1936 Derby 4.25l Distributor Type

Paul Markland · Posted: Fri Jun 17, 2016 4:06 pm Post subject: 1936 Derby 4.25l Distributor Type

Anyone know the make and model of the distributor. Could it be Bentley's own design or a Lucas Special Equipment ?

Anyone fitted Electronic ignition, either optical or magnetic ? if so whose, what spec and fitting kit used

Many thanks

Christopher Carnley · Posted: Fri Jun 17, 2016 4:20 pm Post subject:

It is of Rolls-Royce design and manufacture, and the small brass label states that the distributor and cover (cap) is covered by several R-R patents.

I do not believe that there are any modern electronic systems for it, but the best way for improvement is to fit a modern 12 v coil and bypass the obsolete ballast resistor.
You will see from the essential Green Book, that the central cam is removeable, for final setting up purposes.The spark plugs in the cast iron head are of 1/2" reach, and modern 3/4" ones can used with a 1/4 inch brass washer.

Paul Markland · Posted: Fri Jun 17, 2016 5:07 pm Post subject:

Thats very interesting, I actually have used a modern coil for many years but I use 3/4" plugs with no washer and have done close to 75,000 miles like this! Interestingly, I am just overhauling the engine on another 1936 4.25l and I noticed just yesterday that he uses 1/2" spark plugs and I was going to change these to the ones I use !!. will not be doing that now.

Both these cars have just returned from Panama where mine had some issues with the ignition which I have finally fixed by overhauling the distributor. During the period when we could not find the issue, electronic Ignition was discussed and I am going to have a go at fitting one and test it if only as an exercise.

As this is probably a unique distributor I will have to experiment with various magnetic sensor systems so your info much appreciated.

Christopher Carnley · Posted: Fri Jun 17, 2016 5:38 pm Post subject:

Paul,

Please keep me informed of your experiments, but so many people insist on originality as there are so many trolly carpers looking over their shoulders.
I am now rebuilding a 3 1/2 engine, with lubrication modifications, and a new alloy head, but the plugs that came out of the iron head were 3/4".

Paul Markland · Posted: Fri Jun 17, 2016 6:12 pm Post subject:

I have 12 of the 1/2" ones from this other Derby so I will put them in and test on my scope. I cannot remember the code off hand but they are NGK and I will post them on this forum when I am next in my workshop.

I fitted an alloy head 4 years ago after my old one started leaking through cracks in the walls. It was the original head so hadn't done badly for 75 years, bought the new one from Fiennes and it has been a mega success.

My oil system has an external filter which when you do endurance rallying is, in my opinion, essential to enable regular changes.

Christopher Carnley · Posted: Sat Jun 18, 2016 7:53 am Post subject:

Paul,

Did you drill the crankshaft with the final holing mods, and fit a high capacity oil pump for the filter?

Pictures of the current job are coming up with some enviable high compression pistons.

Paul Markland · Posted: Sat Jun 18, 2016 3:29 pm Post subject:

No, we did not modify crankshaft or pump. I did notice a drop in oil pressure initially but we compensated for this by adjusting oil pump output pressure with shims. This was a very long time ago and since then I have never had a pressure issue and more importantly can change the filter in a few minutes.

The car I have in at present had two big ends go and main bearings and it looks to me as though he had too little oil pressure. I have bought a new pump from Fiennes for this car and will try it with standard 20/50 which I use and see if I can get his pressure up.

Christopher Carnley · Posted: Sat Jun 18, 2016 4:21 pm Post subject:

Paul,

Skimping on oil supply and pressure leads to failure as you see, I would not rebuild an engine and rely on the original low capacity bronze type, worn or not, especially when fitting a remote full flow filter.
I have the cranks EDM drilled by a local firm of spark eroders.

Just having a cup of tea during the awkward job of repinning the horrid sludge trap castle nuts.

Paul Markland · Posted: Sun Jun 19, 2016 9:38 am Post subject:

Chris

Interesting, I will look into this for the current rebuild.

I am also a great advocate of Slick 50 which I found found very helpful in extreme temperatures where it is difficult to maintain pressure on tickover.

I have had a conversation with Powerspark and sent them the distributor drawings, hopefully they will come back with an appropriate sized magnetic donut for the electronic ignition. I decided against an optical system as there is just too much dust and dirt in the engine compartment on occasions and I can see the sensor being disrupted. Will let you know how I get on.

Paul

Christopher Carnley · Posted: Sun Jun 19, 2016 1:35 pm Post subject:

I will be interested to see how they plan to retain it on the cone fixing.