I now know why the gearbox died (or rather what is very dead inside it) but first thing's first...
Before I could split the 'box away from the engine, I had to firmly prop the engine on blocks so that I could release it from the crane and use the crane to lift the 'box off.
I put a stout baulk of wood on the flat area just ahead of the bellhousing
And two props at the front:
This arrangement allows the engine to sit very steady and steady enough to remove the 'box. before the 'box can be removed, the hose to the cooler has to be disconnected and then the TDC sensor is exposed and this must be removed. Then it's just five big bellhousing bolts, a careful lift and pull and...
Then, after an hours-worth of careful cleaning with the 'box hanging high on the crane to ensure no contamination when opened up, it was time to strip and finally reveal the problems within.
Sure enough the oil filter is blocked. No surprise there. Interestingly, there are no magnets on the filter case
There was a lot of debris in the bottom of the gearcase, in the region of the filter intake.
But where had it come from?
From clutch B, that's where...
The friction material has COMPLETELY disappeared
Here's a close-up....
You can see bits of friction material but basically, what now looks like a plain steel plate should be friction material.
Clutch B is one of the main clutches and is engaged in every gear except 2nd and 4th. You can also see from the picture that the drum of Clutch B is jammed against warped plates. This clutch has been very hot.
The rest of the 'box looks OK although the oil pump shows signs of a little wear; the peaks of the teeth are worn and all other clutches show signs of wear compared to my other spares 'box.
I reckon what happened is that the debris from the wearing clutches started to clog the filter, and with a worn pump, oil pressure fell, Clutch B started to slip and burned itself out and triggered a chain-reaction in that more and more debris circulated in the oil to block the filter further until it blocked completely and the 'box died. I think the amount the other clutches appear to have worn is normal enough for the mileage. Clutch F shows wear to the extent that the diagonal grooves that are normally seen on clutch friction plates are only just visible.
So, the spares 'box will be dismantled again and all the clutches and the oil pump internals transferred into my cases. I'll also not take a chance with the Torque Convertor as it too is likely full of debris from Clutch B although I don't think it is the root cause. Had I not found Clutch B wrecked, the TC would have been prime suspect.
Good job I have a spares donor 'box as the parts mine requires would cost an arm and a leg.
I'll try to dismantle Clutch B further but it's so well jammed together, it may not be willing.
At last we now know how a 4HP20 dies at high mileages. Seems 150K is the top whack for it
At 100K I reckon a new set of clutches and oil pump/filter would be prudent as a preventative measure.