange just clears the stud dowels in a couple of spots.

If you want to make a billet big block for the 650 I already looked into it... $500 for the aluminum and another $2000 to machine it, unless you can do it yourself over a few weeks. It might be cheaper to get a run of them sand cast, but I have not looked into that.

So it looks like the practical limit for the stock block is a 70mm bore... with 69mm being the cheaper way to go.

Awesome info! Thanks!