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FASTENERS - (Nuts & Bolts to mortal man)
Well, I think of zips, and hooks and eyes
when fasteners are mentioned. It took quite a while to accept that
the new trade catalogue I was browsing was selling nuts and bolts,
but there you are, that's progress for you, Fasteners. Well they
do don't they, fasten things together.
There are two standards with which we are concerned; they are Metric
and Imperial. (After thirty-five years of working on British motorcycles
and before I mention "Imperials" let me first of all say, having
come to it rather later in life, how much I love the Metric System.
You will soon understand why.)
The Imperial system
as used on our beloved British mounts
includes a selection of the following types.
· Whitworth - Prior to Mr Whitworth, who laid down the first
standard pitch and profile dimensions for given diameters of round
bar, it was left to every village blacksmith or engineer to invent
his own. Interchangeability was virtually unknown. Because it was
"invented" so long ago for fairly "agricultural" machines it is
quite coarse, and was soon complimented by finer forms providing
greater resistance to shaking loose
· BSF and BSC - British Standard Fine, and of course, Coarse.
BSF being most commonly used on our bikes.
· BSC - British Standard Cycle, a fine thread commonly used
all over bicycles, employing Whit/BSF spanners, and used on motorcycles
in a wide variety of diameters from cotter pins to wheel spindles,
and notably, on carburettor mounting studs.
· BA - British Association, used for very fine small diameter
applications, scientific instruments, cameras, clocks, and on motorcycles
mostly in connection with electrical equipment.
· BSP - British Standard Pipe, (or "gas" thread) used for
the "plumbing" applications, petrol and oil pipe fittings.
· AF - By the 1960s when the greater portion of our exports
were destined for America rather than the colonies, to please them,
the more commonly accessed fasteners were made AF. (The easy bit
to remember about AF being that the sizes apply to the distance
in inches - right down to 32nds - between the jaws of the spanner,
easily remembered as Across the Flats of the nut.)
· Oddball or "bastard" sizes devised by some manufactures
for "special applications". Fortunately there aren't too many of
these, but the BSA steering damper for Girder fork models springs
to mind at 5/16" x 28 TPI (threads per inch). Apparently some genius
at BSA figured that this would be less prone to undo itself than
the common 5/16" BSC, which had a mere 26TPI. - Damper rods for
telescopic forks - are BSC.
You will appreciate then that you require a sizeable collection
of spanners to cope with that little lot. Then, if you are restoring
Japanese or European, you have the Metric System, which is boringly
DELIGHTFULLY simple and logical. All spanner sizes relate to the
diameter of the round bar on which the thread is formed. All pitches
we come across are either 0.5 mm (std) 0.25mm (fine) or 0.75mm (coarse)
and at the same angle, - and that's it! Moreover, metric fasteners
are readily available and cheap and usually bright zinc passivated
finish. Don't you just love your nuts bright passivated?
Types
Bolts have a plain portion
of shank beneath the head for a more secure fit into the hole. This
will locate the components which it is "pulling together" more accurately
than a setscrew, which is threaded right up to the head.
Setscrew
Ideally the plain portion of shank should come flush with the component
surface, washer(s) allowing the nut to be fastened tightly before
meeting the plain part of the shank. It is poor practice to use
setscrews in place of bolts because of their sloppier fit. They
are also more prone to wear at stress points and will then allow
components to vibrate.
Avoid using setscrews or threaded bar in engine or gearbox mount
applications. Rapid wear and vibration leading to cracked mounts
and damaged casings will always result.
Cheesehead is the name given to engine casing screws, which
should have a specially small spring washer beneath the head. These
washers often get stuck in the casing recess and are subsequently
lost. It is worth buying a packetful, they never come in wrong.
While on the subject of casings, watch out for the hollow dowels
(or sometimes studs) in some cases. There will usually be two, and
they ensure precise location when fitting, usually because a working
clearance must be maintained, eg for a generator, or to ensure alignment
of oilways. If a screw hole has a recess slightly wider than the
screw shank, there is meant to be a dowel in it.
Care of your nuts, and bolts
It should go without saying, but as I all too often witness
some horrible form of abuse in this application I'll say it anyway.
"Always use the correct spanner for the nut (or bolt) and always
use the right nut and bolt for the job."
Don't be afraid to modify spanners for special applications. By
grinding off corners - (BSA C15/B40 cylinder head nuts) - bending
shanks (Gold star R/H rear cylinder bolt) - or thinning socket spanner
walls (Bantam D3/D7 fork spring nut) are examples which spring immediately
to mind (probably because I've worked on so many of these).
