KZ650 B2 Light Weight rebuild

I am up late thinking about building a light weight KZ650. 

I have that 831cc street monster motor sitting in the corner, glaring at me. 

I have a B2 frame with papers. It was going to build a completely stock B2 (have all the parts) but now I am hankering for the "stripped" down look.

Won't be cutting tabs etc. but will loose weight the old school way (remove steel parts) as I don't own a welder. I still have a kick stand mount tab on my drag bike.

I also want it to look period correct, so its not going to be a "bobber" etc. as I want that iconic Z tail piece. 

I have the wheels stripped down, and new spoke set ready to go. Steel wheels and fenders too heavy to assemble on this one, so I will save them for another time. 

Been holding off on buying the aluminum rims, but recently changed my mind. The rims were in Italy the last time I looked. 

I have a fiberglass 19" front fender, will make my own fork brace. I have a "cat eye" tail light and fender "template" sitting on the shelf. 

I have the lathe and mill in my garage, missing years ago when I should have done this. Will try to fab some alloy replacement bits to get the pounds out of the old girl. 

I have been pushing around that bike I set up for Touring, its a C2 with hard bags/fairing so about as heavy as a 650 can get! 
I want something I can push around a bit easier (says the guy who recently put in a Z1 engine!) 

The B2 was getting to look like it was never going to be a stock 650 (no "complete" new chrome exhaust) It was slated for a 720cc Wiseco barrel (ready to go) and some 29 flat slides (in pieces) which may be in it some day. 

For exhaust I have 4 into 2 MAC head pipes that I modified to fit Universal "shorty" 17" megaphone mufflers (custom baffles wrapped this Summer) I like the "balanced" look of the dual pipes, plus they are spares from the first used $200 special I came home with! If I do find a suitable header that may go on as well.

I have to search my PC for some old pics of that B2, Think I bought it in 2012 at the swap meet. 

It's been a lot of years since I have ridden a stock 650, one day I will build one, and it might be nice to have this lightweight version to stick it in.

looking forward to seeing it take shape, you always do great work

Sounds like a great project, looking forward to seeing the photos as it undergoes its development 😁

Just thought I would get the ball rolling on this but not sure on a timeline. Here are the pics I found of the day I bought the bike.





I took the tank, tail piece, side covers, and the seat off for the pics, all that was missing was the headlamp/bucket. It was already missing turn signals (a common mod)

The engine ended up being a low compression dog but it was free to rotate, just crusty with white corrosion.

I plan to cut down the stock seat foam, maybe do a aluminum or fiberglass solo seat pan. Fabricate a fiberglass headlamp shell. Alloy DIY rear set controls, minimal wire harness and smaller battery etc.

It came with a fairly clean tank and mismatched body pieces. All usable parts.

You could call that a blank canvas.  We'll follow along as you build it.    

It might be a bit before this project takes off in earnest but here is what I am planning :

There are quite a few non critical parts that can be swapped out for aluminum. I will be getting a scale out and tracking the differences. 

Everything that needs replacing will be fabricated from alloy as there are not too many things you can purchase from the aftermarket that work on a 650. 

The steel fenders, rims, and front brake parts are bit ticket items. Modern brake part equivalents are much lighter. 

I may opt to drop the front wheel down to 18" as the tire may weigh less. I still want it to look like a vintage bike so nothing smaller. 

Stock forks but they can be turned down on the lathe to save a bit (have a drag bike like that) 

Lowering it for sure, I do that to all of mine. Shorter shock weigh less. 

Steel seat pan will be replaced, heavy turn signals replaced or eliminated altogether. 

Steel battery box will be replaced, have had a aluminum one on my drag bike since building it. 

On the 831cc engine my 33 flat slides are considerably lighter than the stock carbs. I did a weight check, just need to dig up those numbers. 

Some engine covers can be lightened with holes or machining, prefer the machining over the "Swiss cheese" look.

Starter motor and starter clutch can come off, kick start only.

The stock gauges are quite heavy so they will be replaced. The battery is also heavy, another item I can purchase. 

Some fasteners can be swapped out or heads machined down. Done this on my 1327 drag bike. 

Nothing is off the table as long as its safe. This is not new to me, did this on my first gen 650 drag bike.

I found this 13" Mark Zero road race rotor in my brake parts spares bin. It was on my 831cc 650 back in 1986, I took it off for a smaller diameter when I converted the bike for drag racing.



I think I want to cut the diameter down a bit and machine it to accept floating buttons. From what I can gather the buttons are steel and held on with a cir clip, can make those as well. Will fabricate a alloy carrier to bolt to the front hub. May turn the front hub to get rid of gate flash grind marks.

Looking at used Ducati 4 pot caliper to complete the front brake.

There is a few kgs to save on the frame. 🙂
Maybe not recommended for street use.
If I remember correctly the frame now weights ~10kg.
The whole bike weights 159kg without fuel.
But that’s without generator and starter. Aluminium swingarm and magnesium wheels.

Yeah, my drag bike is maybe 10kg over that (dummy tank/light) The back half of the frame hoop was cut off, and a bunch of steel tabs. Seat foam almost non existent!



Not trying to get down as much because its a street bike. I don't want to do any frame cutting like I did on the drag bike. Plus I can't run total loss ignition because we have a daylight "light on" rule here they enforce.

It still needs most of the charging equipment, and it has to pass a safety inspection, so it needs all that on it till it passes (center stand, passenger pegs, turn signals) Once it passes inspection I can take all that stuff off it.

I think if I can get it down to 420 lbs I would be very happy with that as the engine going in it makes +85 HP at the wheel.

Have you considered titanium rather than aluminum for the replacement parts? Could be used for many of the fasteners and even the axles.

Titanium is expensive for what you get, it has come down in price in the last 10 years, but its still expensive per pound. It's not a option for me because I am on a budget, plus I would not do it anyways.

I worked for a bit in the bicycle industry so I know the Ti weight saving plan is a thick wallet game. Realistically of all of those guys could have just lost 10 pounds, then they would have been miles ahead of the competition!

Same thing goes in the motorcycle world, it only makes sense on +$50K race bikes. I do see people throwing dollars at the machines they own rather than thinking it thru. Hell there are dudes at my local track that have +$30K into there weekend warrior drag bike engines! Not to mention the Pro Stock chassis they don't know how to ride!!

Aluminum is cheap and easy to machine. There are some ways to machine steel fasteners and axles to save weight without replacing them.

That drag bike looks great. Very long intake runners? Did that improve midrange or top end?

About saving weight on axles. I tried once to drill out axles but it was difficult. I found it was easier and cheap to buy hollow axles from a scrapyard.

For the engine mounts I put helicoil in the engine and short screws instead of the long bolts all the way thru.