I got a call from Laszlo asking if I could do a mad cool motor swap.

He'd seen the work I did on Tim's bike and was confident that I could translate his vision into metal. His plan: stuff a desert-racing spec Kawasaki KX 500 motor into a Yamaha TZ 250 chassis.

The chassis is a '91 Deltabox frame, and I'm really impressed with the build quality. If you think about it, Yamaha probably only sells fewer than 1000 of these race-only bikes in a year, yet they went to great expense on the tooling for all the stampings. If you look carefully at the picture, you will see that there are no less than 17 separate stampings in the frame. That's not even counting the swingarm! It's no wonder that virtually every manufacturer has gone to beam frames now - significantly less money in tooling and massively reduced labor for fabrication.

The first task at hand was to hang the motor in the chassis so that the final position of the sprocket could be determined. My way to accomplish that: a clamp-on head stay. Initially, the plan was to use a temporary clamp-on stay for positioning. The finished product fits so well and is so stiff that there's no reason to weld tabs to the cross memeber.

The next task that could be undertaken was the rear motor mount plates. Laz and I discussed this and agreed that beefier is better when it comes to a non-counterbalanced, large displacement two-stroke single. Once the original motor mount tabs and lugs were removed from the frame (notice the pile of gray stuff on the floor? That's what's left of the tabs!), a pattern was developed in wood. The wood pattern allows me to check clearances - particularly for the swing arm through the range of travel.

Once the pattern was done, I started cutting the 1/2" aluminum plates by first welding them together to ensure an indentical pair. A very large pile of metal shavings later, and VOILA! motor mounts!

I turned up some aligning pins to help me locate the countershaft sprocket vertically in relation to the swingarm pivot/axle centerline. This is a pretty critical dimension: too high and the rear will squat badly under acceleration using up cornering clearance, too low and the rear wheel will spin easily. Not to mention that there are chain line and ride height adjustability issues.

Since I got the little Harbor Freight lathe working again, I have turned out *lots* of little press-in bushing for the motor mounting system of this bike. I'm quite pleased with the results. There isn't much nice to say about cheap tools, but these 7x10 lathes will do decent work if you respect their limitations.

Here's the rear motor mount assembly ready to weld into the frame. It's stout and it fits really snuggly to the rear of the motor.

Here's a closeup of the solution for the front motor mounting system. There were 2 considerations: first, it had to be strong and stiff; second, it had to allow for the removal of the motor. The detail picture above shows one side of the system -- there is an identical pinch clamp on the other side. The spacers are turned from alumium and have pressed-in steel bushings to prevent vibration damage.

This is the completed frame showing the front motor mount pylons and the rear mount.

The final part of the project was to make a starter motor drive dog for the kick start lever. The tab which restrained the kickstart lever to half-a-turn was ground off, and this 5/8" hex was added. The idea is that an electric drill (if it's powerful enough) can be engaged on this hex and used to turn the motor, rather than relying on rollers.

The KX/TZ project is almost complete - I just need to do some little things and it can be delivered to the client! He's offered to let me ride it at a track day -- I can't wait, this should be a really fun bike to ride!!! Check back next season for a ride impression.

I wrote the last paragraph over a month ago, and things didn't work out that way. When I attempted to put the motor back into the frame, I simply couldn't get the magneto cover to squeak past the left motor mount pylon: it was too close by 1/16". I considered several options: grinding the magneto cover, grinding the pylon or cutting off the pylon and re-designing it. I opted for the latter option because it was the right thing to do. It would be a shame to get this far, with this level of build quality and do something cheezy to work around an error.

At the same time, my friend Dano, who built a TulDa (Dr. Rob Tuluie's CR500-powered racebike) said he thought the frame would crack due to vibration where the pylons met the frame. We discussed the options, and I decided that some massive braces would help distribute the load across a larger area of the frame.

This photo shows the bike upside down (for welding the bottom) and the new curved pylon design with the braces can be seen. Now, it's finished!