From the recent OCR whilst I was searching for things on 'Camshafts' I found this from 1983-05. Not bad considering it was typed on a wonky typewriter where any typing mistakes had to be scratched out causing the OCR to have problems. Many of the 't's turned into 'l's by the OCR and 'w's always had problems.This is where the Legacy team can start work extracting all the technical articles.
CHECKING & RESETTING THE VALVE TIMING ON PRE-WAR TWINS
1. The 18th edition of the 'Jowett Instruction Book" states that the crankshaft
and camshaft chain sprocket shave centre punch marks on their teeth,
arranged so that the valve timing will be correct l<hen these marks are opposite
each other i.e. at 12 o'clock on the crankshaft and 6 o' clock on the
camshaft.
It also gives a timing diagram showing :
Inlet valve opens at TDC
fully open at 110 after TDC
closes at 40 after BDC
Exhaust valve opens at 50 before BDC
fully open at 105 before TDC
closes at 20 after TDC
This diagram is marked "The above timing applies only when tappet clearances are 0.006"
In 1935 the running clearance for the tappets was 0,002"; it was only in
1936, after engine No 643100, that a new camshaft was introduced designed for
running clearances of 0,006". This new camshaft was also supplied as a
replacement for earlier engines,
2. The "Jowett Owners Maintenance Handbook for Models 1935-1939" issued in
January 1952 clearly states that valve timing should be carried out only
when the tappet clearances have been increased from 0.002" to 0,006" with
earlier camshafts or from 0.006" to 0.012" with later and replacement cam
shafts, The further statement that the timing diagram, quoted above, only
applies when the tappets are set to their running clearances is, in my view,
an error by the editor of the 1952 handbook who failed to realize that the
0,006" clearance referred to in the 1935 timing diagram related to the
earlier camshaft,
My contention is therefore that this timing diagram does apply to both
earlier and later camshafts when tappet clearances have been increased to
0.006 and 0,012" respectively, There would be no point in providing the
timing diagram unless it was to be used for valve timing .
This increasing of tappet clearances for valve timing purposes is common
practice and is presumably to ensure that first or last contact of the tappet
with the cam is clear of the quietening ramps on the cam profile and so
gives a more definite indication of the valve opening or closing point.
3, Even so, it is still difficult to judge consistently the exact point at
which a valve opens or closes, It is easier to judge the point at which a
valve is fully open by watching the compressed valve spring and by rocking
the flywheel to and fro about the correct position as you can feel the valve
spring pushing the camshaft first one way and then the other as the cam goes
"over centre". Moreover, this method has the advantage that it is independent
of the tappet clearances.
4. The 1952 handbook (but not the 1935 one ) mentions that replacement camshaft
sprockets are not marked with a centre punched tooth for valve timing
and have three keyways, the choice of which keyway to use being left to a
process of trial and error. This lack of timing marks came to light in
rebuilding a 1936 Kestrel engine originally fitted with an early camshaft but
known to have been supplied with a later replacement camshaft; this engine
also had a camshaft sprocket with three keyways, The following instructions
relate to this engine but probably apply to the majority of pre-war twins and
possibly to Bradfords as well,
Checking the valve timing.
5 . Having determined the "valve fully open" position for one valve, make a
chalk mark on the flywheel rim opposite the crankcase mark for TDC ; repeat
for the other three valves.
The marks for the two inlet valves should ideally coincide but a gap of a
quarter inch or so will not matter; similarly for the exhaust valves,
6. Owing to the restricted opening above the flywheel, you may not be able
to see both the TDC mark and either pair of chalk marks at the same time, so
stick the end of a flexible tape measure {your wife ' s dressmaking tape?) to
the flywheel at the TDC mark with adhesive tape, then rotate the flywheel
until the marks for the two inlet valves come into view and note the measurement
to their midpoint; similarly stick the end of the tape measure to the
flywheel midway between the marks for the two exhaust valves and measure to
the TDC mark. Convert these measurements into crankshaft degrees (on the 1936
Kestrel engine the circumference of the flywheel is exactly 30 inches so one
inch represents 12° ) ,
Compare your measurements with the design figures o f 110° after TDC and 105°
before TDC for inlet and exhaust valves respectively, If the average error
is more than 3° early or 3° late, then the camshaft should be re limed,
Resetting the valve timing
7, The camshaft itself has a single keyway but the camshaft chain sprocket
has three keyways at 120° intervals in order to enable a close approximation
to the correct timing to be achieved.
