11 July 2008
What is it about exhaust nuts? They seem
one of the parts you can generally rely on to be a problem on an old motor
cycle. Consequently it seems they regularly get 'bodged' at some point in an
Here are my trials and tribulations in the hope they may help
someone struggling alone in their shed.
A short sorry history
of a Sunbeam Lion's exhaust nut
My 1931 Lion
was acquired with an exhaust pipe held in place by a nut that didn't even connect
with the port's internal thread. Two holes had been drilled and tapped through the
cylinder's projecting exhaust port and into the nut. An ingenious solution,
erm?! Through these
holes two small screws
were used to hold the nut, and hence the exhaust pipe, in place ... or had
done once! (photo, left). The nut rocked and wobbled, and lasted one trip around the block before the exhaust pipe blew
out leaving remnants of two old screws fouling the internal thread of the
Luckily, a replacement was
found in the form of a well-worn original item from an identical 1931 engine (photos,
below). It just had enough 'bite', fouled exhaust port thread and all, to
hold the pipe in place. It saw me through a few years until one day the
connection between the internal and
external threads became non-existent. Unsurprisingly, this occurred 20 miles from home! A
few feet of garden wire fashioned into a tourniquet, of sorts, held the
exhaust pipe in place long enough to get me back. (My
late grand-father - a Marston's employee - advised always carry a few feet of wire
as well as string in your tool kit as it won't burn if you need it to be in contact with anything hot. It turned out
to be useful advice.)
A new nut was
needed. I turned to The Marston Sunbeam
Register as they advertise side-valve exhaust nuts as one of their available spares. Unfortunately when I enquired
it turned out the
batch had been made up some time ago and had
now all gone. The Lion was consigned to the shed.
Sometime later a further engine
of 1928 vintage arrived in the shed, from which projected a gleaming chrome exhaust nut.
It was clearly home-made. It was, however, a beefed-up, more robust item than the original
(photos, above). It seemed the Lion would have its missing part and be back
on the road. Unfortunately the ungainly projection of the gleaming nut from the
exhaust port was explained not by
the usual explanation - the laxity of previous owners in tightening up nuts
and bolts when assembling stuff for sale - but by a failure of whoever
fabricated it to cut the correct thread to fit the port. The nut was no use
at all. That's when the measuring started in earnest! Time for a bit of
Getting serious: measuring and cleaning-up the
It seems that
the 'blind' cylinder, side-valve engines up to 1931 have a 1½ inch diameter
exhaust pipe, whilst the side-valve engines introduced in 1932 with a
detachable head have a 1¾ inch diameter pipe. At least engines I've had a
good look at seem to be consistent with this.
Out with the thread
gauge to measure the screw pitch. For the 1½ inch
diameter exhaust pipe (the blind cylinder engine) the internal thread for the port
t.p.i. (threads per inch) as did the original, much-worn exhaust nut
(photos, above). This explained why the shiny, home-made nut at 25
t.p.i. wouldn't fit. Thankfully it hadn't been forced so the internal thread
to the port on the engine with which it came seems fine.
However, the condition
of the internal thread in the exhaust port of my 1931 Lion engine was a cause for concern. There was a
momentary dread that it was worn beyond any practical use and I would need to renew
it, possibly with some form of insert. I spoke to people about this. It was
all worryingly complicated! And, very few of the engineering companies that
the old bike press wanted to touch the job.
Before going down
this road, I thought I'd clean up the thread with the correct-sized tap and
see what I'd got. A huge 1¾
inch diameter tap in British Standard Cycle thread pitch of 26 t.p.i. was
called for. Thankfully, Tracy Tools Ltd of Dartmouth were able to supply
just the thing (tel. 01803833134 or web site: www.tracytools.com). And, even
more thankfully, the exhaust port thread, complete with its bodged remnants
of broken-off old screws, cleaned up well (photo, left). This required a
very gentle hand, and involved lining the tap up and letting it 'bite' by
itself rather than forcing it into the port.
All it needed now
was an exhaust nut that fit. Time to fire up the lathe. The shed had
recently acquired one of those very useful combined lathe and mills from
Chester UK (www.chesteruk.net). I had promised myself one after my
access to a Colchester student lathe ended with the closure, a few years
ago, of the model
engineering evening classes at the local college.
Sunbeamland nut appears to be made of mild steel. Its maximum outside
dimension is 2 inches with a 1½ inch hole for the exhaust pipe, which meant
I ideally needed 2 inch steel tube with a ¼ inch wall. Finding mild steel
tube with a ¼ inch wall proved impossible. As I didn't fancy removing all
that metal from a solid bar I decided to use brass tube which was available
in this size.
Mallard Metal Packs
Ltd of Birmingham supplied
me with a 6 inch length of brass tube at 2 inch
diameter with a ¼ inch wall (web site:
www.mallardmetals.co.uk or tel. 0121 624 0302).
I started the fabrication
of the nut by sawing
off a few ¾ inch
brass tube with a hacksaw in the bench vice. These would allow me a few
attempts to get it right! The attempts included a replica of the original,
Sunbeamland-made nut with its more slender flange as well as a nut with a wider, more robust flange for the four
slots for the C-spanner, similar to the home-made jobby that
had arrived with the 1928 engine (photo, above left).
In the lathe, I faced-off back and front
of each section of tube. Next, I marked off a length of 7/16th inch to be
turned down. This was turned down by an ⅛ inch deep cut,
leaving a reduced diameter of 1¾
inch to take the screw thread (photo, above left). N.B. I use the formula in
Lawrence H Sparey's very useful 'The Amateur Lathe' (first published 1948) for setting the lathe speed for turning brass.
