Author Topic: Stuck piston in BSA B21 (like a B31 sort of)  (Read 1316 times)

Online KiwiGF

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Re: Stuck piston in BSA B21 (like a B31 sort of)
« Reply #45 on: 03.01. 2020 23:21 »
I've lost track of this, a bit, but, after all this time and the various methods, why not just destroy the piston by whatever means and move on? Others may correct me, but it seems to me that attempting hydraulic means of loosening the puston has as good a chance of fracturing the barrels as moving the piaton.

Richard L.

It’s back soaking in the diesel and I’ll give pressing the piston out (I’ve got a 20 tonne press) another go, but not so much pressure to risk damaging the barrel or cases.

I’ve come to terms with the prospect of destroying the piston.

I’ve been busy in the shed installing and also “converting” a 3 phase car hoist to single phase so the b21 is on the “back burner”, been out riding a bit too :-)
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1956 A10 Golden Flash (1st finished project)

1949 B31 rigid “400cc hot rod” (2nd finished project, + favourite bike)

GL1800 Goldwing, well, the wife likes it

KTM 950 ADV, cos it’s 100% nuts

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Online KiwiGF

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Re: Stuck piston in BSA B21 (like a B31 sort of)
« Reply #46 on: 03.01. 2020 23:44 »
As Klaus has already said
Drill a 14mm bolt to take a grease nipple
Give the valves a quick lap so they make a good seal.
I like to fill the head with old ball bearings as it reduces the amount of grease needed
Then fill the cylinder with grease through the plug hole.
You can use any grease or any other liquid for that matter.
Once full as you can get it screw the adapeter in loosely and pump more in till it starts to ooze up the thread  then tighten the adapter.
Pump as much grease in as your gun can then add heat.
I use a gas torch.
The person who originally showed me this used a BBQ set to 200 C .
Never had one that would not come loose but some did take a day or two.
Once you get it apart play very close attention to the crank case between the cam holes
They are prone to splitting.
When the B series was redesigned post WWII they moved them further apart and of course added the outrigger support plate.
You can modify a B/M series plate and the timing cover to make it fit and if you intend to do any sort or spirited riding it is worth while doing.
M/B series cams can be made to fit but they need to be ground smaller or the followers will foul on the cases.
And yes the frame is very light , although BSA called it the "medium weight" frame and they do go like the proverbial scaled cat  .
Std C series wheels drop right in as B series wheels are near impossible to find.
No one was too worries about offering the 250 for war effort scrap so there are not many around and very few in the UK.
Early C or B ( tapered mainshaft ) clutch fits and it is a single plate job.
I think the early 2 plate B series clutch will fit  but the latter one will foul the chain case.
And we are talking about the single spring clutch here.
One member cut down a WM20 clutch to fit

Without checking I am fairly sure they used the same "medium weight " 4 speed box as BSA revived for the 46-48 B31/33 ( probably found a stillage full of them round the back of the factory ).

Thanks for all the info, very useful, I’ve got wheels, cams, a hand change AND and a footchange gearbox, the “proper” clutch is missing I think, but there are some clutch parts of some origin there.

I’m really just “assessing” this b21 project for what needs buying/fixing at present, as well as being too expensive it may also be simply too time consuming for me to get it in the road again. Welding the crank cases welded up is way beyond me, and probably fraught with risk if I subbed the welding out.

I mainly bought it because I thought it was a rare bsa (based purely on the premise it is pre war) and also because it was selling at auction at a price not much higher than a similar b31 project bike. The number of “correct” missing parts may ultimately make it unviable from a cost view at current values, but I’m in no rush to move it on, so you never know. Maybe it’s future is as a “bitsa”.

