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The Heart of Many Dennis's - The Rolls Royce B80
Fire Engine Photos
No: 14013   Contributor: Dr. B.A. Hutchinson   Year: 2009   Manufacturer: Dennis   Country: United Kingdom
The Heart of Many Dennis's - The Rolls Royce B80

From the early 50's to the 80's the heart of most petrol engined Dennis appliances was the Rolls Royce B series engine. The B60 (the six cylinder engine) was fitted to the smaller machines like the F8 and the B80 (8 cylinder engine) was fitted to the larger appliances like the Belfast ET (OZ9606) which has been the subject of recent discusion. The B40 (4 cylinder) engine completed the range but was never used by Dennis. All of these engines feature interchangeability of parts accros the range, making then particularly appealing for use by the armed forces.

The engine in the above picture is that belonging to OZ9607 the Belfast Dennis Metz TL. You will see the dual ignition coils which are selected from a switch on the dash panel. Only one at once is ever used, but this redundancy gave greater reliability. (It was almost a 'hangover' from earlier days when most appliances had selectable coil or magneto ignition. Mechanical and electrical fuel pumps are present again selctable by a changeover switch).

B80's and the later slightly improved B81, are very soft engines and can easily be started by a light pull on the starting handle. They are slightly unusual in that they have overhead inlet valves but side exhaust valves. Their only weakness is that the cylinder bores are quite close together so that the head gasket can blow through between two adjacent cylinders. Unless caught soon enough this leads to a a chnnel being burnt into the cylinder head which then leads to quite an expensive repair. The later B81 will drop straight onto a B80 block and this is the configuration shown in the photograph.
Picture added on 09 January 2009
add commentComments:
The Brange of rationalised engines were designed in 1946 and were to power military vehicles, these engines were designed to have most parts interchangeable between each other.

The B80 seen here has the 3.5" bores and gives a swept volume of 5675cc RAC rating was 39.2hp or 165BHP with 280lbft torque the overhead inlet side exhaust was done for quietness as it made for less valvetrain noise and also because the exhaust valves are water cooled around the seats to facilitate cooling and lubrication under high revs.

The B81 was a direct spin off of the B80 the 1 indicates 3.75"bore size giving a swept volume of 6522cc power ratings ranged from 180bhp to 235bhp depending on cylinderhead carburettor and inlet valve/manifold sizes thes featured 3/4 length liners and an extra compression ring this engine produced 336lb/ft of torque at 3800 rpm compression was 6.5-1 or 7.25-1

the range fitted to early f range fire appliances were of the commercialised type having aluminium heads (BSF) this led to warping and gasket issues so later b80/81 engines had cast iron heads (UNF) also this indicated thread types

normal running temp is 170/190 %f
this unit is a simple to maintain engine and provides good power and torque the favoured engine of the range was the later B81 An underrated engine and under stressed found its way in to motorcars too with head work and 4 sidedraught carbs give around 300 bhp with 450lb ft torque

i have specialised almost in these engines and any info or help needed regarding such engines dont hesitate to ask!

Added by Jason Lester on 10 January 2009.
Can anyone help with sourcing a sound B80 engine? I am one of the owners of a 1954 Thornycroft Nubian, ex Staffordshire Fire Brigade (a rare civilian model - you can find picture elsewhere on this site). The machine is undergoing further renovation: we fear its engine problems may make replacement more practical than overhaul. Any advice most welcome.

Added by Ian McBride on 23 March 2009.
I know we got a B81 from a Government Surplus outlet...the MOD had stacks of them.

Added by Barrie Green on 24 March 2009.
Marcus Glenn ( have fully reconditioned ex MOD B80 and B81's although they are not cheap!

Added by Barry on 24 March 2009.
There is a supplier of military spec b80/81 near to me called c&c military services he had them in abundance or there is a stolly being broken in bicester in oxon that has a b81 in it

Added by Jason on 24 March 2009.
Lovely picture... well done.

URGENTLY REQUIRED.... water pump for a B80 as fitted to my Ex ECFB (Grays Fire Station) Dennis F12 WTW 749. This vehicle is based at the Essex Fire Museum which is at Grays fire station in Essex. Please call me on 01708 861003. Thanks.

