How not to B33!

[Rog1]

There's always something when you are getting a project on the road, just when you think you've finished; as I found out this year.

There are two types of piston listed in BSA's piston clearances service sheet (No. 704) for the B33; the two types have very different clearances:

Non-split skirt: 0.0045" to 0.0065"
Split skirt:    0.0006" to 0.00275"

So which type of piston is it, and what clearance was the cylinder bored to? I had to research this in June last year, when I had my M33 rebored to +40 thou.

There's also the piston ring end gap, which should be 0.010", says service sheet 701. I got the firm that rebored the cylinder to check and is needed adjust the end gap.

[iansoady]

Modern pidston clearances are far too tight for old air cooled engines, and many engineering shops do not understand this. Some years ago I rebuilt a 1930s Sunbeam which the previous owner had rebored and I had exactly the same problem as you.

I had an old school reconditioning workshop take another couple of thou out and it was fine.

I always err on the side of more rtaher than less clearance as few of us are going to wear out rebuilt engines.

[Dan_B33]

Thanks Ian.
I have called the guy.
He proposed solution:
1. Can send him barrel and piston hi will rebore to higher clearance.
2. Try to wear-in by short rides (perfect temperatures) if no joy after 20-30 hours send back for rebore.

Now thinking if I shall waste time trying to wear-in?

I had couple of short rides (2km) without stopage

[Rog1]

I think you'd be better off removing the head and barrel so that you can make sure you know what's wrong. It is most probably too tight a bore but what if it is too small a piston ring end gap?

Also, 20 to 30 hours of riding is 800 to 1200 miles/1300 to 1,950 kilometres, at an average speed of 40mph/64kph. How long would that take you, in short rides?

Send him the barrel and piston.

[Dan_B33]

Hello,
Many thanks for advising.
I will likely send it back.
For last 3 days I jus enjoyed short 2-3km rides. Just to award myself.

I have reanimated old installation and I have lights (12V bulbs and dead battery).
Dynamo works perfectly but no light on low rpm which is normal with battery flat.

I will enjoy few more rides and start strippin top end.

I will get there finally some day with all working....not yet though - no drama

[Dan_B33]

Here is link to the link of my son's Instagram. He has made short video:
BSA B33 Dan first ride

[Rog1]

Great to see the complete bike, and in action too. It starts well and sounds good.

[scifi]

Must admit I don't like the idea of re-boring the cylinder to take 1 or 2 thou off.  This is practically impossible to set up.   You would be better off polishing 1 thou off each skirt of the piston, with some 1000 grit paper.
Most pistons have different diameters on them.  The gudgeon pin sides can be less diameter, or even flat,  and the area above the rings is usually a couple of thou less than the skirts.
Remember it is the rings that do the sealing, not the piston skirt.

[Dan_B33]

Hello ,
I will have look inside and decide what to do next. Many thanks for that advice.

I will remove this home made carrier at the back.

We're there BSA made carriers ?

[neil1964]

As per sci-fi above is what I did (a more impecunious time for me) and it seemed to work in a low tuned motor doing low miles. Cleaned the piston up (high spots off with fine file and then fine carburundum/wet/dry type paper always following correct profile wrapped around the file) and correctly gapped rings. Of course it will depend on bore finish etc too.

[iansoady]

A decent machine shop can easily take one or two thou off using a hone. Not one of those hand-held things but a proper one.

[Dan_B33]

Small update.
Since I have the motorcycle in one piece I have started applying for legalization of it.
Will do MOT this week and then I could strip it agai

[Bigsingle]

There's always something when you are getting a project on the road, just when you think you've finished; as I found out this year.

There are two types of piston listed in BSA's piston clearances service sheet (No. 704) for the B33; the two types have very different clearances:

Non-split skirt: 0.0045" to 0.0065"
Split skirt:    0.0006" to 0.00275"

So which type of piston is it, and what clearance was the cylinder bored to? I had to research this in June last year, when I had my M33 rebored to +40 thou.

There's also the piston ring end gap, which should be 0.010", says service sheet 701. I got the firm that rebored the cylinder to check and is needed adjust the end gap.

Hi Roger , when you rebore a motor the new bore size is,  standard bore size plus the over size and sometimes you can add an extra one thou when honing , the piston is cam ground to provide the necessary clearance , also to add more clearance it is only necessary to put the barrel back on the cylinder hone . No one will try to remove .001"  or .002" with the boring bar because you need .0025 to hone to remove tool marks . Cheers .

[Dan_B33]

Thanks All,
This is comprehensive explanation.
I have not removed head yet.
Turned out I need to wait for the paper from certified specialist before Im allowed to make MOT. Papers under preparation.

I have tried to fit new central stand spring (from Dragonfly). Failed to extend it with thick coins an washers (not long enough even I stuck to whole available slots).

Any suggestions? How to fit central stand spring. Do not say zip ties cuts as that is far too thin to extend to the sizes need.

[neil1964]

I use a stout round body screwdriver through open spring and lever against convenient bit of frame/gearbox.
When you are lined up you slide the spring along the screwdriver to its lug and use another screwdriver to slide it onto the lug. It takes three or four attempts but the neither the wash/coin method (they kept falling out) nor the zip tie methods worked for me.
Did it couple of weeks ago on my off roader.  It was a pattern stand with poorly drilled holes and it sat low on the stops in the raised position.  So line bored out the 'in between' hole sizes and fitted new custom tube then serial adjustments to the stops so that the stand is up parallel with frame.  Long way of saying beware of new Indian supplied hardware unless you are prepared to do some serious fettling (I had to pay local co to do the accurate line boring. I'll post pics of the 'finished' machine on the Britbike Goldstar thread.