650 to 750 conversion

I got a hold of a set of KZ750 cylinders off a 83 spectre. I'm also rebuilding a 77 KZ650B motor. I'm wondering if I can put the 750 pistons and cylinders directly on the 650 bottom end and use the 650 head with no problems or are there going to be issues with fitment?? What about the cam chain tensioner?? If it all works then I might just send them off to ape and go for 810. Might as well go the whole nine yards right?? Any help is appreciated. Thanks guys.

Also what is the advantage of a 750 head?? If any. I'm trying to build a solid street engine that will put a smile on my face but is still dead solid reliable. Balanced and smooth is much better then high horsepower here.

Becker wrote:

I'm wondering if I can put the 750 pistons and cylinders directly on the 650 bottom end and use the 650 head with no problems or are there going to be issues with fitment??

The short version: no.

The long version: Apart from the 750 sleeves being bigger than the 650 sleeves meaning you will have to grind the cases at the cylinder base to make the 750 block fit physically the 77-80 650s use a single roller cam chain with cam chain sprockets, one at the rear cylinder block top, one at the front cylinder top and one at the top of the head.

The 750s use a multilink (hy-vo) cam chain with sliding blocks, one at the rear of the cylinder block, one at the front of the cylinder block and one sitting in the valve cover.

Apart from the 80 750 (and possibly a few 81 models) the 750 cylinder blocks do not have the provisions for the cam chain sprocket shafts so they won't work with a single roller cam chain. The only way to get around this is to use a 750 crank, post-80 750 cylinder block, 750 valve cover, 750 cams (or cam sprockets) and a 750 multilink (hy-vo) cam chain which basically means that you have to rebuild your 650 engine into a 750. It can be done if you really want it, but it would be far cheaper to get hold of a complete 750 engine that will bolt right in.

Or even better, a complete GPz 750 engine, it bolts right in your 650 frame with a few spacers (Steell will know) and gives you 85 crank hp out of the box.

OnkelB, I've done some more research, and had the opportunity to tear down a few more 750 four motors, since we had this discussion.

At this point in time it looks like the 80-82 KZ750E and H models have the provisions for the 650's roller cam chain. The 83-85 GPz750 models definitely do not have these provisions, 83 Spectre I don't know.


Becker, post a pic of the top of the cylinder block and we can tell you for certain if it will work, and at the same time expand our knowledge base.

I have my camera sitting next to me, and a 81 750H cylinder block sitting on the floor next to me, so I took a pic

If the top of your cylinder block looks exactly like this one, then you're good to go. The difference is the front of the cam chain tunnel.

Rear may be different also, I forget.

Here is a photo of the block and pistons that I got. It looks like it must use a hyvo chain because I can't see any provisions for the rollers. I knew there was something goofy about the chain fitment but I wasn't sure what it is. Well I guess I'll just have to keep it a stock 650. not that big of a deal. Thanks for all of your help though.

you can always have the 750 sleeves installed in the 650 block.
but you will still have to have the upper case milled to accept the larger sleeves.

Now we know that 83 750 Spectre cylinder blocks won't work on 650's.


All 750 fours use a HyVo cam chain, it's just that some of them also have provisions for the roller chain of the 650's.

steell wrote:

OnkelB, I've done some more research, and had the opportunity to tear down a few more 750 four motors, since we had this discussion.

At this point in time it looks like the 80-82 KZ750E and H models have the provisions for the 650's roller cam chain.

Good info Steve, I didn't know that the provisions on the 750 blocks were present as late as 82.

Oddly enough, while the 400s/650s/900s/1000s and the GPz line were very popular over here (Denmark), the 750s never really caught on - I'm 49 and I've only ever seen two in person, one four and one twin, guess the CB 750s covered that market.

All you need is a crank to make it a 750, and some cam sprokets. Right now there is a ebay kz750 crank with chainsand rods for a buy it know for $24 and shipping is $34. Not too shabby of a price if you ask me.
You can bore the top of your case with a dremmel dont have to be perfect cause you have dolls to line every thing up.

cgi.ebay.com/ebaymotors/1981-KZ-750-Cran...fPartsQ5fAccessories