Automatic gearbox overhaul

In the first of a two-part feature we show how a Borg Warner 35 is stripped down. Part two covers the rebuild.

During the ‘Sixties, the Borg Warner 35 Automatic Gearbox was used in numerous cars. It consists of a three- element hydro- kinetic torque converter and a hydraulically operated gearbox comprising a planetary gear set providing three forward ratios and a reverse gear. Generally, as on the Daimler V8 (from which this particular BW35 is from), a cooler for the hard working transmission fluid is incorporated in the bottom of the engine cooling system radiator and consists of a pipe running through the water- based coolant.

The BW35 is generally a pretty reliable unit but as with all auto boxes, after a while it will require rebuilding. Basically, this job is best done under the supervision of someone who has experience working with automatic gearboxes or religiously following the appropriate workshop manual. This month we start with the strip down, which is a fairly straightforward process. No special tools are needed for this part of the overhaul but a good tip is to always have a selection of trays available to keep the various nuts, bolts and minor components safe. Also, it’s a good idea to label all the parts and the trays too. That way, when your spare parts arrive a few weeks later and the rebuild gets underway you hopefully won’t be caught out knowing what bit goes where.

It’s also a good idea to fabricate a cradle to rest the gearbox on, so when you turn the gearbox upside down the case will remain stable. This will also assist accessibility and avoid damaging components. Before starting work on the gearbox, be sure to check the torque converter. Fortunately, in this case the converter was free and wasn’t seized as some commonly are. When removing the front clutch, watch out for a broken diaphragm. This can often break up, as has happened in this gearbox. Eventually, a broken diaphragm could break up and it would then very probably jam the forward clutch.

Another point to check is where the linkage connects, as this model Borg Warner gearbox tends to suffer wear on the cross- shaft.

2Next, the transmission oil filler tube and dipstick should be removed and set aside. Now disconnect and remove the kick- down cable. The gearbox is will be less clumsy and more manageable when the casing is inverted.

1The first thing to remove should be the bell housing. Using the correct sized socket, undo all the bolts that secure the housing to the gearbox – all the way round.

3Now the reversing light and inhibitor switch should be removed. You don’t want to leave this item in place, as it is easy to damage the terminal connectors whilst you are working on the box.