The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: ppanichelli on 08.06. 2015 15:13
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Hi all!
I start talking about the bike of my avatar, a BSA A7 1949 I started to restore.
I don't know if it's all same around the world, but here in Argentina, ALL the BSA cylinders are cracked and patched up. My bike was not the exception (of course!).
After verifying it, the mechanic adviced me to get another cylinder...(you know it's not just going to the supermarket)...anyway, after a hard research I found a guy who was selling a god blessed BSA cylinder. It was cracked and fixed, but usable.
This guy was an old school mechanic, who once upon a time talked with an engineer that used to be the BSA Technical Country Manager about this issue.
The engineer told this guy that the problem (in Argentina) with the cylinders was that everybody used to rebore them at 90°, ignoring that the angle of turn of the cylinders was not 90°!!! This error generated an extremely thin cylinder wall, which eventually would break.
However, if it's not 90° the question is: which is the right degree of the cylinders?? Anybody has this information? If not, it's still a nice data to bear in mind!!
regards
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Hi ppanichelli
congratulations on your excellent English and hope your restoration goes well for you, the issue of the 90° thing, I think you mean +90 which is the measurement of piston size ( = 90 thou over standard), yes it is too much +80 is done but even then some people think that is also getting near to the limit.
When I restored my Flash it was on +60 and needed a rebore, I used liners to bring it back to standard then put in standard pistons.
so to answer your question +80 can be done but +90 a bit much
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Hi, thanks for your reply Bill! I'm glad that somebody understood what I wrote!! :D
Regarding your comment, I was not referring to the piston diameter, which is 62 milimeters indeed. I referred the angle of "inclination" of the cylinder:
If you put the cylinder on a table, you would think that the piston would enter through the tubes in a perpendicular way (90°)
What I've been told is that the inclination of the tubes (where the pistons works) are not 90° degrees, but a little bit less: there's certain inclination at both tubes to the front. What I'd like to know it's the right angle of the tubes.
If this tale it's true, then we'd have the reason of why everybody cracks their cylinders and also the knowledge not to do it anymore.
Thanks in advance!
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Oh right - well that's a new one to me, never heard of it before.
I would assume 90 degrees to the base of the barrel otherwise how would shops know how to set up their boring machinery, also how would thery machine heads to match barrels.
However would be interesting to hear other views on this (as I'm sure there will be)
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OK, let's dispell this, it's 90 degrees. Perhaps the reason the cylinders are cracking around where you are in Argentina is because someone started a rumor that it was something else and local practice was to go in off of perpendicular to the base.
Richard L.
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*dunno*...no offense p-panno, but I guess we won't be buying too many spares from Argentina then....? *whistle*
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Hi All,
Yes, 90 degrees to the base of the cylinder, NOT the head surface !!
Maybe BSA sent all the "reject" cylinders to Argentina *eek* *eek* *eek* *eek*
John
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Geez- one'd think head and base would be parallel... That could make some people nervous. . *eek*
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This thread is raising a whole lot more questions than answers right now. Keep going ...
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Hi All
Geez- one'd think head and base would be parallel..
They should be parallel,
but if you think about it, it doesnt matter if the head/cylinder joint is a little "off"
The cylinder base must be parallel to the crank and at 90 deg to the bores
The cylinder base must be used as the reference face when reboring the cylinder
HTH
John
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Fairly sure there was some discussion about this in a thread a couple of years or so ago....?
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Hi All
Geez- one'd think head and base would be parallel..
They should be parallel,
but if you think about it, it doesnt matter if the head/cylinder joint is a little "off"
The cylinder base must be parallel to the crank and at 90 deg to the bores
The cylinder base must be used as the reference face when reboring the cylinder
I don't think Parallelism is the best way to tolerance this cylinder business. When I took my A10 cylinders off, the rings had worn a large "step on the outside of each bore and the inside (where the cylinders were closest together) had no "step" at all. Clearly these bores were canted toward the center of the motor. Parallelism would simply have both cylinders wear the same, although both cylinders would be incorrect. I would say that "True Position" of the bore centerline in reference to the crank center and "Perpendicularity" in the X & Y position. When you assume that the bottom of the cylinder is a good reference point for boring, also be certain that the mating surface on the block is qualified correctly with the crankshaft. It may be in some instances that the cylinders were made correctly and the blocks surfaces were the problem. From what I've seen, nothing is outside the realm of possibilities.
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This thread is raising a whole lot more questions than answers right now. Keep going ...
sure is, including from my point of view(see what I did there) not assuming the writer means something he/she did not state *red*
Thinking further - the only thing I took to my machine shop was a piston and the Barrels, so the reference point they use has to be the base of the barrels so the onus then has to be with BSA that they got that and the crankcase top face machined at 90 deg (no degree sign on this keyboard) to the centre line
The cylinder base must be parallel to the crank and at 90 deg to the bores
as John has said
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I always tend to presume with this older technology that a whole lot of things really aren’t that important. Metallurgy was nowhere near as precise, tooling wasn’t that much better – even when it was new. Given those factors, I get kind of surprised when we worry about a few degrees difference in ign. advance between cylinders, or when we discuss the need for an intake bias gasket etc. My presumption now, is that excepting the odd low mileage barn find we are now running these bikes waaaaaay beyond their design life, and some of us also somewhat beyond what might in the day have been considered a sensible state of tune.