The crankshaft sprocket has 20 teeth, each spanning an angle of 18°; the
camshaft sprocket has 40 teeth so that the 120° angle between keyways spans
13 1/3 teeth; this enables the valve timing to be adjusted in steps of one
third of a tooth, i.e. 6° of crank angle,
8 , If the camshaft sprocket is rotated clockwise, relative to the camshaft
itself, by 120° its teeth will have advanced by 13 1/3 teeth ; the camshaft and
sprocket (now keyed together again) must then be rotated either clockwise
(advanced) by 2/3 of a tooth (12 of crank angle) or anticlockwise {retarded)
by 1/3 of a tooth ( 6 of crank angle) to enable the sprocket to be re-meshed
with the chain.
Similarly, if the sprocket is rotated anticlockwise by 120° relative to the
camshaft, the valve timing can either be advanced by 6° or retarded by 12°
of crank angle. Greater errors can be corrected by moving one "hole tooth
(18° of crank angle ) .
9. Consider the situation where the engine is assembled except for the induction
manifold, timing cover , camshaft sprocket, key and timing chain,
The valve springs will force the camshaft into a position in which the
springs have a minimum of compression; this occurs at 10° after TDC when one
cylinder has just completed its compression stroke and the inlet valve of the
other cylinder has just started to open while the exhaust valve is just closing.
If the camshaft comes to rest with the keyway pointing downwards, turn
it through 180° so that the key can't fall out.
Fit the camshaft key; if the hub of the camshaft sprocket has a centre punch
mark in line with one of the three keyways, fit the sprocket on the shaft with this keyway engaging the key.
Check which cylinder has both its valves fully closed and rotate the dynamo
sprocket until the contact breaker points have just opened and the distributor
rotor is pointing towards that cylinder,
Now set the flywheel to 10° after TDC and fit the chain over all three
sprockets so that the two ends meet half way between the dynamo and crankshaft
sprockets, taking care not to move any of the three sprockets more
than is necessary to engage the teeth with the chain,
Stuff some clean rags into the open mouth of the timing case so that if you
drop part of the connecting link it can' t fall into the sump. Fit the connecting
link, remove the rags and tension the chain by moving the dynamo ,
10, Set the flywheel to TDC and stick a pointer into the split pin hole in
the camshaft (a small rat-tail file makes a good pointer as its tang will
wedge in the hole ) . Make a mark on the rim of the timing case in line with
the pointer. Remove the pointer, Check the valve liming as described earlier
and decide how much change, in steps of 6° of crank angle , is required to
get as close as possible ( i.e. within 3°) to the correct valve timing { the
odds are at least 2 to 1 against getting it right first time) .
11. Suppose your measurements indicate that the inlet valves are fully open
9° late and the exhaust valves 11° late. Split the difference and say that
the camshaft is 10° late. Working in steps of 6 ° , the correction required is
12°; the camshaft needs to be advanced by 12° of crank angle which is equivalent
to 6° of camshaft angle.
Cut a narrow V-shaped cardboard template with an angle of 6°; put the point
of the V at the centre of the crankshaft and, as you wish to advance the
timing , line up the left hand edge of the template with the mark you made
earlier on the rim of the timing case and make a second mark on the timing
case in line with the right hand edge of the template,
Turn the crankshaft to TDC on the compression stroke of the same cylinder as
before to ensure that the key is on top of the camshaft, slacken the chain,
stull the rags back into the mouth of the timing case, remove the connecting
link and get the chain clear of both the dynamo and camshaft sprockets, withdraw
the camshaft sprocket, turn it through 120° clockwise (see para
andreplace it on the camshaft.
Fit the pointer into the split pin hole and turn the camshaft (using a spanner
on the fixing nut) till the pointer lines up with the second mark made
on the rim of the timing case, Refit the chain making sure that the flywheel
remains at TDC, the pointer in line with the second mark and the dynamo
sprocket such that the contact breaker points are just opening ( the exact
ignition timing can be adjusted later by swivelling the distributor) and the
rotor is pointing to the correct cylinder.
Refit the connecting link making sure that the closed end of the spring link
points in the direction that the chain travels,
Remove pointer and rags, tension the timing chain correctly and then recheck
the valve timing to make sure it i s within 3° of crank angle to Jowett' s
specification, Fully tighten the camshaft nut and secure i t with a new split
pin, Complete the assembly of the engine.
*Ben adds in his note that the above is based on his experiences in retiming
the valves on Bob Guille's '36 Kestrel which they hope to drive to this years
rally, I hope you both have a good run and enjoy the weekend.