Rob Bunyan's YZF600/FZR400 Conversion

Rob Bunyan managed to stuff YZF600(thundercat) engine internals into FZR400 cases, and into the FZR400 frame. It's a bit faster than stock........

Here are some pics: Picture1 Picture2

Here's the lowdown:

Cases: Most important is the oil gallery. Drill the oil gallery as you would on the 400, but don't put the restrictor jet back. The reason is that this oil port, on the 400, supplies only the main bearing, but on the 600, it supplies the main brg, and both bigends, and the oil squirters. The oil squirters themselves are fitted to late model 400s, but they can easily be fitted to early ones.
- The clutch drive gear on the crank is biggger on the 600 than on the 400. You need to grind some clearance in the bottom casing for it.
- The YZF clutch basket is bigger. Grind somme clearance for it in the cases, and also the clutch cover. You could use the YZF clutch cover, but the oil filler will be difficult to use in the 400 frame.
- The 400 barrel studs are too short. Use FZZR or YZF600 studs. CBX550/750 studs will also work. In all cases you will need to drill and tap the cases for the fatter studs.

Crank: Find a machine shop that can grind tapers. Industrial chromers can usually help. Grind the flywheel end of the crank to use the FZR crank seal and flywheel. At the same time, if you are using the YZF clutch, get the little "T" piece that fits in the end of the gearshaft ground down.

Starter: Either 400 or 600 starter clutch can be used. The 400 one is considerably lighter, though, so the motor will rev faster. Either starter can be used, but note that the YZF engine inlet water pipe has a flat on it to accommodate the bigger starter.

Oil cooler: The YZF oil cooler can be used. Make an adapter plate from 4mm alloy, cut a keyhole shaped hole in the middle, and seal everything with silicone. Get some 8mm steel pipe, and make a tapping off the water pump discharge pipe, to copy the YZF part. Do likewise on the radiator tank, using 8mm alloy pipe. Obviously, you will need a bigger radiator with this engine, irrespective of whether you use the YZF oil cooler.

Airbox: The YZF airbox won't fit. Cut the base off of it, and glue it to the underside of your FZR airbox, then fit as normal. Cut a hole below the filter and duct air in, and vent the float bowls to the airbox too. Increase the ram effect until the stock jetting works.

Gearing: The 600 has a bigger gear on the crank, and a smaller one on the clutch, therefore the gearbox turns faster. This allows the 600 to use the same oil and water pump. THe 400 trans has shorter internal gearing, so you will run "taller" gearing on this conversion than on a stock YZF. I have the same final gearing as a YZF running 15/49. I run 15/41.

Other stuff: Dont forget to use the YZF timing chain guides. Note that the YZF conrod bolts are "upside down" ie the nut is at the top. This is for clearance between #4 rod and the clutch. The YZF cam timing marks are weird. The exhaust cam is set at 12 o'clock, but the inlet cam is set at 1:30. The ignition coils need to be moved up and forward. You need to use the YZF water outlet pipe and thermostat, the 400 stuff will fit the motor, but the carbs won't go on.

After you've done this, it all bolts up. What could be easier?

That's about all I can remember. I didnt take notes, and I started this project blind, so most of these things caused delays and frustration.

Any questions regarding this project please ask Rob Bunyan.

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