Advise on fitting a S1 engine to a Derby

Oliver Wright · Joined: 23 Jan 2018 Posts: 6

Hello All

I have recently bought a 1934 2 door Thrupp and Maberly Derby. Sadly at some point in its life (whilst in the US) it lost its engine. It currently has a GM engine fitted Sad

I have been offered a 4.8 S1 engine and on the basis it originates from the 3 1/2 litre unit, I wondered if anyone has any experience of fitting a 4 1/4 manual box to the later engine.

A 4.8 would make quite a lively car.

Any advice gratefully received.

Thanks
Oliver

Oliver

Christopher Carnley · Posted: Thu Feb 08, 2018 7:49 pm Post subject:

I can provide all and any advice, so ask the questions.

Have you got a starter for the engine? Flywheel? Bellhousing? Gearbox and selector?

The 3 1/2 litre engine derives from the R-R 20 and 20/25 engines, and far distant from the rationalised post war cars.

The production 3 1/2 engines when factory tested with the complete exhaust system fitted only generated between 82 and 87 BHP.

The rather weedy back axle may not cope as the halfshafts are on the slender side, you should see the size of the pinion and crown wheel.

Oliver Wright · Joined: 23 Jan 2018 Posts: 6

Thanks for your reply, based on your answer I expect I am asking for trouble fitting the later engine without upgrading the rear axle etc but this certainly could be an option.

Unfortunately I do not have any of the original engine or box so I am starting from scratch.

The box fitted currently is linked up to the original selector but it's an auto unit so the clutch pedal has been removed. I have located a new clutch pedal and I have found a 3 ½ litre gearbox requiring rebuild but that is it. I have also got wind of a dismantled 3 1/2 litre engine in the US but further investigation is needed. Is there a resource to check engine numbers anywhere?

If anyone has any parts to assist in this project that is not going to cost me an arm and a leg, please let me know, I am not in any rush.

Many thanks
Oliver

Christopher Carnley · Posted: Wed Feb 14, 2018 10:39 am Post subject:

Oliver,
Start your Derby experience by obtaining "The Derby Built Bentleys" by Bernard King, second edition, available on ebay, and Amazon.

Parts are very scarce and any available are expensive. Had you considered a post war 4 1/4 litre MK VI engine?

I recently spent many many hours over a 3 year period building up a Derby
4 1/4 litre from a jumble of parts for a client, and I had a difficult time finding all the missing bits. Also, there are some bottom of the barrel scrapings up.