I have since seen a few for sale in the UK and other countries, at similar prices to rigid b31’s, so rarity does not relate to “desirability” or higher value I guess, I wonder how many b21’s are left *dunno*
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1956 A10 Golden Flash (1st finished project)

1949 B31 rigid “400cc hot rod” (2nd finished project, + favourite bike)

GL1800 Goldwing, well, the wife likes it

KTM 950 ADV, cos it’s 100% nuts

1952 Armstrong Siddeley Whitley for rainy days (with wife)

Online KiwiGF

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Re: Stuck piston in BSA B21 (like a B31 sort of)
« Reply #47 on: 09.04. 2020 01:10 »
As a follow up to this, the diesel soaking did not work, the piston still would not budge under my press after months of soaking, so I resorted to drilling numerous 10mm holes in piston top, then chiselled away the piston using a modified punch, ground to have a small chisel type end 1/8” across. This actually only took an hour and is what I should have done in the first place I guess  *problem*

Whilst the bottom part of the piston was wet with diesel the ring area was completely dry, so the diesel did not penetrate there, despite numerous heat cycles etc.

It’s pretty rusty inside the cases but the rod, crank and barrel look salvageable, the cases maybe not so, it looks like the hole in the cases (s) was caused by the frame punching a hole through the cases, maybe in a crash. A repair would involve re creating new lugs for “through” engine bolt, that also bolts the engine into the frame. There are two other small cracks in the cases, one small around a sump bolt (maybe the press caused that) and one on the “protrusion” at the front for bolting to the frame (the b31 has engine plates instead of this “protrusion”, I guess to over come these problems.

My next problem is the crank is stuck in the drive side case, despite the spacer over the drive spline spinning freely, as does the main bearing *doubt* *dunno*


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1956 A10 Golden Flash (1st finished project)

1949 B31 rigid “400cc hot rod” (2nd finished project, + favourite bike)

GL1800 Goldwing, well, the wife likes it

KTM 950 ADV, cos it’s 100% nuts

1952 Armstrong Siddeley Whitley for rainy days (with wife)

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Re: Stuck piston in BSA B21 (like a B31 sort of)
« Reply #48 on: 09.04. 2020 10:54 »
wow perfect example of pistons being oval and long term rusting is as good as a weld *grins*
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Online KiwiGF

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Re: Stuck piston in BSA B21 (like a B31 sort of)
« Reply #49 on: 09.04. 2020 12:02 »
wow perfect example of pistons being oval and long term rusting is as good as a weld *grins*

Fortunately....the piston (or what remained of it) was not too badly “welded” to the gudgeon pin, so the last two shown in the pic bits bits were freed off the barrel by moving them over towards the con rod, incredibly those fragments were still holding tightly onto the barrel before I worked that out  *eek*

I’ve cleaned most of the rust off the parts now, and now it looks a lot better proposition for a restore job (not necessarily by me tho, as it will take a lot of skilled work) and I’ve also got crank out of the drive case (lots of heat required to free the main bearing in its housing).

The crankcases of this pre war single range are VERY lightweight compared to a B31, even a C11, it’s not surprising they need major work I suppose, given their age, in fact the whole engine is pretty lightweight, as is the frame (with flimsy girder forks) come to think of it.
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1956 A10 Golden Flash (1st finished project)

1949 B31 rigid “400cc hot rod” (2nd finished project, + favourite bike)

GL1800 Goldwing, well, the wife likes it

KTM 950 ADV, cos it’s 100% nuts

1952 Armstrong Siddeley Whitley for rainy days (with wife)

Offline RDfella

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Re: Stuck piston in BSA B21 (like a B31 sort of)
« Reply #50 on: 09.04. 2020 12:04 »
Nightmare job. Pressing on piston whilst restraining crankcases would have been a waste of time, as the conrod would prevent movement. What was needed was to press on piston whilst gripping the cylinder – but how to do that, given the head is a through-bolt fixing? Were that not so, a thick flange bolted to top of barrel would have served.
Those flywheels look pretty hefty – should be able to count the firing strokes on tick-over. But how to get that crank out? If the main bearing is located with a circlip, pressing out could do even more damage. Guess I’d try supporting the case with crank underneath and plenty of heat to main bearing area and hope it drops.
Good luck with those cases – definitely worth saving in my view.
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Online RichardL

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Re: Stuck piston in BSA B21 (like a B31 sort of)
« Reply #51 on: 09.04. 2020 13:44 »
I don't know how others feel about this, but I'm starting to get the impression this engine may not have been stored properly.

Richard L.
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Re: Stuck piston in BSA B21 (like a B31 sort of)
« Reply #52 on: 09.04. 2020 21:56 »
I don't know how others feel about this, but I'm starting to get the impression this engine may not have been stored properly.

