The engine rebuild is now complete
and it looks a bit happier than when I first received it a
bit over a year ago.
A few more details below of the top-end rebuild and final assembly.
The Cylinder Head
The bronze cylinder heads - part number K-1/6 - fitted to
the Mk IV and V KTTs really are a work of art. This is the 2nd one
that has come into my possession and when cleaned up and vapour blasted it
could be mistaken for gold.
I had cleaned up the springs, the inlet valve and the other
bits and pieces in the tumbler some time ago
and bought a new exhaust valve from G&S valves. However,
the eagle-eyed might have spotted that there is one part missing in the last
picture above, namely a top collar that holds the collets – only one was
present in the parts that I had acquired.
Luckily, this is a straightforward component to make and so
a new one was turned from a piece of O1 tool steel and heat treated. New one on
the right in the pictures below.
For the record, the internal hole that supports the collets
has a half-angle of 60 and although I have a carbide cutter
of this angle it is more accurate and the surface finish is better if it is
machined using a small boring bar.
New collets were fitted to both valves which were lightly
ground before reassembling into the head.
With the valves in place, I could now measure the TDC combustion
chamber volume and check the compression ratio,
The volume of oil required to fill the combustion chamber up
to the bottom of the spark plug hole with the piston at TDC was found to be
57cc. With a bore and stroke of 74mm x 81mm this gives a compression ratio of
7.1:1.
As I mentioned in my last blog, I have used 2x paper gaskets
to sandwich 2x shims between the base of the cylinder barrel and the crankcase
(this engine did not have these when originally built – I believe that
something such as shellac was used as a sealant). I estimate that, when
compressed, the addition of these gaskets has added a volume of ~2cc to the TDC
volume – ie without the gaskets the volume would be 55cc. This would give a
compression ratio of 7.33:1.
The value quoted for the compression ratio of a Mk V KTT is 7.75:1 (see eg. Ivan Rhodes Passion of a Lifetime, page 265) and whilst I can account
for a 0.2 difference I cannot see where another 0.4 of a ratio comes from …not
that it really matters.
Cylinder Head Sealing
I am quite familiar with the fine/coarse grinding paste
procedure for fitting iron heads onto iron barrels – see here, for
example, which describes the process for the heads/barrels on the V-Twin. However,
neither the bronze head nor the iron barrel on KTT 581 show any witness marks from applying such a method. Measurements of the height of the
spigot on the barrel (0.370”) and the depth of the recess in the head (0.374”)
indicate that either the head sealed directly onto the main top surface of the
barrel or there was a very thin gasket (that disappeared long ago) between either
the top of the spigot and the bottom of the head recess or the main flat
surfaces of each component. I can find no mention of any cylinder head gasket
in the Mk IV parts book, which has the same bronze head.
As measured, there is a 0.004” gap between the top of the spigot
and the head recess. I decided that I would apply the seal to these surfaces
using a very thin circular copper gasket. A quick search showed that I
could get copper foil in a variety of thicknesses from 0.01mm to 1mm from a
Chinese-based vendor on eBay. I ordered a 12cm x 1m long roll of 0.15mm (0.006”)
thick foil and 2 ½ weeks later this arrived in the post.
It could have been packaged a bit better for its 6000-mile
trip but there was plenty of unbent foil to use. It is not so easy to make a thin
circular gasket from this material! I tried various combinations of scalpel,
scissors, tin cutters, sharpened point of dividers and with both “as-received”
(hard – combination of age and work hardened from the manufacturing process)
and annealed pieces of the sheet but was unable to make a satisfactory gasket.
In the end, I clamped a small piece of the copper sheet to
the rotary table and with a 1/8” diameter long end-mill used the milling
machine to cut out the centre first
and then the outside.
This worked pretty well - the gasket was made much more
accurately and it fits into the recess perfectly. After annealing, the head was
fitted.
The Cambox
Although I had taken a brief look inside the cambox when I
first received the engine, I had done nothing further with it until now. The 3
pictures below are of the “as-received” cambox.
As a restorer, this is exactly how I like to find parts –
covered in oily grime and oily inside – because everything will have been
preserved without deteriorating.
The cambox was stripped and all of the parts were cleaned.
A few observations:
The rockers have been highly polished.
There are aluminium quills in both the top cover
and the cover for the bevel drive
with 1mm diameter holes to provide oil to the cams and upper
bevel gears.
A K-17/6 cam is fitted
and which shows little signs of wear. The rocker skids were
also OK and were left in place.
The original SKF bearing on the oil pump end of the shaft
was as-new and was refitted.
The oil pump assembly has 3x 0.008” brass shims (K-109/6 in
the Mk IV parts list) which have been inserted to get the correct axial
positioning of the pump relative to the end of the camshaft.
The cambox was reassembled with new felts to help keep the
oil inside and was now ready to fit.
The cylinder head nuts/pillars on which the cambox is seated
were missing and new ones were made, together with the cambox retaining bolts,
using ETG 100 steel bar.
This is not a steel that I have ever used before and I don’t
know if it is even available in the UK. A few lengths of this were given to me
by a buddy from Sweden (Thank you Christer) and it turns out to be a high
tensile steel with excellent machining characteristics and I have to agree that
it is a very straightforward steel to machine.
The cylinder head nuts/cambox pillars were initially made
around 0.050” longer than required to check that the cambox would “sit” on the
4 pillars without rocking (it does). The thickness of the washers beneath the nuts/pillars
were checked to ensure they were exactly the same size. The nuts/pillars were
then reduced in length to give an exhaust tappet clearance of 0.025”. The
resulting inlet clearance was 0.015”. This is a few thou more than the
recommended 0.020”/0.012” but I’ll leave it like this for the moment until the
engine has run a while and settled down before resetting. The pillars all have
the same dimension of 0.75” +/- 0.0005” and were chemically blacked prior to
assembly.