However, we are still running them. And hence this kind of dialogue is almost infinitely fascinating (no irony intended).
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Don't want to get too involved but. ...
(no degree sign on this keyboard)
On my Macbook it's not 'on the keyboard', as such- I found it by pressing many combos of buttons until I found it: 'option + K '... depends on the 'puter..
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ON a PC it's alt+ 0178 I think, however by the time I've done that I could have typed "degrees" (or something close ;) )
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ON a PC it's alt+ 0178 I think,
Bill you can use ALT+ 0176 or ALT+ 248 either will do it. What a sad life I lead.
John
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Heh Dutch, my barrel base was not even flat....it needed over 020 milled off to make it flat, luckily it was a thick flange type. Now not so thick.....
My engineer spent quite a while figuring out the best way to ensure everything lined up as well as possible, and found out that the boring/honing work and top face machining work were most likely referenced off the top face and not the bottom base of the barrel........
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My engineer spent quite a while figuring out the best way to ensure everything lined up as well as possible, and found out that the boring/honing work and top face machining work were most likely referenced off the top face and not the bottom base of the barrel........
Yo Kiwi, I guess it's the obvious thing to do with out thinking about it. It likely explains why I have a difference of a few thou from top of pistons to top of barrel- but as I said earlier, I'd read a discussion(maybe yours) about the boring method, and may or may not have measured head face to flange, but put it down to the rods being a bit whacko anyway....so changed them and got on with it.. *dunno2*...
I guess the obvious fixit, is to set the barrel on the flange and bore it and skim the head interface at the same time (if it can be done on the same machine)
Started this a couple of hours ago...better send it before I forget... *eek*
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Heh dutch, the machinest made up a solid ally mandral which fitted inside one bore exactly and which had a perfectly square end which was bolted to the mill bed abd the barrel slId over it.....then he machined all round the barrel base....thus making the base 90deg to the bores....
the cylinder face was left "wrong" as he felt it woukd make little difference fixing that up
he had to make the mandral sightly tapered to fit the bore perfectly *eek*
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wow, this forum is amazing! that was my first post (after the introduction) and it generated a really cool interaction.
Thanks so much to everybody for the answers!!! Also to the funny ones ;)
Well, in order to bring some additional confusion, I could mention that after reviewing the cylinder with the mechanic he noticed that the barrels below the base had one side 2 mm wider than the opposite side.
This fact remarks that someone was wrong regarding the barrels angle. We assumed that the the boring machine was set 90°, which could give some credit to the version of barrels not at 90°. Or it also could be that they swallowed the fake version and ruined the cylinder.
So far I only have a collection of broken cylinders and a forgotten bike that doesn't roar anymore :(
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surely the way to go forward is another set of barrels. i have seen a10 barrels come up on u.s. ebay which must be within postage distance. barrels are expensive and probably the rarest part of the engine so make sure you buy a good low milage set.
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*pull hair out*
Heh dutch, the machinest made up a solid ally mandral which fitted inside one bore exactly and which had a perfectly square end which was bolted to the mill bed abd the barrel slId over it.....then he machined all round the barrel base....thus making the base 90deg to the bores....
the cylinder face was left "wrong" as he felt it woukd make little difference fixing that up
he had to make the mandral sightly tapered to fit the bore perfectly *eek*
*pull hair out*
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hmmm,
The bores have to be at right angles to the centre line of the crank lengthwise.
however i'm not so sure about the 90° bit when you are looking from the side of the motor.
if you google 'cylinder offset from crank' you will find some engines do not have the bore directly over the crank.
At top dead centre you can push the piston down with your finger due to this offset.
Maybe this is what Panni's mechanic was referring to.
I have a vague idea musky talked about offsets once. (pistons maybe ??)
cheers
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Yo Olev, I'm pickin' up what yo're puttin' down, and it's not on my boot....I think Musky was talking about piston/gudgeon/wrist pin offset..but I'll let him sort that out when he sobers up... *smile*...
...for the rest of it I'll have to draw some artwork on the back of a drink coaster...(might've finally found a use for them *ex*)
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i m a little lost on this one as i thought all modern boring bars bolted to the cylinder head surface and relied on the manufacturers machining being correct at the base flange.
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Unfortunately I sold an a7 barrel with standard bore for $200 a while back, in New Zealand I thought myself lucky to find a buyer!
Is this barrel being sold by boa any use?
http://en.vintage-motorcycle.com/index.php?language=en&site=4&pid=369&id=9603&limit=0