[David C]

I have just been asked for details of fitting an overdrive gearbox to a V8. A fair bit of the information will be elsewhere in the messages but I have tried to bring it together as follows:


I fitted an A500 42RH transmission to a 410 that I think was originally in a Dodge Durango. Another 410 has an A518 46RH, probably from a Dodge Dakota.

A500 and A518 are series numbers with subsets 40RE, 42RE, 44RE, 42RH etc and 46RE and 46RH. The higher the number the stronger the transmission. For a 411 or 603 you are probably better off with a 46RH. The RH part is very important as it is hydraulically controlled as opposed to the RE which is electronically controlled. The RH models can be simply automated with mechanical switches but the RE models will need a separate electronic controller to operate overdrive and lockup. My conversion uses two pressure switches to enable overdrive and lockup, a vacuum switch to disengage overdrive and a half throttle switch to disengage lockup. PATC in the USA provide a kit of switches but I managed to source some in the UK which I think make for a neater and more convenient installation.

These transmissions bolt directly to a 410 - although I had to trim the bottom corner off the block to get the starter to fit - but will need an adapter for the bigger block 411. They should also bolt directly to the small block 360 in a 603 but that needs confirmation.

The crossmember needs the top half removed in the middle. I had a thicker 6mm plate welded in place to restore the structural integrity and then adapted the Dodge mount to fit.

The vertical rod for the shift lever needs to be further from the centre line of the transmission than the original. On the 410 with 46RH it was canted over but this made adjustment a little tricky. My gearbox tunnel was significantly modified to keep the shaft vertical and works well mechanically but is not so neat. I think the ideal solution is between the two choices.

The speedometer cable is an easy fix. Ready made short extensions are available but I chose to fit a right angle drive which works perfectly with the original cable.

The original cooler lines are 5/16" but the overdrive gearboxes increased this to 3/8". However the pickups they were used in were nearly twice the weight of our cars and regularly used for towing. I have retained original lines and cooler and added an increased capacity finned sump but am fitting a transmission temperature gauge to confirm whether this was the right decision.

The replacement transmission will need to come with a lockup torque converter. This introduces the choice of stall ratio which caused me some worry. The unit which came with my gearbox is made for 2300RPM which I thought a bit high but it seems to work well.

The prop shaft will need to be shortened and balanced as the overdrive transmissions are about 10cm longer than the 727. I had this done by Propshaft Services near Heathrow and it was reasonably priced and quite a quick turnaround.

There is only one overdrive ratio available as far as I know. This is 0.69 : 1 and results in a very relaxing 1600RPM at 65MPH with a 3.07 rear axle ratio.

The 42RH has a lower first gear than the 46RH or the original A727 so it's a win /win with sharper take off and considerably lower cruising revs.

It's a bit early to give an accurate indication of fuel saving but first signs are promising.

Having located a donor transmission there are a few minor modifications that are worthy of consideration if rebuilding the 'box.
Sonnax make a manual valve that allows the torque converter to fill in Park. It is suggested that this is fitted along with a lube pressure regulated valve.
The plastic pistons fitted to these transmissions can fail with time. Billet aluminium servo and accumulator pistons seem a good plan and the accumulator springs are known to fail so should be replaced with new.