Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Engine number problem.

Hi Guys,

I need some help with an engine number. I have a spare engine with the # 18V 836Z. I cannot find a reference to it in the Haynes manual or in the Moss catalog.

The engine in the project 1971 GT is an 18V 584Z which is correct for 1971 . I also have a 25D distributor. Problem is I do not know which engine it came from. Is it compaatible with both of these engines?

Any help would be appreciated.

Regards Neil Peniston.
Neil Peniston

Yes, I run 25D distributors on all my B's - from 1966 to 1980.
Steve Buchina

Neil. The general reference for things MGB is Clausager's "Original MGB With MGC and MGB GT V8", a book that should be in the library of any enthusiast.

On page 64, on the table "18V Engines (1974-80)", Clausanger notes that the 18V-686-Z-L engines, from number 101-5401 were installed from Sep 74--Dec 74. Under notes is listed, "North American export '1974 1/2 model year, two HIF-4 carburettors, without overdrive'".

You have a fairly rare engine which might be worth something to a museum, but is of little value to the rest of us. Easy enough to find a quite similar (i.e. identical to) engine, pull the tag off, and have a tag made up to suit the Heritage Certificate if the number on the installed engine does not match the Certificate.

But, a good, rebuildable engine is worth keeping around, if you have the storage space, as a "just in case" thing. About a year ago, I used my best engine block as the basis of an engine rebuild and made the mistake of using the Moss connecting rod bolts, resulting in a total destruction of all the parts below the cylinder head. Quite glad I had a spare block to throw another $2K+ into and will begin to assemble it when our weather gets a little cooler.

The distributors used in the MGB were the Lucas 25D4, the Lucas 45D4, the Lucas 45DE4, and the Lucas 45DM4. All with, physically, interchange. The latter two were "electronic distributors" which means that they had some form of electronic points replacement system. The Lucas 45DM4 was not bad. The Lucas 45DE4 was quite bad. I like a points type system myself and the Lucas 45D4 distributors (four, purchased over a period of a number of years from Bob Ford at Brit-Tek) have worked well in my three cars and my older daughter's car.

Please, let us talk a little about distributors. As "usens old folks" know, the various Lucas distributors tend to interchange within a model (e.g. DM2, 25D4, 45D4, etc, et al) and, mostly, within the Lucas range. Thus, a Lucas 25D4 will fit into an MGB block, an Austin-Healey Sprite block, an MGA block, an MG midget block--almost any BMC engine with four cylinders will use, physically, the same distributor. Mechanical and vacuum advance curves may differ and that is a more important point than the physical interchange.

All of the Lucas distributors, because they were the "sole source" manufacturer for a number of other companies, will have a "specification number" stamped on the side of the distributor. This specification number will allow you to determine what the mechanical and vacuum advance characteristics of the distributor were when it was new. Paul Hunt, in is excellent "The Pages of Bee and Vee", has a list of the MGB factory distributors and shows their mechanical and vacuum advance characteristics.

Modern engines run on modern fuels, which may be quite different than the fuels our cars were originally intended to run on. This changes the ignition system requirements and the only way to know exactly what any engine requires is to have a rolling road (chassis dynamometer) session concentrating on the distributor's mechanical advance requirements. Then, to have a custom distributor built to meet, as close as is possible, the ideal requirements.

Most of us cannot afford this type of thing and use what seems to be best without audible detonation within the engine.

Not a perfect solution, but it works fairly well for most of us.

Les Bengtson

Any 25D4 or 45D4 distributor will physically fit any MGB engine, and will run with it. However there were many different curves depending on era and market, denoted by the reference number after the 25D4 or 45D4. Until 1967 the curves were generally chosen to give the best compromise between performance and driveability, but after that for North America in particular they were chosen more and more to get through the emissions tests of the day, and the performance can and did suffer as a result. The further the curve is away from the ideal for your engine there will be a greater loss of performance and/or economy at certain parts of the operating range. Even getting the original curve isn't ideal, as the fuels today are very different to those the engine was designed to operate on. Unless you are going to spend a lot of time on a rolling road determining what the ideal curve is for your engine and fuel, then tweaking curves to obtain it, the best you can do is run the most advance you can without pinking at any combination of throttle, revs and load. However if you have a low compression engine they sometimes don't pink even over-advanced to the point of stalling the starter, so the rider to the foregoing should be not to go more than about five degrees more than the original spec. You should be able to do this, as even originally the specs were designed to cater for the worst case scenario in a range of component tolerances, as well as have a safety factor, and many engines would run much better on several degrees more advanced, as I found in the 70s.
Paul Hunt

Thanks for your response,

Regards, Neil P.
Neil Peniston

This thread was discussed between 05/09/2009 and 06/09/2009

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.