Richard L.

It really does look worse in the pics than it is, but yes it was not kept in a dry place ALL of the time  *conf*

The barrel is already sleeved, there is a slim chance the sleeve will clean up without having to be replaced. I’d be very surprised if it does not need a new big end crank pin, due to rust. The plain main bearing is loose in the case, no surprise there.
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1956 A10 Golden Flash (1st finished project)

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GL1800 Goldwing, well, the wife likes it

KTM 950 ADV, cos it’s 100% nuts

1952 Armstrong Siddeley Whitley for rainy days (with wife)

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Re: Stuck piston in BSA B21 (like a B31 sort of)
« Reply #53 on: 09.04. 2020 22:24 »
if the sleeve needs replacing it will have to be turned out if its as welded as the piston was *bash* *whistle*
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Online KiwiGF

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Re: Stuck piston in BSA B21 (like a B31 sort of)
« Reply #54 on: 10.04. 2020 05:33 »
Nightmare job. Pressing on piston whilst restraining crankcases would have been a waste of time, as the conrod would prevent movement. What was needed was to press on piston whilst gripping the cylinder – but how to do that, given the head is a through-bolt fixing? Were that not so, a thick flange bolted to top of barrel would have served.
Those flywheels look pretty hefty – should be able to count the firing strokes on tick-over. But how to get that crank out? If the main bearing is located with a circlip, pressing out could do even more damage. Guess I’d try supporting the case with crank underneath and plenty of heat to main bearing area and hope it drops.
Good luck with those cases – definitely worth saving in my view.

The main (ball) bearing WAS held in place by a big circlip, I was lucky (before I knew about the circlip) that the heat I applied to get the bearing actually freed off the crank (interference fit) in the bearing centre, so I did not accidentally break anything   *eek* the ball bearing was pretty rusted into the case but still turned surprisingly, anyway it’s out now and I was able to read the Hoffman part number off it.
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1956 A10 Golden Flash (1st finished project)

1949 B31 rigid “400cc hot rod” (2nd finished project, + favourite bike)

GL1800 Goldwing, well, the wife likes it

KTM 950 ADV, cos it’s 100% nuts

1952 Armstrong Siddeley Whitley for rainy days (with wife)

Online Greybeard

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Re: Stuck piston in BSA B21 (like a B31 sort of)
« Reply #55 on: 10.04. 2020 09:53 »
I don't know how others feel about this, but I'm starting to get the impression this engine may not have been stored properly.

Richard L.
You think? Should have put a bag of silica-gel beside it.
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Offline RDfella

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Re: Stuck piston in BSA B21 (like a B31 sort of)
« Reply #56 on: 10.04. 2020 12:25 »
Or better still inside it.
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Online Butch (cb)

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Re: Stuck piston in BSA B21 (like a B31 sort of)
« Reply #57 on: 14.04. 2020 10:51 »
... Hoffman part number off it.

I was apprenticed at the Hoffmans (actually RHP by then) plant in Chelmsford.
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Online KiwiGF

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Re: Stuck piston in BSA B21 (like a B31 sort of)
« Reply #58 on: 26.08. 2020 13:21 »
Just for the record I sold the B21 project as I decided it required skills beyond what I have (especially welding up cases), and more time to locate rare parts than I was prepared to give it. The final straw came when I found out the frame originally belonged to a B23 (side valve 350cc).

The new owner is probably going to cannibalise it to make up a very rare military WB30 replica (this being a sort of “prototype” of the B31). He has the B30 head with hairspring valve springs, the head is pretty unique to the B30, but most other parts of the B30 were taken from the pre war B series, or later became B31 parts. See:

http://www.vintagebike.co.uk/pictures/bsa-wb30/#.X0ZR78qxWhA
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New Zealand

1956 A10 Golden Flash (1st finished project)

1949 B31 rigid “400cc hot rod” (2nd finished project, + favourite bike)

GL1800 Goldwing, well, the wife likes it

KTM 950 ADV, cos it’s 100% nuts

1952 Armstrong Siddeley Whitley for rainy days (with wife)