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Bsa B31 engine #770865
04/12/19 5:11 pm
04/12/19 5:11 pm
Joined: Feb 2012
Posts: 110
Brum, West Midlands, UK
SmoothPierre Offline OP
BritBike Forum member
SmoothPierre  Offline OP
BritBike Forum member

Joined: Feb 2012
Posts: 110
Brum, West Midlands, UK
Hiya I have sold my B40 and now have progressed to a B31, I would interested to find out if a B33 barrel and piston would swap directly onto the B31 crankcase. Thanks

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Re: Bsa B31 engine [Re: SmoothPierre] #770883
04/12/19 8:07 pm
04/12/19 8:07 pm
Joined: Aug 2015
Posts: 1,356
Bolton Lancs UK
A
Andy Higham Offline
BritBike Forum member
Andy Higham  Offline
BritBike Forum member
A

Joined: Aug 2015
Posts: 1,356
Bolton Lancs UK
Yes, as will a 350 or 500 gold star
This is a B31 with gold star 500 barrel and head
[Linked Image]20180609_152516 by Sigma Projects, on Flickr


BSA B31 500 "Stargazer"
Greeves 200 "Blue Meanie"
Greeves 350
Greeves 360
Suzuki GSX1100 EFE "Sorcerers Apprentice"
GM500 sprint/LSR bike "Deofol"
Jawa 500 "Llareggub"
Aprilia RSV Mille "Lo Stregone"
'35 & '36 OK Supreme
Kawasaki ZZR1400 "Kuro no senshi"
Re: Bsa B31 engine [Re: SmoothPierre] #770940
04/13/19 3:55 pm
04/13/19 3:55 pm
Joined: Sep 2001
Posts: 2,919
Comox BC Canada
Gordo in Comox Online content
BritBike Forum member
Gordo in Comox  Online Content
BritBike Forum member

Joined: Sep 2001
Posts: 2,919
Comox BC Canada
It depends on what you are trying to do and which B33 bits you have. While the cylinder will fit the B31 cases you need to look at the conrod and piston to see if they will work together. The B31 kept the 7.375 inch conrod through it's complete life while the B33 changed to a 6.875 inch conrod in 1953 and moved the gudgeon pin lower in the piston. I do not think you can mix and match the long conrod with the short rod piston.

Gordo


The roadside repairs make for the best post ride stories.
Re: Bsa B31 engine [Re: SmoothPierre] #771002
04/14/19 12:47 pm
04/14/19 12:47 pm
Joined: May 2004
Posts: 4,020
Sydney Australia
B
BSA_WM20 Offline
BritBike Forum member
BSA_WM20  Offline
BritBike Forum member
B

Joined: May 2004
Posts: 4,020
Sydney Australia
Not unless you are trying to make a 740cc M20


Bike Beesa
Trevor
Re: Bsa B31 engine [Re: SmoothPierre] #771615
04/20/19 3:34 pm
04/20/19 3:34 pm
Joined: Feb 2012
Posts: 110
Brum, West Midlands, UK
SmoothPierre Offline OP
BritBike Forum member
SmoothPierre  Offline OP
BritBike Forum member

Joined: Feb 2012
Posts: 110
Brum, West Midlands, UK
I have a 1959 B31 with alternator on crankshaft (no mag/dyno), what I would like to know is ... can I swap this engine for an earlier mag/dyno type engine and still swap the alternator onto the earlier engine. Is there much grief in doing this?

Re: Bsa B31 engine [Re: SmoothPierre] #771618
04/20/19 4:33 pm
04/20/19 4:33 pm
Joined: Sep 2001
Posts: 2,919
Comox BC Canada
Gordo in Comox Online content
BritBike Forum member
Gordo in Comox  Online Content
BritBike Forum member

Joined: Sep 2001
Posts: 2,919
Comox BC Canada
I cannot speak from experience as I have never worked with an alternator type B31. However If you have both in front of you just need to look at the crankshaft of the Magdyno type engine to see if it can take a rotor. Normally the alternator rotor is keyed to the crankshaft and not on a spline like the cush drive setup. That would be the big issue that would be hard to overcome.

The other issue would be to see if the inner chaincase will just bolt onto the engine case and what modification would be needed to mount the contact breaker assembly. Does the alternator mount on inner or outer chaincase?

Gordo



The roadside repairs make for the best post ride stories.
Re: Bsa B31 engine [Re: Gordo in Comox] #771622
04/20/19 5:26 pm
04/20/19 5:26 pm
Joined: Feb 2012
Posts: 110
Brum, West Midlands, UK
SmoothPierre Offline OP
BritBike Forum member
SmoothPierre  Offline OP
BritBike Forum member

Joined: Feb 2012
Posts: 110
Brum, West Midlands, UK
The alternator mounts on the outside of the inner chain case,on the crankshaft there is no (mag/dyno) of course just a distributor. I cant compare engine because i need to buy an engine to swap and I can obtain an earlier model engine. And yes you could be right about the impossible job to mount the alternator on a splined shaft. If any other ideas come to mind i would appreciate the input. I could swap cranks but paying a small fortune for a second hand engine just to strip it, and I dont know what is wrong with the alternator model yet, I found a piece of piston ring on top of sump plate gauze.Thanks.

Last edited by SmoothPierre; 04/20/19 5:30 pm.
Re: Bsa B31 engine [Re: SmoothPierre] #771623
04/20/19 5:39 pm
04/20/19 5:39 pm
Joined: Sep 2001
Posts: 2,919
Comox BC Canada
Gordo in Comox Online content
BritBike Forum member
Gordo in Comox  Online Content
BritBike Forum member

Joined: Sep 2001
Posts: 2,919
Comox BC Canada
So the issue is really about a sick B31 engine. I would think finding out what is wrong with your engine would be the first thing to do. It might be a simple fix and the repair could be a lot easier than trying swap to another engine. I have always thought that the alternator B31 is the best of the lot. You could upgrade the alternator to a later unit and even fit electronic ignition into the points housing.

Gordo


The roadside repairs make for the best post ride stories.
Re: Bsa B31 engine [Re: SmoothPierre] #771653
04/20/19 11:22 pm
04/20/19 11:22 pm
Joined: Feb 2014
Posts: 545
Cork Ireland
C
chaterlea25 Offline
BritBike Forum member
chaterlea25  Offline
BritBike Forum member
C

Joined: Feb 2014
Posts: 545
Cork Ireland
Hi,
The alternator engine does not have the magneto mount machined, the case machining is different for the distributer
the early crank would take a lot of re engineering to make it work
The top end components of the earlier engine will fit the alternator engine so would provide the parts to repair

It would not be unheard of that some DPO would either break a ring on trying to fit the cylinder or drop a part of a ring into the cases
while using it to clean the ring grooves

John

Re: Bsa B31 engine [Re: chaterlea25] #774559
05/23/19 9:17 pm
05/23/19 9:17 pm
Joined: Aug 2001
Posts: 131
United Kingdom
P
PaulM Offline
BritBike Forum member
PaulM  Offline
BritBike Forum member
P

Joined: Aug 2001
Posts: 131
United Kingdom
why not just bore out and fit aa 79mm piston


A7, A10, B33, B31, A65, C15, T100
Bantam

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