Added by Roger Pickett on 07 July 2009.
i can put you in contact with my supplier he has them my email is

Added by Jason on 07 July 2009.
Hi Jason

I have sourced a B81 from Marcus Glenn and would like to tun it into civilian spec to put into a bentley special, I have also sourced a Lucas DU8A distributor which I believe is correct, but I am missing a small aluminium casting, which fits in between the distributor and engine block. Would you be able to help with dimentions which would enable me to make one? Or better still put me onto a supplier. Thank you.

Added by Mitch Harrison on 02 November 2009.
i have one in my cupboard if you like mate!
the bell housing needs to be changed also and water pump as the multiple pully set up will be too long to fit behind the radiator also the inlet manifold needs work blanking off parts etc
check the sump it may be a dry sump on the military too

Added by Jason on 03 November 2009.
Thanks for the advice, your right I have many problems to overcome, but would like to address them one by one, I would like to take you up on your offer can you email me direct?



Added by Mitch Harrison on 03 November 2009.
no problem mate drop me an email on and we will sort it out
where abouts are you in the country mate?

Added by Jason on 04 November 2009.
I have a Saracen APC, which as standard had a B80, fitted. My Saracen is a Mk5 uparmoured one and could do with a little more 'oomph', I was wondering what would be involved in fitting the uprated B80/81 to it, and would I be right in thinking that my fuel consumption would improve? -even if only slightly?

Added by Matt Kill on 21 January 2010.
Wat is the stroke of a B81 engine?
On engine arfolowing numbers.
UE 41616 PV 145 160
SERIAL NO B84 14180.

It is in a Racing Bentley
With twoo HD 8 (2 inch ) carburettors.
Wat do you think the power "estemate" wil be?

Added by Charles. on 15 March 2010.
Can anyone please answer the following questions re the Rolls Royce B80 engine?

What are the piston dimensions? eg diameter?

Can I prove an engine type from the engine number?
Is engine number UE7255 a Rolls Royce B80 engine?
Does anyone know of a register of Rolls Royce engine numbers?

Added by Glenn Cumming on 07 May 2010.
3.5 inch diameter for the rolls B80, 3.75 inch for the B81. Both have 4.5 inch stroke.

Added by Jason Lester on 17 September 2010.
Hi Jason , we have a 1955 F12 Dennis with a B80 MK50.C engine , we have a copy of a manual that tells us most spec, s we need to know except the timing setting . wondered if you maybe able to help with this ?

Added by Andrew N.Z on 25 April 2011.
I have a 1954 Denis Fire Engine with the B80 engine looks exactly like the one in picture above.
Engine No: 389W1B80E20L..........where is the best place to source parts?
I may consider selling this engine.
Just put some gas in it after 11 years of sitting and started right up. I have own this truck for about 25 years

Added by George Ullyett on 13 September 2011.
Can anyone suggest the best source for B80 oil filters? The Grimsby Dennis Metz engine is experiencing low oil pressure although the engine is super smooth. Can it be as easy as a stuck oil pressure relief valve? Has anyone had a similar experience?

Added by Barry on 12 January 2014.
Do you happen to look for a unused brandnew distributor cap for a B80 or a B81 engine for the DU8A distributor? I still do have one, and I do also have the contactsets for this kind of ignition. I do live in the Netherlands, but I can ship anywhere. Send me a mail if you are interested.

Added by Ben Silvius on 12 December 2014.
What type of spark plugs should be used in a B60 engine fitted in a Dennis fire engine

Added by David Millar on 08 April 2015.
Ones that work!?

Added by on 09 April 2015.
Does anyone know where I can source a rotor arm for a du8a distributor fitted to f12 Dennis.

Added by Colin Timberlake on 22 June 2015.
My B61 in the Dennis F38 suffered low oil pressure as in Barry's comment.The O rings on the pickup tube between the floating strainer and the oil pump were shot, thus it was drawing more air than oil.I get my B series parts from W.J.R.Banister 01797 253211.

Added by Howard Godsland on 12 September 2015.
The full story of my problem with the oil pick up tube on the Grimsby Metz TL B80, can be found here;


Added by Dr. B.A. Hutchinson on 15 September 2015.
Hi Jason, I've just bought a B80 Mk6 engine for a Bentley special and I am trying to convert to 12 volt. I need a distributor and starter motor. As Mitch mentioned in his message to you, he has used a Lucas DU8A distributor. Is this the correct item? Are you able to help in locating one? Also, can you tell me what starter motor I should use?
I will be selling the current mil spec 24 volt items as soon as I have secured replacements. The are unused, MOD restored components out of Hong Kong in 99, all marked as rebuilt in 83.

