Hydramatic gearbox on the Bentley S

This is part one of hopefully just two parts, although that may be optimistic. The plan last summer was to buy the 1957 Bentley S, get it transported home, bleed the brakes and take it for a drive. Several months later, the going-for-a- drive part of that plan is still a distant dream. The previous owner, my elderly friend Robin, had been a marine engineer and had owned the car for a significant percentage of his life. He used to drive it to work, but took it off the road to sort out the brakes and a few other minor issues. He vaguely thought that had been ten years or so ago, but the paperwork says it was in 1989. Time flies when you’re having fun.

The tranny fluid had been drained in the initial flurry of activity, and Robin believed he had replaced it. I wasn’t able to test it out though because the ancient brass carb floats had split, scuttled themselves and sunk rather than surrendering. I soldered them up to functionality, refitted them and persuaded the engine to run. It now mumbles quietly to itself, accompanied by an exhaust blow. The supposedly re-rubbered brakes were as yet un-bled, but I could at least drive the car for a few feet on the driveway.

Only no, I couldn’t. The elegant one-finger electric gear selector seemed to select as suggested, but forward motion came there none. Backwards with some revs got the hint of a feeble lurch, but that was all.

Usually, a lack of drive from an automatic transmission means low fluid. Okay, Robin said he had replaced it, but let’s check: there’s a dipstick under the carpet. A dipstick with no fluid registering even on its very tip. Either Robin forgot to fill it up again in 1989, or it has all drained out unnoticed over the ensuing 30 years. So I had been trying to drive the car with the transmission bone dry. That, as they say in medical circles, is contra-indicated.

The guts of automatic transmissions are well outside my comfort zone, although I have tackled some major engine work with possibly misplaced confidence in the past. I know vaguely how trannies work, but this one apparently has a fluid coupling rather than a torque converter. As I don’t actually know why a fluid coupling is not a torque convertor, I decided to find some tranny magicians to overhaul the box.

The first recommended transmission repair shop kept the car for a bit and then called to tell me it wasn’t a GM Hydramatic gearbox. Oh yeah? Bentley Motors Ltd would beg to differ. Given that shop’s scary level of expertise, I retrieved the car and wrangled it past the two-year restoration queue into RWM and Co in Delta.

Bentley parts prices can sometimes make Porsche look like Walmart, but there are options. The General Motors Hydramatic is an American transmission, fitted to various Cadillacs and Oldsmobiles from 1939 on, and used with minor mods by Rolls and Bentley from 1946 until 1967 in Silver Dawns, Silver Wraiths, Silver Clouds, Silver Shadows and Bentleys from S1 onward. There’s a mechanical clutch on the side of the gearbox that operates the mechanical brake power assist mechanism, so there are some different internal parts, but Dave at Autotran in Massachusetts sells an overhaul kit which is specifically for the Rolls/ Bentley version of the box. It’s more expensive than the Cadillac kit, but cheaper than what it would cost shipped to me in Canada from the UK.

I’ve taken the engines out of a couple of Bentley chassis myself, but getting the complete engine and transmission out of the Bentley and back in exceeds the capacity of both my engine hoist and my cracked garage floor. In any case, I’m supposed to be busy chopping the roof off the doomed Silver Cloud that rolled straight into my garage when the Bentley left. So I added that work to the bill for stripping and de-rusting the transmission and removing the brakes, which came to 114 hours and $10,000 (£6000), even with a discount.

Sitting empty of oil in an inevitably damp garage had resulted in major corrosion inside the gearbox, and all the valves that should move under the pressure of a pinky finger had to be delicately drifted out of their aluminium housings before being de-rusted. The alternative would have been £3500 from Flying Spares for a rebuilt transmission, but shipping it to Canada would have meant being gouged by UPS or Fedex and the border tax clerks, and my rusty core may not have been accepted.

I later learnt that it would have been okay if complete, so that could maybe have been the cheaper option in the end, but mixing it with the RR big boys is expensive any way you play it. I keep reminding myself that this car is a demonstrator for the Ayrspeed RR Cloud and Bentley convertible two- door conversions, and selling a couple of those will make ten grand look irrelevant.

Buying the rebuild kit in Massachusetts meant paying Oldsmobile prices rather than RR prices, but even so it was US$450, or £350. The early Hydramatics also have a few obscure different parts from later ones, so there was more chasing down for the final seals and springs in the US and UK, with phone pictures and measuring- calliper action. Hopefully though, I now have everything I need.

The Bentley is filleted. Iain must remember to check the condition of the water gallery plate on the back of the cylinder head while the engine is out – it almost touches the bulkhead, so if it starts leaking, the engine will have to come out again.

The Bentley really is magnificent and very handsome piece of kit. Iain reckons it will look even better with two doors and a soft top that follows a slightly lower version of the same roofline.

The automatic gearbox is a Hydramatic unit from General Motors, but with Rolls-Royce alterations such as the mechanical power assisted brake servo on the side at the back. If regularly used, it’s a good reliable box.

What goes on inside an automatic transmission makes sense when you look at how the pieces work, but that’s one aspect of life Iain is more than happy just to leave to the transmission magicians.

Looks as though he needs a new front oil seal on the diff. It can be updated to a modern oil seal with some machining, but a new felt seal is six quid and that will do for the moment.

Bentley B171DE must have been built in late 1956, and there were minor changes in the transmission used on the straight six, while the SII with its V8 uses a different Hydramatic. Getting all the right parts is possible, but it can be a tricky undertaking.