The Bevel Drive
Now having the fasteners to actually bolt down the cambox it
was time to set up the bevel drive. This has to be the last task because the
distance between the upper and lower bevels depends on everything that has preceded
– cylinder base gaskets, cylinder head gaskets, cambox mounting etc.
With all the main engine components bolted in place the vertical
shaft (K-49/X) can be put in position without any Oldham couplings and the
distance between the end of the shaft and the bottom bevel pinion (K-33) can be
measured.
The screwdriver is simply to hold the shaft hard up against
the upper bevel to enable the feeler gauge to be inserted. This dimension is to
be filled by the flanges of both Oldham couplings + clearance.
Now, I seem to have acquired plenty of Oldham couples over
the years
but I never seem to have ones with the flange thickness that
I need. I also prefer to distribute the available space for the flanges as equally
as possible rather than having one thick and one thin. As a consequence, I
invariable end up making new ones and I now have an established process for this.
New Oldham couplings were machined in O1 tool steel to
within a couple of thou on the milling machine
before finishing/fitting by hand with emery cloth,
coating with “KEEPBRYTE” anti-scale compound and heating to
8200C in the furnace,
oil quenching and tempering at 2600C
and cleaning up before final checking and fitting.
I checked the clearance of the vertical shaft again before
final assembly and it was 0.012” - about the minimum acceptable value.
I have used the nitrile O-rings and PTFE rings that I
described for building
the engine of KTT 305 for sealing the cover for the vertical shaft (K-50/3).
It is important when fitting the vertical shaft that the
positions of both the crankshaft and the cam are in the right places. If the crankshaft
is positioned so that the piston is at TDC and the orientation of the slot in
the lower bevel gear is aligned parallel to the axis of the bike (ie,
orthogonal to the crankshaft – it might take a few rotations of the crankshaft
to achieve this because of the hunting tooth) and the upper bevel slot has the
same orientation (ie the slots are mirror images of each other) with both
valves closed (which would correspond to TDC firing – again, a few rotations
may be required) then the vertical shaft can be fitted in place.
After assembly and before tightening the gland nuts (K-52)
the valve timing was checked. The “book” values for a Mk V engine with the
K-17/6 cam (see Ivan Rhodes Passion of a Lifetime page 265) are:
IVO: 510 BTDC
IVC: 570 ATDC
EVO: 710 BBDC
EVC: 430 ATDC
and checked with a 0.012” valve/tappet clearance. I have
made allowance for the checking clearance by using appropriate thickness feeler
gauges but without physically changing the clearances with the resulting valve
timings of:
IVO: 550 BTDC
IVC: 680 ATDC
EVO: 700 BBDC
EVC: 500 ATDC
These are in pretty reasonable agreement with the book values
and I’m quite happy that the setup is good.
The gland nuts were tightened and it was now time to set up
the ignition timing.
The BTH TT magneto has been overhauled as I mentioned in a previous blog and it has also been set up with a locked advance/retard, ie the timing is
fixed. As it was like this when I received the engine, I assume this was done
at the factory to avoid the possibility of either a broken cable messing up the
timing or the timing being inadvertently retarded during the race. I have
retained this setup and the timing has been set to 350 BTDC which
should be an adequate level of advance without being over-advanced.
Carburettor
The carb is a 1 1/16” 10 TT 34 AMAL instrument – 10 denotes
the body size and 34 the year of manufacture. This is the correct (see Ivans
book) and almost certainly the original carb for the bike
It was stripped in its entirety
to clean, check for any damage and to replace the fibre
washers with new ones from AMAL
All the parts were in excellent condition however the main
jet that was fitted was a #210 rather than a #360 that would have been fitted
to a Mk V engine (see Ivans book). Now, I can imagine that some tuning/fettling
on the dyno or the track could easily have resulted in a different jet size from
a published value but this jet is too small for a 350cc engine and the
difference is too great to be accounted for in this way.
A clue to the reason for the small main jet lies in the
history of the bike. As I mentioned in a previous blog,
Geoff Monty acquired the original complete bike around 1950 and used the
chassis for a 250cc racer. And, guess what, a #210 main jet is perfect for the
smaller capacity engine. So, my instinct is that the carb was “borrowed” for
another, smaller engine and subsequently reunited with KTT 581 when Geoff sold
it.
Anyway, I don’t have a #360 main jet in my stock of jets
(I’ll get one when I next put in an order to AMAL) so have inserted a #380 in
the meantime.
The last little job before completing the engine build was
to strip and clean the 1:1 rev counter drive.
After repainting and repacking with fresh grease it was
reattached to the engine.
And that concludes the engine rebuild. The engine is now on
the bench and I’ll get back to this project later in the year when I have
completed the final builds of both KTT 55, 305 and the DOHC 250.
A final couple of observations about this engine:
Having now seen the engine in its entirety, I can honestly
say that I don’t believe it has done more than 500 miles in its life – practice laps
plus the 7-lap Junior race (37.73 miles/lap) back in 1935. There is minimal
wear for any of the major parts – crankshaft/big-end, small-end, piston-bore,
cams and followers. It is also clear that this was built as a race engine
judging by the amount of careful polishing of the con-rod and rockers.
From necessity I have had to replace a few bits – piston rings,
make new cylinder head nuts/cambox bolts and Oldham couplings, replace a few
3/16” BSW screws and repair some damage to the piston that was caused by a careless
stripper (that could be misinterpreted!) in the past. But to all intents and purposes
the engine is pretty well exactly as it left the factory 91 years ago.
