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#9318 - 03/21/06 10:24 pm b31 to b33 conversion  
Joined: Jul 2004
Posts: 346
pooch Offline
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pooch  Offline
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Posts: 346
Lake Conjola NSW Australia
gday, can a (1948) b33 barrel be put on a (1956) b31 crankcase?

I can see the hole where the barrel lower goes in must be made larger.

Is the bolt pattern and the clearance to pushrod tube the same?

I think I have come across different pushrod tubes occasionally, maybe the b33 one is narrower, dunno.

Any other problems?

If this is all that is needed, can the B31 head be modified to fit over and seal on to the B33 barrel?

I would presume the compression ratio will be upped a fair bit, but should be controllable by decompression plates under barrel.


56 B31 with B33 barrel
51 Golden Flash with Dusting sidecar
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#9319 - 03/22/06 3:37 am Re: b31 to b33 conversion  
Joined: Sep 2001
Posts: 2,766
Gordo in Comox Offline
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Gordo in Comox  Offline
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Comox BC Canada
Are you sure the crankcases have to be opened up? I looked at some cases and the BB34/ZB34GS/BB31/BB33 all seem to have the same size opening for the barrel at about 89mm. The alloy barrels I have seem to be interchangeable on all of the above crankcases. On the 500 barrels only the liner fits in while on the 350 there is aluminum around the liner where it fits in.

There seems to be only one pushrod tunnel listed for those engines.

I do not have any iron heads to look for comparison or fitting ideas.

Gordo


Without frequent roadside repairs there is no fun in riding!
#9320 - 03/22/06 11:12 pm Re: b31 to b33 conversion  
Joined: Jul 2004
Posts: 346
pooch Offline
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pooch  Offline
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Posts: 346
Lake Conjola NSW Australia
gday Gordo, yeh my case hole is 89 mm.

B31 cast iron 71 mm bore with a lower extension piece which must be about 9mm thick then.

With your info, am I am guessing that a 500 alloy barrel has a liner about 2 mm thick?

85mm bore and extends into a 89 hole?

What does the cast iron B33 have on the bottom to go into the hole, or is it flat and just locates on the head bolts?

Surely there would not be a piece of cast iron only 2mm thick?


56 B31 with B33 barrel
51 Golden Flash with Dusting sidecar
#9321 - 03/23/06 11:27 am Re: b31 to b33 conversion  
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BSAPhill Offline
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UK
Just one word of caution the crank balance isn't spot on for this conversion, sure it works but it ain't great. If you want a bigger B31 you can take the bore out the std barrel to give you a 400cc machine, this with some goldie scrambles cams is a nice little motor, with alot less hassel. Porting the head is a simple and cheapish job as is upping the Vv size a little.

Oh yes the head can be modded to fit the B33 barrel but its not cheap, if you look at Pearsons Goldie page look at the costs for doing a Goldie one in soft alloy, then they Vvs should be upped in size and so it goes on.

#9322 - 03/24/06 9:07 am Re: b31 to b33 conversion  
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pooch Offline
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pooch  Offline
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Posts: 346
Lake Conjola NSW Australia
From eBay sellers, I am trying to get info.

Is the lower part of a B33 barrel that fits inside the case only about 2mm thick?

Is the reason for crank balance not being spot on because the B33 piston is heavier?

I have a honda sidevalve 10 HP piston here, which measures 85.6 mm and is nice and light and short, flat top, and a 3/4 gudgeon, the gudgeon is 3 mm lower (to the top) than the B31, but that is easily overcome.

Thanks for info Phill, I am not burning my bridges by trying it, as I have a spare B31 head and if I buy this 48 barrel and it does not work, I can put it back to standard.

ATM, the barrel that is on it is 73mm which makes about 370cc.

I also have another B31 barrel which I will bore to 3 inch and try also, 3 inch makes 401cc.


56 B31 with B33 barrel
51 Golden Flash with Dusting sidecar
#9323 - 03/24/06 2:55 pm Re: b31 to b33 conversion  
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BSAPhill Offline
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BSAPhill  Offline
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UK
Yep Piston heavier, like i said its not spot on, it works and i know of a chap sprinting on the cheap with a B31 crank in a B33 engine that has been taken to an advanced Goldie spec over the years.

As far as i am aware the B33 barrel just pops on in place of the B31 barrel, i know for B31 to DBD34GS barrel you have to drill and tap the barrel / head studs out to a larger size, cos i have just done that. But the throat was ok and the barrel slipped on / in no probs.

#9324 - 03/25/06 8:06 pm Re: b31 to b33 conversion  
Joined: Aug 2001
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Josef2 Offline
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Josef2  Offline
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Posts: 29
Germany
<What does the cast iron B33 have on the bottom to go into the hole, or is it flat and just locates on the head bolts?

