As I continue to develop/test/crash my 3TJ/Thundercat/Aprilia hybrid the inevitable has happened and it has shat second gear. Fitting another motor could easily end up in the same place after a couple of track days but swapping the gearbox looks like a family sized pain in the backside, not to mention expensive so I want to make sure the whole gearbox is good once its all finished. I can get a mint late model FZR 600 gearbox but don't know if this is the same or at least a suitable swap for the Thundercat.

Can anyone advise?

Also apologies for a 600 question on a 400 forum!

Not sure about the FZR / T'Cat changes.

To be honest why not get a cog made up by Nova that way it will stronger than the standard. Stripping the motor means everything off (unless you didn't put in the 2 bolts only accessed when the barrels are taken off), it's a straightforward job though.

The axles and their bearings for the 4TV (Thundercat) are different than the 400s, ie 1WG, 2TK. If you have measurements of the sizes of the axle, bearings and such, confirm it against the 400/600 bottom end. I heard the Thundercat trans has been beefed up to fix the common issue with the 600 on a 400 gearbox. Don't know the specifics on dimensions, me.