Well unfortunately I wasn't running the stock carrier, i was running the vertigo one
with brand new bearings, shimmed up with minimal play and didn't even get time to heat up in most cases (I am talking literally 3-5 mins runtime in some instances). I tried a different carrier and it still happened, tried stock gearing, different bells, overhauled drivetrain with new bearings and checked for drag or binding - all to no avail. Basically the drivetrain (which includes that pesky little 4mm bolt) was designed to take the stock engine's 2 odd HP and low torque, now we run 4-5 HP engines with truckloads more torque without even blinking an eye. For me, with my setup the 5mm bolt fixed the issue, and it will for all those in a similar situaton to me.