He suggests a cutting speed of 200 ft. per min. for brass. To set the r.p.m.
for the lathe, he advises take the cutting speed (200) and divide by a quarter of the
diameter of the work in inches (0.5 inch for 2 inch tube). Which in this case
requires I set the lathe to 400
r.p.m. Finally, once the length was turned down, the 'shoulder' between the two diameters was faced-off.
The result was a 'blank' that didn't quite fit over the exhaust pipe, so
boring its internal surface to remove 1/16th of an inch was called for -
i.e. a 1/32nd inch cut (photo,
'blanks' fitted over the 1½ inch diameter exhaust pipe a treat. They were
bored out sufficiently to cope with the various curves in the pipe whilst
not being so wide internally that they failed to contact against the exhaust
pipe's terminating flange (photo, left showing three nuts on the exhaust pipe).
This leaves two
operations, screw-cutting a 26 t.p.i. pitch on to the nut (requiring a 60° lathe cutting
tool, required for British Standard Cycle thread) and milling of the four
the outer flange of the nut used for tightening it with a C-spanner.
Milling used an 8mm
(9/16th inch) end mill to cut slots to the full depth of the outer flange.
Advice in Sparey on cutting speed for milling brass simply advises that end
mills can be run at higher speeds than other types of cutter, and brass at
higher speeds than steel and cast iron. It seemed, the faster the better!
Care is needed in
lining up the work, too, to ensure that the cut is square and occurs at 90°
intervals around the circumference. I marked the nut out accordingly before
milling. I used a length of narrow steel rod in the lathe chuck as a guide to line up
the end mill and exhaust nut.
The end result is an
exhaust nut missing only its screw thread. Unfortunately, my lathe arrived
without the necessary gear for cutting 26 t.p.i. - this was annoying as I
advised last year, when I went to look at the lathe that I eventually bought,
that it came as standard! I stressed that the
ability to screw cut at 26 t.p.i. was essential for me. Thankfully, Chester
UK have been able to supply
the appropriately-toothed gear quite quickly and at a reasonable price.
The exhaust nut was
firmly clamped in the three-jaw chuck and gears set up as per the
instruction helpfully provided by Chester UK. I was ready for screw cutting.
The power feed lever on the lathe proved invaluable, allowing me to turn off
the power feed to the carriage with the cross-slide and the tool post. This
meant I didn't have to remove the tool from the job and remained permanently
engaged with the lead-screw of the lathe. The reverse button on the lathe
then allowed me remain engaged with the lead-screw whilst reversing the
direction of the cut. By this means I was able to re-position the tool at
the start for the next cut (photo left above).
Spacey's advice on
screw cutting is to take light, shallow cuts of no more than .005 inch (5
thou) for each pass. I went down to 4 thou which seemed noticeably smoother.
The cutting tool has to be the right profile (60° in this case) and at the
exact centre and 'square' with the work (the tool is just visible in bottom
centre of the left-hand photo, above). The lathe should set at its slowest
speed - in my case 160 r.p.m.
The total depth of
cut was monitored on the cross-slide's dial. But how deep to cut? A letter
from David Hull in 'The Classic Motorcycle' of June 1985 helpfully advises
that for a 60° thread, the depth of thread is 0.5237 multiplied by the
pitch, i.e. 1 divided by 26 in this case. (For 55° threads, the figure is
0.6403 multiplied by the pitch.) That gives a cut of .020 or 20 thou. I
tried this on the first example I cut. It was just too big for the port, so
on the second version I cut to .025 or 25 thou (photo above right). This
fitted a treat!
The finished result, firmly holding the
1931 Lion's exhaust pipe in place is shown in the photo (left).
And finally, a bit of Sunbeam research
Out of interest, Marston's 1931 'Price
List of Spare Parts', identifies the part as 'exhaust pipe union', reference
D.423, used only on the Lion model that year. It cost 3/- (three shillings).
The 'D' number is a specific reference for the 1931 spare parts list.
The factory reference number for the part
is also given, it is part no. 16. Marston's 1929 spare parts list shows the
same part was used on pre-Lion saddle tank Models 1 and 2 (350cc side-valve)
and Models 5 and 6 (500cc side-valve). The reference number from the 1929
spare parts list is C.308.
An earlier spares part list of 1927
indicates it was also used on flat tank Models 1 and 2 (350cc side-valve)
and 5 and 6 (500cc side-valve). The reference number is A.363. I have no
earlier spare parts catalogues, but as 1927 saw the introduction of the
cross-over exhaust pipe for the side-valve engine - to replace the earlier
'coffee pot' exhaust - I cannot say with certainty that a 1¾ inch, 26 t.p.i.
union nut was used earlier than 1927.
The spare part lists all refer to an
exhaust union washer with the nut. I have not seen this on the three
Longstroke engines I have removed exhaust pipes from. Illustrations in
Marston spare parts list show it as roughly the same size as the exhaust nut
but I have read no reference to it in Pitman, etc. I am not sure whether
such a washer would sit in the port prior to inserting the exhaust pipe, or
be placed on the pipe itself between flange and union nut. I haven't used a
washer but any thoughts welcomed.
The union nut was not used from 1932
onwards as a 1¾ inch exhaust pipe was introduced, requiring a larger union
nut (factory part no. 6828 for those interested in these sort of details!).