Motor & Engine Repair 5
Rebuilding a BMW R80 Airhead Transmission
Seven years ago I dragged this 1978 BMW R80 out of a basement in Massachusetts. I immediately found out it had badly rusted cylinders and the pistons were frozen. I didn’t document what happened next, but the top end got completely rebuilt. The cylinders were sent out to be serviced and to accept R90 pistons, upping the compression to 10.5, and the displacement to 900cc. The motor purred like a kitten and ran beautifully. However, about 50 miles later the transmission failed and the bike had to be towed home. Since then the bike has been sitting on a lift with the transmission spread out on a bench.
The time had finally come to get this old girl back on the road. Let’s build a transmission!
Opening up the Motor
As I mentioned the XR isn’t so healthy. Aside from it’s bent axle, oil lines, and the dozen other worn out and broken parts, the motor is also toast. There’s no compression, so I got to tearing it down in a makeshift outdoor garage.
KTM 500mx Piston
This is the piston from the 1988 KTM 500mx I picked up (see the previous post).
You can see the giant gouge that was created as coolant flooded into the cylinder from the blown head gasket. When metal changes temperatures that quickly there’s really no predicting how severe the damage is going to be.
The cylinder has a matching scare on it but is otherwise in good condition.
Unfortunately these pistons are a bit hard to come by these days. Some custom piston houses do limited runs from time to time. There are also a few remaining NOS pistons on dealer shelves and the guys over at KTMTalk.com bump into them from time to time while fishing through bins like old vinyl.
XL600 XR600 Motor Transplant and Revival
In just over 4 hours today I tore down the bike, replaced the motor, cleaned the carbs, flushed the oil, lubed the cables, cleaned the throttle, adjusted the brakes and inflated the tires. The replacement motor starts easy and runs well. I can’t wait for the morning so I can go ripping around.
1982 Kawasaki KZ440 Motor Rebuild
I picked up a Kawasaki KZ 440 that needed some work, though I didn’t know what kind. The bike came with a parts motor, and chances are, if a bike comes with a parts motor, something is wrong with the motor in the bike. When I got home I checked the compression and bingo, 90+- psi on each cylinder. Terrible. I got lucky though that this is the only problem I found with the motor.
Over the next 6 hours or so I completely tore down the top end of both KZ440 motors I had. I visually inspected each part and chose the best pieces I had. I didn’t measure ANYTHING. I even pulled off all the rings, mixed them up, then visually inspected the wear and varnish on each to select the best ones to use. Both motors were used a fair bit but not awful. The motor in the bike had 25,000 miles on the clock, the other I’m not sure.
So I rebuilt the motor in the bike using all the mix-matched parts. I didn’t buy a single new part. I even reused the gaskets. The gaskets got ripped to hell when I was pulling the heads and cylinders, but as long as you clean them carefully and apply generous amounts of gasket sealant on both sides, they can (can, not should) be reused. Doing what I did isn’t generally considered good practice, but for an old bike that isn’t worth much, there’s no point in dumping money into it.