B31 engine

[Gadorey]

Hi everyone,

Does anyone know if there are any differences between a 1946 XB... and a 1953 BB... engine?

I have a 1946 B31 with engine Nr XB31... which is in a very bad state.  Stood for ages in the rain with a half opened engine and everything that moves completely seized up.  Not far from me a 1953 BB31.. engine came up for sale in good condition and I'm considering buying it.  The idea would be to move all the internals to my cases in order to keep the original matching nrs on my bike, or just to forget about matching nrs and use the 1953 engine as a replacement.

As far as I know in 1953 the cylinder and head fin area was increased, but other than that I believe everything remained the same.  Can anyone please confirm or let me know what are the differences?

Many thanks,
Gui

[Pete C]

as far as I know, apart from the increase in finning (?) on later engines, the only real difference is the amount of material around the main bearings and the webbing support there. Of no consequence unless you are going racing.

As long as you are not fanatical about originality, I'd be inclined to install the later engine and only really consider doing all the work of swapping everything over if I got really bored or wanted to sell it and the original numbers could increase the value.

[MGI]

Whilst the engines are undoubtedly interchangeable, the individual constituent parts may not be - I think BSA altered the length of the con rod and also the position of the gudgeon pin in the piston at some point in the earlier fifties. I may be completely wrong and if so I apologise, if this is the case rest assured there will be a plethora of contradictory posts emanating from members considerably more well informed than I, and I shall slink off to a dark place!

[idie]

The B31 con-rod never changed in it's length during it's production life. The B33 had the same rod as the B31 until 1953 when it was shortened by 1/2".

[Pete C]

The top picture is from a 1947 B31 and the lower a 1954 so you can see the difference in support for the main bearing.

As I said, no real difference for normal road use.

[Gadorey]

Thanks everyone.
Reading through Roy Bacon's books, I also didn't find any mention of further differences. The B33, sports models and the GS were more complicated and had several changes - the shorter con rod being one of them - but the B31 remained the same.

If I buy the engine, I'll then decide what I shall do, but it wouldn't be a waste anyway.

Of course, if anyone has an early B31 engine in reasonable condition laying about and would like to sell it, do let me know.

Cheers
Gui

[MGI]

Interesting... does that mean that the additional internal webbing as illustrated for the slightly later B31's also applies to the slightly later B33's? - BSA must have realised, despite this upgrade, that more strengthening was required as they increased the internal webbing again in the late fifties early sixties! I don't know whether that was for both models or just the larger capacity types.

[Gadorey]

Interesting... does that mean that the additional internal webbing as illustrated for the slightly later B31's also applies to the slightly later B33's? - BSA must have realised, despite this upgrade, that more strengthening was required as they increased the internal webbing again in the late fifties early sixties! I don't know whether that was for both models or just the larger capacity types.

I might be completely wrong as I'm not really familiar with the later models, but didn't BSA introduce the alternator, hanging on the crankshaft in the late 50's / early 60's? Wouldn't this be the reason for reinforcing the internal webbing at that time?

[Pete Gill]

Hi Gui

Have a look at this engine/gearbox in Car and classic, I was considering buying it myself but it sounds like you need it more?

https://www.carandclassic.co.uk/car/C1289517

Pete

[Gadorey]

Hello Pete,

Thank you very much for passing it on.  I've contacted the seller to see if he is willing to ship it to Germany.

Cheers,
Gui

[Pete C]

where are you in Germany? I'm in Würzburg

[Ian C]

where are you in Germany? I'm in Würzburg

I've happy memories of staying just down the road in Bad Mergentheim whilst having company training in Lauda Konigshofen.. 

[Gadorey]

where are you in Germany? I'm in Würzburg

Up north near Helmstedt.

[Gadorey]

Well, someone was faster with that engine on Car and Classics,  so I'm still on the hunt for one if anyone knows of anything available.