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[andy w]

I have a cylinder barrel for one of my bikes (BSA B40)which looks like it has not been bored true to the origonal centreline.I have been looking for information about allowable tolerances for this but cant find any,was hoping someone on this site might have some experience.
The barrel was bought second hand and looked ok .When cleaned up I noticed the chamfer at the bottom was not equal and on measuring the thickness of the bottom extended part found there was 0.6mm difference at the side to side.The top spigot is same all round.
Bore size is ok.I have to decide if I need a liner,look for another barrel,or give it a go as is.
Would appreciate any comments,experience,etc.

[Alan R]

Hi ANDY W -------------- just to start the ball rolling then }----I assume it's an "oversize" bore ??? If so, which size ??-----Is the bore at 90 degrees to cyl. or is it lopsided?? Do you have access to accurate measuring equipment OR a friendly Machine Shop?? Might be worth thinking of a re-bore to the next size, perhaps? Have you consulted Rupert Ratio's book for tech. details ?? I used to be a Self-Employed contract machinist back in the day and also have two friends currently running a pair of Ex-WD B40's. Where are you in the country ?? We might be able to help you ???

[Dennis C]

Hi Andy. This is fairly common to find on a rebored barrel, the bore does tend to wear more on one side than the other, often the machinist will put the barrel in the boring machine run a clock down one side and the front or back to set it up, with a bore that is worn one sided the result is a bore which is not true, although this is not an ideal situation, I don't think it will cause any problem.

[Les H]

Hi Andy. The cylinder bore is referenced and machined to correspond exactly at right angles to the cylinder base that mates with the crankcase mouth. The centre line of the bore should also correspond exactly and perfectly to the centre of the big end eye, it must not be out at all in the sideways direction as this will cause side thrust and rapid wear on the big end bearing. However it does not matter so much if the bore is set forward or backwards a touch as regards to being forward or backwards as regards the perfect TDC position. (De-Saxe engines are designed this way) The maximum wear on a barrel is on the thrust face of the barrel which is at the back of the cylinder. When the barrel is bored it is set up to sit on the face of bottom of the barrel and unless this was not achieved correctly perhaps if there was some swarf under the face, which is very unlikely, then the barrel would have been machined perfectly vertical from the top as well. You say the bore in relation to the top spigot is perfectly symmetrical so that is a very good sign that the bore is also perfectly symmetrical all the way down. It could be that the casting of the barrel spigot was off centre and as it has no importance on the actual bore it will not matter. Is there are way you can use an engineers square to check the bore runs down perfectly squarely from the top to the bottom or visa versa? By using the square on the top clean spigot face (you might have to use a steel plate to support it across the bore) the blade of the square should make contact on the bore all the way down its length in any position you place it. If it does then the bore is perfectly vertical and in line and you have nothing to worry about.

[Les H]

Just to make it more clear. Should have said: "It could be that the casting of the barrel's BOTTOM spigot was off centre and as it has no importance on the actual bore it will not matter ect etc"

[andy w]

Thanks for the replies to my post.I did use an engineers square to check the bore against the top and bottom faces and there was some sign it was not square,will try again and come back with figures.
The bore is +40 at present and with the different wall thickness at the bottom rim I think going up to +60 will not true it fully.The different thickness is at left and right sides.
I did think i could run a dial gauge down the bore but found it impossible to read so specialised measuring gauges are needed.
Location Devon.

[Les H]

Mornng Andy. from what you say I would seem that the rebore has been mucked up so I would now get a new cylinder liner pushed in. Liners are avilable from: http://www.westwoodcylinderliners.co.uk ... p.....make sure you take it to someone who has a good reputation for quality work to bore out the barrel and fit the liner..

[Les H]

Forgot to mention that you get the benefit of being able to use an old piston (with new rings) if in good nick so you save around 100 quid there. Before you get the liner fitted, have the chosen standard size piston available so the the mechanic can hone the liner for the the correct clearance.

[Alan R]

Hello again matey !!------------- just a thought---have a word with your local BSA Owners Group as to the location of a known engineering company that understand the particular needs of the air-cooled engine ( Also perhaps the VW people might have some leads ??)The machinist will need to have the preferred clearances for final honing. Rupert ratios book/CD OR the HAYNES manual should give this info. For my recent re-bore of the Honda PC 50 I just photo-copied the actual page in HAYNES.... yellow-highlighted the relevant sizes and gave that in with the barrel and new piston. Job done---bike running. Cost £40. Restored compression---instant starting---worth the money.

[Dennis C]

Hi Andy.... I have read with interest other peoples comments and advice on this, and a lot of sensible and relevant comments have been made, however when you consider your bottom flange shows as being 0.6mm thicker at one side than the other this in real terms means that if the bore is off the vertical the the real amount is only 0.3mm, you can easily end up with a true bore further off the vertical simply by fitting a poor quality gasket or tightening the mounting bolts incorrectly, I am sure this small amount will not cause any problem whatsoever, the free movement on the type of big end fitted to this bike and the amount of end float on the crankshaft is far greater.