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B33 Piston #18704
12/12/06 1:15 am
12/12/06 1:15 am
Joined: Dec 2006
Posts: 8
Melbourne Australia
Royce Offline OP
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Royce  Offline OP
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Joined: Dec 2006
Posts: 8
Melbourne Australia
I have a 1951 B33. I bought a new piston for it, but I only found out just recently that there is a long and short conrod version of the engine.
I gather that my B33 is a long conrod as BSA changed to the short version in 1952.
The dealer didn't ask about what year or short/long conrod so I am wondering if I have the correct piston.
There is a shim under the barrel about 1mm and the top of the piston reaches 6.8mm from the top of the cylinder. Is this the correct piston?
I guess a short rod piston in a long rod motor should go up higher and perhaps hit the valves.
Unfortunately I had the valves re-seated and they managed to pocket the valves so I don't know if the valves would hit the piston if they were in the correct place.
Can you get inserts to restore the valve position?
What is involved? I better do it myself as I am unlikely to take them back to that shop.

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Re: B33 Piston #18705
12/12/06 1:54 am
12/12/06 1:54 am
Joined: Jul 2004
Posts: 346
Lake Conjola NSW Australia
pooch Offline
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pooch  Offline
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Joined: Jul 2004
Posts: 346
Lake Conjola NSW Australia
Gday Royce,

Admittedly, mine is a 56 B31, but my piston goes up dead level with top of cylinder before and after changing to B33 barrel.

I am using a small fin barrel prob 51, until I can find a big fin one.

I used a 10 HP sidevalve honda industrial engine piston and it dropped straight in(with a rebore), perfect pin dia and height.

The honda piston had no valve reliefs, so I dolloped a heap of grease on piston and loose assembled and saw there was miles of clearance.

It runs great.


56 B31 with B33 barrel
51 Golden Flash with Dusting sidecar
Re: B33 Piston #18706
12/12/06 4:34 am
12/12/06 4:34 am
Joined: Dec 2006
Posts: 8
Melbourne Australia
Royce Offline OP
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Royce  Offline OP
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Posts: 8
Melbourne Australia
Hi Pooch,
my piston is 40 oversize. What is the diam of the Honda piston?

Re: B33 Piston #18707
12/12/06 6:57 am
12/12/06 6:57 am
Joined: Jul 2004
Posts: 346
Lake Conjola NSW Australia
pooch Offline
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pooch  Offline
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Posts: 346
Lake Conjola NSW Australia
I did not measure it exactly.

I know I should have.

But I think it was about 85.6 mm, which means about 25 thou OS .


56 B31 with B33 barrel
51 Golden Flash with Dusting sidecar
Re: B33 Piston #18708
12/12/06 7:45 pm
12/12/06 7:45 pm
Joined: Apr 2002
Posts: 962
North East England
beezageezauk Offline
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beezageezauk  Offline
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Joined: Apr 2002
Posts: 962
North East England
Hi Royce,

The difference between the two types of B33 piston is the height of the gudgeon pin in relation to the piston crown. The earlier piston has the pin positioned further down the piston than the later one. This compensates for the difference in the conrod lengths.

You should not need the shim under the barrel as the correct piston should be level with the top of the barrel and will not hit your valves. In fact, if the valve seats are pocketed, shouldn't this give a further clearance between the piston and valves?

Could you contact the dealer and ask him to swap the piston for the correct one? It would be helpful if you had the correct part number.

If I can find the part number I will post again.

Beezageezauk.

Re: B33 Piston #18709
12/12/06 11:31 pm
12/12/06 11:31 pm
Joined: Dec 2006
Posts: 8
Melbourne Australia
Royce Offline OP
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Royce  Offline OP
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Joined: Dec 2006
Posts: 8
Melbourne Australia
Thanks Beezageezauk.
So it looks like I have a short rod in my engine and by putting the 'correct'piston in the compression is a lot less than it should be. I'm surprised that the skirt didn't hit the flywheel.

I'll remove the shim, but I'm sure I won't get the 6.8mm difference to the top of the barrel.
Do you know the difference in the con rod length? Perhaps that isn't standard as well.

Unfortunately the piston was ordered in specially and bought some time ago, so I don't think I can swap it. Looks like an job for eBay.

Re: B33 Piston #18710
12/13/06 1:25 am
12/13/06 1:25 am
Joined: Sep 2001
Posts: 2,795
Comox BC Canada
G
Gordo in Comox Offline
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Gordo in Comox  Offline
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G
Joined: Sep 2001
Posts: 2,795
Comox BC Canada
The early B33 rods had a 7.375 inch rod and the 53 on used a 6.875 inches according to the Bacon restoration book. That would be about 12 mm difference if the barrels stayed the same length. Could be that the rod is something else altogether. The Gold Star rods were 7.375, 6.875 and 6.468 for example.

What is the number on your piston? For the long rod engine the centre of the pin should be approx 1 7/32 inches from the top of the piston side.

From a look at my pistons, the early pins are closer to the top of the piston than the later versions.

Gordo


Without frequent roadside repairs there is no fun in riding!
Re: B33 Piston #18711
12/13/06 1:55 am
12/13/06 1:55 am
Joined: Jul 2004
Posts: 346
Lake Conjola NSW Australia
pooch Offline
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pooch  Offline
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Posts: 346
Lake Conjola NSW Australia
Can others give references to long and short rods besides Bacon's book?

Is it an absolute fact they went from long to short or short to long?

I presume the B31 is same ?

My 56 must have a long rod because the pin is only a smidgeon below oil ring.

The cases are date stamped late 55.

But who knows what after 50 years of tinkering.