Surely there would not be a piece of cast iron only <2mm thick?

hi,

I own a cast iron B31, converted to B33 by one of the preowners.

The cast iron cylinder has an cylindrical extension, which fits in the hole of the crankcase. This is necessarly for the piston movement to BTC.
The fixing bolts are the same, on the 500ccm cylinder is less space between cooling fins and the bolts, but its interchangeable.

There are different conrod lengths over the years and models, my 1954 engine has an 6 7/8" length. So you have to look for a piston of the correct heigth.

Unfortunately I lost the excel sheet sheet with the length calculations.

Josef

#9325 - 03/25/06 10:19 pm Re: b31 to b33 conversion  
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Josef2 Offline
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Josef2  Offline
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Posts: 29
Germany
hi,

I have to correct my message. The cast iron cylinder of the B33 has no collar on the underside, its just flat. Found an old picture. ( Its different on late short stroke Gold Stars).

I found a sheet with some dimensions of my B33, based on an B 31.

Crankcase height 4 1/2" 114,3 mm
Gasket 0,3 mm
cylinder height 149 mm
gives an overall height of 263,6 mm

Crankshaft, half stroke 44 mm
conrod length 6 7/8" 174,6 mm
Piston crown height 48,5 mm
gives an overall height of 267,1 mm

This means, the piston stands some 3,5 mm above the cylinder.

The conrod lenght ist 7 3/8" for all B31s and B33s up to 1953. Then the length of the B33 changed to 6 7/8",
while the B31 remained unchanged in their lifetime.

The main length dimensions should be the same for B31 and B33, the spare parts catalogue lists only one type of pushrod,
pushrod tube and cylinder bolt.

About your flat Honda piston, IMHO it will not work properly, the compression ratio is quite poor and you need inlet and exhaust valve pockets in your piston.


hope that helps
Josef

#9326 - 03/27/06 4:08 pm Re: b31 to b33 conversion  
Joined: Jul 2004
Posts: 346
pooch Offline
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pooch  Offline
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Posts: 346
Lake Conjola NSW Australia
Hi Josef,
good info.

If the B33 piston protrudes 3.5mm, where does this fit?

Is it a flat or domed piston?

My B31 head is flat for a bit where it sits on the cylinder, with a slightly concaved piston , which edges end up level with top of cylinder.

The combustion area starts furhter in than the piston sides.

My B31 piston would not fit if it was above cylinder.

It would hit the head.

This honda piston, I am hoping to end up same level as B31, and cut a couple of valve scoops in it.

I will have to machine a little more out of the B31 head so it fits over the B33 cylinder top ridge.

My calcs with an 86 mm bore are 511cc, and I am presuming a B31 head is smaller than a B33?

I roughly measured the chamber and it is about 73cc, so this will make a comp ratio of about 8 to 1.

Will be fun seeing how it goes on todays gas.


56 B31 with B33 barrel
51 Golden Flash with Dusting sidecar
#9327 - 03/27/06 4:25 pm Re: b31 to b33 conversion  
Joined: Jul 2004
Posts: 346
pooch Offline
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pooch  Offline
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Joined: Jul 2004
Posts: 346
Lake Conjola NSW Australia
And also, after 53, if the B33 conrod was shortened, am I correct in presuming that the gudgeon was lowerered this 1/2 inch, and cylinder and then piston top height remained the same?

If this is the case, did BSA try and reduce vibration or whatever by lowering the gudgeon height, or would it just be that less conrod is less reciprocating weight?

Has anyone got a pic of a post 53 B33 piston?


It would be easy to see, cos the B31 gudgeon is only just under the oil ring by about 2 mm.

Im in Aussie, so this may be an alternative too, I had a look at a grey motor holden piston yesterday, and it a nice light 3 inch, with 3/16th available too, but the gudgeon was way too low for my B31 conrod.

Maybe with a post 53 B33 crank/rod, if this B33 conversion does not work , I can uss this holden piston.


56 B31 with B33 barrel
51 Golden Flash with Dusting sidecar
#9328 - 03/27/06 10:29 pm Re: b31 to b33 conversion  
Joined: Aug 2001
Posts: 29
Josef2 Offline
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Josef2  Offline
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Posts: 29
Germany
hi pooch,

the measures are old, so I can't remember, if they are 100% correct. And the Excel-Sheet with the proofed measures is lost in the comnputer nirvana :-(.

The piston is domed, of course, to reach some 8:1 compression. The combustion chamber is of a perfect half circle shape with the circle medium point some mm's below the piston crown.

The change of conrod length was a further development for the Gold Star and you should know, that BSA was very keen for a modular range and interchangeable parts.

Josef


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