M40, M41 and J-type overdrive
When the gears start to sing
When I opened up the gearbox, I found exactly what such a noise often betrays: metal particles and clear wear on the components. Not a matter of a quick adjustment, but of thoroughly investigating what was really going on. It was an M41 with J-type overdrive, but it was in a condition that no longer really justified a further rebuild. As it happened, I still had an M40 of my own with components in considerably better condition.
That sounds simpler than it is. The mainshaft of an M41 is considerably shorter than that of the M40, because of the space the overdrive needs — and since almost everything in the gearbox is built around that mainshaft, this meant both gearboxes had to be completely dismantled. Making one healthy gearbox out of two incomplete ones calls for something other than a standard rebuild: assessing each part separately, deciding component by component which of the two gearboxes had the best condition, and then bringing everything back together in the right combination. One of the most difficult parts was tracking down the right bearings — not every bearing is still readily available, and finding the right size and quality took considerable time and perseverance.
The J-type overdrive itself naturally received an equally thorough rebuild. That is a different kind of precision work from the gears and synchromesh in the rest of the gearbox, because the overdrive works largely on hydraulics and oil pressure. I replace all bearings and O-rings as standard — a leaking O-ring may seem trivial, but can easily undo an otherwise perfect rebuild. I also check the pistons extremely carefully for wear and a good fit, and assess whether the brake ring is still serviceable or due for replacement. The latter is often a challenge in itself, as parts of this kind are hard to come by. I also inspect the planetary gear set thoroughly, component by component. And because the entire operation of an overdrive revolves around oil pressure, working cleanly is at least as important as rebuilding: dirt is a real killer here, so I also carefully clean the micro filter.
The result is a gearbox built from the best parts of two gearboxes, with the overdrive fully functional once more. The owner is now driving again with a confidence he had lost for a while — and, as is usual after a rebuild of this kind, has a few "teething troubles" to get through. The running-in period now needs to do its work to make everything operate smoothly together, but the foundation is solid again.
Jobs like this show that a rebuild is not always simply a matter of replacing parts. Sometimes the solution lies in combining what is there, provided you take the time to judge each part on its own merits — whether it is an M40, an M41 or another gearbox from the classic Volvo range.