It seems if Royce has a long rod, a long rod piston would be level, but a short rod piston would stick above barrel.

For it to be 6.8 mm down, he must have the short B33 rod and long piston, but as Gordo states, this would be a bout 12 mm diff, not 6.8.

Well, that's the long and short of it.


56 B31 with B33 barrel
51 Golden Flash with Dusting sidecar
Re: B33 Piston #18712
12/13/06 3:35 am
12/13/06 3:35 am
Joined: Dec 2006
Posts: 8
Melbourne Australia
Royce Offline OP
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Royce  Offline OP
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Joined: Dec 2006
Posts: 8
Melbourne Australia
So removing the shim at the bottom gives 7 to 7.3mm or 0.276 to 287" halfway between the long and short rods
I rode around on the old piston for years, but thought that I would rebuild the motor.
The original piston has no makings that I can see.
It also has a flat top without the reliefs cut in for the valves.
The centre of pin to top of piston is 40.8mm (1.61")
I thought the old guy I bought it off, many years ago, said he used to race Goldies. It originally had a car tyre on the back so I assumed that it was just an old paddock basher.
I wonder what model Fergie it is out of, TEA20 perhaps.
So I don't need a new piston, I need a new rod.

Re: B33 Piston #18713
12/13/06 5:14 am
12/13/06 5:14 am
Joined: Jul 2004
Posts: 346
Lake Conjola NSW Australia
pooch Offline
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pooch  Offline
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Joined: Jul 2004
Posts: 346
Lake Conjola NSW Australia
Post the height of barrel from bottom of flange to top of cylinder, and check if stroke is 88mm.


56 B31 with B33 barrel
51 Golden Flash with Dusting sidecar
Re: B33 Piston #18714
12/13/06 11:30 am
12/13/06 11:30 am
Joined: Dec 2006
Posts: 8
Melbourne Australia
Royce Offline OP
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Royce  Offline OP
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Posts: 8
Melbourne Australia
I'm out of town for a couple of weeks.
I'll check when I get back.
Thanks for all the help

Re: B33 Piston #18715
12/13/06 8:24 pm
12/13/06 8:24 pm
Joined: Sep 2001
Posts: 2,795
Comox BC Canada
G
Gordo in Comox Offline
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Gordo in Comox  Offline
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G
Joined: Sep 2001
Posts: 2,795
Comox BC Canada
Pooch: My early 49-52 engines have the long 7.375 inch rod. A crank from a 57 B33 engine has the short rod at 6.875 inches which seems to support the numbers in Bacons books. There two barrel sleeves available so there must have been a barrel height change at some point in the development. While the pin position is much lower in the later GS engines I do not know how this compares to the B33 engines.
Gordo


Without frequent roadside repairs there is no fun in riding!
Re: B33 Piston #18716
12/13/06 8:27 pm
12/13/06 8:27 pm
Joined: Apr 2002
Posts: 962
North East England
beezageezauk Offline
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beezageezauk  Offline
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Joined: Apr 2002
Posts: 962
North East England
Pooch...BSA decided to alter the length of the conrod on the B33 models but for some strange reason left the B31 engine as it was. Therefore this conrod/piston problem only occurs on the B33.

Furthermore, I am sure that both the B31 and the B33 barrels are the same height and remained the same throughout the model range. Like Pooch, I've played about with barrel swaps on these models.

I'm curious the know why the B33 conrod length was changed in the first place. BSA obviously had a valid reason. Does anybody here know?

Beezageezauk.

Re: B33 Piston #18717
12/13/06 8:55 pm
12/13/06 8:55 pm
Joined: Jul 2006
Posts: 887
newcastle australia
trevinoz Offline
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trevinoz  Offline
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Posts: 887
newcastle australia
Royce, here's my two-bob's worth. Check your barrel height as it was common for raced B33's to have barrels machined down to accomodate other conrods e.g. Matchless/A.J.S. Original untouched ones are getting scarce. Maybe thats the reason for the compression plate at the bottom of your barrel. You didn't give a thickness. You should get new valve seats fitted to your cylinder head. Any good engine reconditioning shop should be able to do it. Get unleaded seats. Good luck.

Re: B33 Piston #18718
12/13/06 9:54 pm
12/13/06 9:54 pm
Joined: Jul 2004
Posts: 346
Lake Conjola NSW Australia
pooch Offline
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pooch  Offline
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Joined: Jul 2004
Posts: 346
Lake Conjola NSW Australia
Gday Trev, if the barrel was cut down, the problem would be the piston too high, but the piston is too low.

It seems a drastic move to change or think that the rod is wrong when it is probably only the piston.

When Royce gets gets back, he should post ..

barrel height.

distance from pin centre at TDC to top of barrel.

stroke.

and state where old piston sat before re TDC.


Beezergeezauk, I thought a long rod gave more angular torque to flywheel, but at the expense of more rotating mass, but a shorter rod gave a lower engine height , but it seems BSA did not save the 1/2 inch by cutting down engine height.

Maybe the bean counters saved a 1/2 inch of conrod metal only given that the piston is the same mass regardless of where piston pin boss is.


56 B31 with B33 barrel
51 Golden Flash with Dusting sidecar
Re: B33 Piston #18719
12/14/06 4:48 pm
12/14/06 4:48 pm
Joined: Aug 2001
Posts: 31
Germany
J
Josef2 Offline
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Josef2  Offline
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J
Joined: Aug 2001
Posts: 31
Germany
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 dimensions should be the same for B31 and B33, because the spare parts catalogue lists only one type of pushrod,
pushrod tube and cylinder bolt.

Josef


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