TOPIC:

I'm building a mild kz750 twin engine, and i need some help determining how to setup the top end. I'm using custom forged pistons that I got used from a member here that are slightly higher compression, but same diameter as stock. In the image the forged piston is on the left, stock on right.

I measured everything and I then assembled the new pistons, and did a dry install of the top end.

The combustion chambers were not exactly the same size, one was 53.9cc and the other was 53.2cc. stock is supposed to be 50.9-52.1 I had a valve job done so I'm assuming that is why its bigger. Is it worth the trouble of trying to make them the same?

Assuming 53.9cc for the combustion chamber I am at about 8.9/1 compression ratio. I would like to be closer to 10.25/1. There is about 0.090" clearance from the piston to the combustion chamber, so there is ample room to deck the head.

if the combustion chamber were a cylinder I would have to remove about .060" to get the combustion chamber small enough, but its hemi-spherical so even more would need to be removed to hit the mark. That seems like a lot to remove off the head. If I remove .040" it will be pretty much cutting into the cooling fins. At .080" the intake valve would get hit.

My new pistons have a squish band on them I'm wondering if I should instead fill in the combustion chamber and machine it back to give the head a proper squish band, this would not only help me reach my combustion chamber size goal, but a proper squish band will make the engine more efficient, with a "bath-tub head". Or should I just deck as much off the head as possible? I could also deck the cylinders to bring the piston to zero deck height. That is much easier at this point since the head is assembled.

Thoughts?

I would bring the pistons up to zero deck first.
Doesn't the KZ750/B have a ridge at the edge f the combustion chamber?


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I double checked all my measurements and I realized when I measured the deck height I forgot to torque the cylinders down, so i measured incorrectly. They are actually at zero deck height. Updated numbers are below, now at 9.27/1. This is better, but still makes it a ways to go.

There is a small ridge in the middle of the combustion chamber. if I weld the combustion chambers up I think I would smooth that out while I was at it.

My main concern is that the squish clearance is huge right now (.090"+) I have an extra head to mess around with, so I may try welding up the combustion chambers on that one and see how it goes. I'm not sure the best way to cut the squish band. I have a brand new 4-jaw chuck for the lathe, but I'm not sure if the whole head would fit on there. That seems like the easiest way to cut a taper for the squish band but that much mass off-axis freaks me out a bit. I'm not sure how I would do it on the mill.

In case anybody is following along here is the latest update.

I got the head back from the machinist. He removed .050" which is as much as is possible considering the last pass ever so slightly skimmed the edge of the intake valve. I think it is only possible to remove this much because I had a 3 angle valve job done, which sunk the valves in a bit.

New combustion chambers are 47.7cc and 47.8cc which brings the compression ratio to about 10.6/1 which is a little higher than I was aiming for but totally fine.

I clayed the head and the combustion-chamber-to-piston clearance is now about .030-.040" around the edge which is much better. The clay i used was too sticky so it was hard to be super accurate, however it looks like there is plenty of room.

The only problem now is that because the head sits closer to the pistons the exhaust valve has pretty much a zero tolerance fit on the edge of the valve pocket, however there is ton of room for valve lift, 0.220"! I'm thinking about taking a dremel with a small sanding drum and just whisking the edge of the valve pocket to make room.

That is close but it looks like there is some meat you can remove.