To avoid damage, always try to use not only the correct size spanner,
but a ring or socket where possible. Open-ended spanners should
have thick jaws for as broad a grip as possible. So-called "slimline"
types can "bite" into mild steel nuts and wreak havoc. Damaged hexagon
heads may be refurbished if you haven't got the correct replacement.
I used to file burrs away, but now I prefer to lay the damaged article
on the back of the vice and "persuade" damaged metal back into place
with subtle application of a hammer, it's much more satisfying.
I recommend you buy a pair of Thread Files too, these are
square bars with thread profiles in the form of a rasp. With eight
profiles on each you can "dress" and reclaim any imperial thread
you're likely to encounter. (Don't forget to use it at a slight
angle, ie follow the thread you are reclaiming.) They are quite
cheap, and they are invaluable when dealing with damage such as
deformed wheel spindle threads where replacement would be expensive.
More expensive thread files incorporate "chasers" at the ends for
dressing internal threads.
Washers and locking devices
This is fairly straightforward but some pointers need to be made.
· All nuts and bolt heads should have a washer under
them. This will be at least a plain washer to protect the component
surface and to spread the load of the nut.
· Most applications will also require a spring-washer
under the nut. (You'd think that a plain washer beneath a spring-washer
would negate its effect, trust me, it doesn't.)
Spring-washer
· Cylinder head bolts or nuts should have double thickness
plain washers. (Haven't you ever wondered why standard washers warp
in this application?)
· There is a new kind of
spring washer available now called a "wave" washer, unlike
the split spring washer they don't dig in and damage the surface.
(Generally metric sizes only, but 13mm is compatible with ½" , and
7mm with ¼" for example.)
· Tab-washers should be used where specified for locking
purposes, and only need bending over on one flat of the nut, not
all round.
Tab-washer
· "Thackwray" washers look like double coil spring
lock-washers but are not intended to be fastened tight. They are
found in applications where constant movement is necessary and are
done up half-tight, acting as a "spring" to absorb vibration and
prevent rattle and are often used in conjunction with a nut and
split pin.(Eg brake torque arms.)
Locknuts
Locknut
· Nylocs are nice but are ineffective after being undone
a few times
Nyloc
· Metal self-lock nuts can be made effective again by closing
up the split portion, but will deform the threads of the bolt.
· Two nuts tightened against each other work well, but look
messy and take up extra space.
· Castellated nuts with a split pin are not meant to be done
up tight, and are used in applications where a little movement is
desirable. They are found on rear brake torque arms to allow movement
for adjusting the chain without disturbing the brake-plate. Some
Japanese wheel axles use castellated nut and pin, but always in
conjunction with some other device for retaining wheel location
(screw adjusters, snail cams etc) they should be "firm" but not
too tight.
"Gentlemen, fasten your nuts please" A Cautionary tale.
A pal with a very nice and indecently quick 1939 500cc BSA had done
a hurried adjustment prior to setting off on an event. With his
little lad on the pillion pad, he roared off to catch the others
at the start venue. He never made it.
Applying the rear brake some miles later, the rear end locked up
and they all slid down the road. Fortunately they were unhurt, but
the bike was a mess. The bolt securing the rear brake torque arm
had gone, dropped out, or forgotten in haste?
The brake plate, with nothing to retain it, had revolved with the
wheel, wrapping the rod round the axle and applying the brake both
hard and permanently. The frame had also bent where the brake plate/torque
arm (as a unit) had jammed after rotating as far as it could. If
the torque arm had had the correct nut and bolt, there'd have been
no need to undo it, would there?
| Comments |
| Ross 8th July 01 | Very useful guide - thanks.
| | Lacy 27th September 01 | An interesting article. One point, Surely metric spanner sizes relate to the measurements across flats in millimetres?
| | Spannerman 16th November 01 | OOPS! - Yes, I meant to say that "all bolt screw and nut quoted sizes relate to the diameter of the round bar etc -, and yes, spanner sizes do refer to the width of the gap "across the flats"
Thanks for pointing the error out.
Regards, Spannerman.
| | dru 8th October 02 | How then do you work out which spanner for which bolt . you have to know both the bolt AND the head sizes. Not so simple after all ! Do the head sizes vary, for the same diameter bolt ? At least if I use a 5/16 BSF bolt the spanner says on it, 5/16 BSF !
| | arjun 10th November 02 | Really very good information about british standards.
plz can you let me know sir what is the difference national threads.
thank you
| | Spannerman 19th November 02 | arjun - American National 60 degree threads had a sharp root and crest and were made obsolete post WW1 by the introduction of Unified (UNF) which have a rounded root and crest. Unless you are restoring pre WW1 American bikes, it won't concern you.
| | J Wagner 11th March 03 | It's not "Thackwray washers" ...
It's spelled "Thackeray washers"...
I thought you might want to make the correction.
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