IPS System,

Can anybody tell me how to lock the gearbox shaft to remove the clutch plates?

Are you asking how to hold the vertical shaft so you can remove the lock nut on the top of the piston disc assembly?

Yes Sbsga,
dosen't seem any obvious way to lock the shaft. Any ideas?

Is the drive still in one piece or have you separated the upper and lower gear housing?
May I ask why you want to get to your clutches and just so you know there are special tools involved in removing the vertical shaft from the upper gear housing. I use an impact gun to remove the nut. The rolling resistance of the gears is enough to keep the shaft from spinning much. I don't recommend self service on IPS drives, I've built a boat load of them and I've also seen the parts bills reach 20,000 plus dollars to rebuild them more than once.