Is your PZ4 a metal hull or an ABS one, If it is ABS then fitting Idler adjusters is a doddle and makes the back of the tank look right and work right.
If it is a metal hull then good luck,
I have spent too much time getting the darned thing fixed to an acceptable standard which is why I go searching for ABS hulls as and when I need them,
Trust me cutting out the battery box and leveling the floor with Epoxy resin (Thanks RAD) and dropping the rear roller (x2) and fitting an Idler adjuster (x2) and a gearbox output shaft bearing (x2) is nothing in comparison to the work that goes into fixing Taigens Metal Hull,
Just about every part of the metal hull needs a lot of work,
In the words of Kenneth Williams "It's a Disgrace".
The idler adjusters and the gearbox output shaft bearings are available from
http://www.rctank.deHeng Long PZ4 ABS Hulls are available from
http://www.forgebeartanks.com