Added by Jayde Deverson on 26 December 2015.
Hi All can anyone tell me what oil should I use in my B80 engine cheers Gavin Palin.

Added by Gavin Palin on 24 April 2016.
It's an old fashioned engine and therefore it is happy with an old fashioned oil. I have always used Castrol GTX without any problems on many B80's.

Added by Dr. B.A. Hutchinson on 26 April 2016.
Gavin Palin for your B80, use SAE 30 single grade motor oil, with no additives, as the extra additives in modern multigrade oils have been known to slowly attack and dissolve bronze/and some white metals found in main bearings of older engines like the Rolls Royce B Series engines

Added by Matt Kill on 26 April 2016.
A mineral sae30 or 20/50 suits them, there were no fancy synthetic oils around in those days and there would be no point in wasting your money on it.
What is your B80 installed in ? do you still have our old Peterborough Merryweather TL ?
Howard Godsland.

Added by Howard Godsland on 27 April 2016.
I have a B80 Mk. 2 D engine installed in a Bentley mark6 special. I'm seeking advice on what plugs I should use.At the moment I'm running with B6ES. NGK.

Added by Donald on 14 May 2016.
In Finland I have a B80 5P engine, ex-Swedish army, only 197 hours on it. Has been winch engine in a big Centurion.
Price 1200£. Phone +358-400-206000

Added by Pete Wahlberg on 08 September 2016.
Did Donald sell the B80 engine or is it still for sale, needed fort mk6 B special

Added by John Fraser on 15 May 2017.
Last unused brandnew distributor cap available for a B80 or a B81 engine for the DU8A distributor! Also have the contactset for this 8 cyl ignition.
Also available distributor cap + contactset for the B60 engine. For all vehicles and motorcycles made between 1930 and 1975 I do have NOS distributor caps, rotors, ignition points, condensors and ignition coils. Send me a mail (preferred) if you are interested: OR PHONE +31 - 622-580039 (between 10.00 and 17.00 h. only please). Use English, Francais, Deutsch, Nederlands.

Added by Ben Silvius on 28 September 2017.

Added by Pete Wahlberg on 30 September 2017.
Hello! Searching for dry sumps to my B81 engines witch soon are going too be installed in my motoryacht build 1912 and 18 meters long.

Best regards/Tony

Added by Tony Duda on 04 November 2017.
Hey ,
My name is Florian from Berlin.
I m looking for a B81 Engine for my Bentley. If anybody has one for me or an idea where I could get one please write an email to
Thanks very much

Added by Florian on 20 July 2018.
I'm currently overhauling the cylinder head on B81 with the big valves. Is it recommended to have the head skimmed ?
Any info would be much appreciated Bill Haworth.

Added by Bill Haworth on 11 October 2018.
Hi Florian, I have a B80 Mk6, dry sump engine. It has been fully rebuilt by the MOD and ready to run. It was destined for a Saracen. If interested, I can crate it up and ship it out to you from Australia.

Added by Jayde on 11 October 2018.
I'm overhauling the valves on my B81 engine. The cylinder head was no problem, but the exhaust valve collets are proving impossible to refit due to the restricted space even with the exhaust manifold removed. the engine is in the vehicle (Thornycroft Nubian). Any ideas or help would be appreciated.

Added by Tony Mills on 15 January 2019.
Hi Tony,
Have you tried some grease on head of valve, holds cotter in place. Stuff a clean rag below to stop loose cotters heading south. Worked with my series 1 Land Rover (same IOE arrangement although access easier)

Added by Peter McCombie on 18 January 2019.
Hi Guys ,
I am looking for a B81 to power my vintage car , would anyone know where I can buy one from ? Happy to travel or collect from anywhere . Thankyou

Added by Matthew Moore on 05 July 2019.
Ask Richard Bannister, Northiam, Rye, he is a good bloke who sells all 'B'range parts.01797 253211.

Added by Howard Godsland on 09 July 2019.
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