Author Topic: B31 again  (Read 2260 times)

Offline Dynamo Regulators Mike

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B31 again
« on: 13.05. 2009 18:48 »
In my role as secretary of Hants Branch BSAOC I have had an enquiry from a member who has his B31 crank apart after big end failure. He says that there is a hole both ends of the crankpin, one has a grub screw fitted, but for the other end there was no form of plugging (might be why the bearing failed *conf*).
Does anyone here have detailed knowledge of this engine, or perhaps e-mail the contact of someone who has? (bsa'at'manortec,co,uk). Chris Jefferies the club tech consultant for B31 etc does not have an e-amil or phone number listed in the Star *sad2*.
Apologies for the off subject post.
Mike
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Mike Hutchings
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Online Brian

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Re: B31 again
« Reply #1 on: 14.05. 2009 09:46 »
Mike, there should definitely be a "plug" in the end of the pin. One end has a slotted head screw in plug and the other end a plain plug. I am not sure if the plain one is threaded or pressed in. I have an old crankpin I can have a look at if necessary. On these motors the oil comes in through the end of the timing side mainshaft and through a drilled gallery in the flywheel and then through a hole in the pin and to the big end bearing. If one plug is missing the big end would not be getting any oil.

Brian.
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Offline Dynamo Regulators Mike

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Re: B31 again
« Reply #2 on: 14.05. 2009 09:58 »
Thanks Brian.
Will pass your comments on. If you can look your old pin out and see just how it was blocked off it would be great. And a photo would be icing on the cake if at all possible.
Cheers
Mike
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Mike Hutchings
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Offline fido

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Re: B31 again
« Reply #3 on: 14.05. 2009 13:21 »
I would have thought the pin would not need to be drilled right through, just far enough to reach the radial hole to the bearing surface. I wonder if he is mistaking a centre drill hole for a through hole? The parts list shows just one plug, 65-479.
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Online Brian

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Re: B31 again
« Reply #4 on: 15.05. 2009 00:16 »
Unfortunately I cant find the old pin I thought I had, I must have thrown it out. But wether they screw in or are a push fit doesnt matter, the ends of the pin must not let any oil out. The screw in plug is there so it can be removed to clean out the oil way just like on the A's.

The big ends simply do not fail in these engines unless they are starved of oil or have done a zillion miles. The B and M series BSA's are built like tanks and the design and engineering of the engines is second to none.
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Offline Dynamo Regulators Mike

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Re: B31 again
« Reply #5 on: 15.05. 2009 08:54 »
Thanks for looking anyway Brian.
One minor question is why the pin was drilled right through at the factory, then plugged?
The owner of the broken engine had someone speculate that the open end was designed to aid lubrication of the drive side bearing sharing lubricant with the big end. This could not be sound engineering but he was part way convinced.
Anyway he will probably have it all done and back together for our next club night on Thursday. These engines are pretty simple after all.
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Online Brian

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Re: B31 again
« Reply #6 on: 15.05. 2009 09:21 »
This got me interested so I rang a friend who has a M20 in pieces at the moment and his has a screw in plug on one side and is solid on the other, only drilled half way through. I have my B31 motor on the bench at the moment so I took the crank out of the cases to have a look and my one is plugged on both ends.
Obviously there are some differences between them. The parts book just shows one plug as Fido says so I think the original ones are drilled halfway with a scew in plug and the aftermarket ones could vary. Mine is drilled through with non removable plugs in both ends. I did fit a Alpha one to a engine some time ago but cant remember what it had.
Whatever the case if the one your friend had was letting the oil out one end then thats why it failed.
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Online BSA_54A10

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Re: B31 again
« Reply #7 on: 15.05. 2009 11:51 »
Pre WW-II M20's had hollow journals.
Wartime ones had smaller holes and only in one side ( saved time money & materials).
Post WW-II reverted to the original journal.
99% of M20 parts are NOS WW-II surplus stock
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Bike Beesa
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Offline fido

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Re: B31 again
« Reply #8 on: 15.05. 2009 12:45 »
Pre WW-II M20's had hollow journals.
Wartime ones had smaller holes and only in one side ( saved time money & materials).
Post WW-II reverted to the original journal.
99% of M20 parts are NOS WW-II surplus stock

Does the M20 crankpin fit the B31 crank? If so, a NOS pin sounds like a less risky purchase than a pattern one.
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Offline LJ.

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Re: B31 again
« Reply #9 on: 15.05. 2009 19:58 »
Coincidently I've had my WW2 M20 bottom end apart this last week and can say that there is only the screw on the timing side of crank, no hole on the drive side and as you can see in the photo the screw is accessable through the case without removing the crank, bloomin job to get at though!

Brian I think you should drill yours out and put a grub screw in because mine was completely gunged up! There's no telling how bad it is untill accessed.
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Ride Safely Lads! LJ.
**********************
1940 BSA M20 500cc Girder/Rigid- (SOLD)
1947 BSA M21 600cc Girder/Rigid-Green
1949 BSA A7   500cc Girder/Plunger Star Twin-(SOLD)
1953 BSA B33  500cc Teles/Plunger-Maroon
1961 BSA A10  650cc Golden Flash-Blue
1961 BSA A10  650cc Golden Flash-Red

Online Brian

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Re: B31 again
« Reply #10 on: 15.05. 2009 20:07 »
LJ, the one i have just put in my B31 is the one with both ends plugged with plain plugs, its a new one.

Fido, they are all the same, B31/33 and M20/21.
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