|Confirming the Valve
Timings and Compression Ratio
|Anyway, the next task in the rebuild was to
fit the piston and ensure that the piston to valve clearances were adequate,
given that I was going to use full Manx longstroke timing, that being
much ‘lumpier’ than the road going ‘International’
This bought back memories of my earlier racing days, when I spent endless hours farting about with overstressed pushrod Norton engines, trying to ensure my home made cams did not result in the valves clouting the top of the piston, or even worse – playing pattacake with each other! I have a clear memory of missing a gear down the back straight at Mallory in the mid eighties and getting no second chance – a crescendo of exceeded revs, followed by instant loss of power and on sheepishly returning to the pits, finding I had no compression whatsoever. Subsequent strip down revealed the expected ‘cleaning’ of the piston’s inlet valve cutaway and a valve (brand new one as I remember – bugger!) that looked like a plate being wobbled on a stick, i.e. it was well bent!
|From this I learnt that a minimum safe distance
between each valve and piston should be 0.060”. In practice I found
there was only one easy way of setting and confirming this distance, which
was to place plasticine on the piston head, assemble everything up, set
the correct valve timing, then very gently turn the engine over from the
crankshaft – stopping as soon as any resistance is felt (just in
case there was negative clearance and the engine would not even do a full
four stroke revolution without valve coming into contact with piston and
locking the engine).
Providing this stage works successfully the next stage is to strip the top half down again which should reveal a piston with ‘marked’ plasticine where the valves have come into close proximity. It is then simply a case of carefully prodding the plasticine with a depth gauge and hoping you have at least 60 thou clearance at any point. If not, it will be necessary to mill larger valve cutaways for any shortfall – see my Vincent rebuild section for an example of doing this.
Anyway that’s the theory, but I’m jumping ahead a bit here, first I needed to fit the piston, assemble the head, then set the valve timing in the cambox correctly, so read on.
Fitting the Piston
Fitting the piston is a straightforward operation, I bang it in the
oven for about 10 minutes (I’m not going to tell you what gas
mark – this isn’t a cooking program . . .), which ensures
it has warmed evenly, then the gudgeon pin should slide easily. Before
fitting though I scribe a small mark somewhere on the crown that indicates
which way is front. This is very important on a racing piston, as often
the cutaway required on one side is different to the other (my inlet
valve is noticeably larger than the exhaust).
|Actually, that reminds me of a mildly amusing
story from my motorcycling past, this time from about 20 years ago. At
the time I was a single bloke with very little money living in a dilapidated
rented flat (paradoxically though, I remember that period being a very
happy time, isn’t that always the case?). Anyway, at the time I
was using a very well worn Honda CB125 which was already the best part
of 15 years old when it came my way (cost me £55 from a mate at
work as I remember, complete with one and a half spare engines –
both with the cambox’s knackered!).
This bike was much abused and used to run on the oil that had come out of the race bikes – yes I know, shame on me! Everywhere was travelled to at full throttle and generally it never complained. On this particular morning though I was travelling out to see Titch Allen and had noticed it was labouring a bit more than normal. Anyway, don’t worry, push on etc etc. Eventually though while trying to ascend a steep hill it became apparent it was not going to make it, so rather than stopping and it not restarting, I turned round and gently coaxed it back the next few miles to home. By the time we arrived back the poor bloody thing was vibrating like a Jackhammer on steroids and strange fumes were emanating from parts of the engine that should normally never see the light of day!
On stripdown the cause of the problem became clear, one of the piston circlips had broken, which had allowed the gudgeon pin to work itself out, where it had gauged a furrow the whole length of the barrel. Amazingly though despite such abuse, it had still carried me the 10 miles home, despite the depth of the furrow approaching 6mm by this time! Say what you like, but old Mr Honda certainly knows how to build bikes!
That old hack was a bit like one of those poor abused donkeys that worked down the mines and never complained, but I still sold it for more money than it cost me and had a queue of people ready to buy it at the end, oh happy days.
Finally, fit the previously built up head, complete with valves etc, and tighten down, but again don’t give the sleeve bolts quite the grunt you would with final assembly, remember this is just a trial fitting.
Setting the Correct Valve Timing.
Ok, here I am going to go out of order a bit, as I know I haven’t
included a section yet on how to assemble/set up the Oldham coupling
(incidentally, my wife like’s to spot this kind of error when
watching movies, I think she calls them continuity errors. Mind you,
if she was here now she would probably correct me on that as well -
that’s one of her other favourite hobbies, correcting my grammar
. . .). Save to say the Oldham coupling will be covered in a (near)
future article, so lets assume this is already assembled and you have
placed the cambox on top of the sleeve bolts and bolted it down using
the four cambox bolts (long and short). Incidentally, you should ensure
first the two cam lobes are positioned such, that the rocker arms are
both in the valve closed position (akin to top dead centre).
Back in Section 3 I described the order of assembly for the cambox.
Having then set the cambox aside, I worked on the final build of the
rest of the engine, to get to the point we are at here – where
the bottom half is complete, the barrel and piston is fitted and the
built up cylinder head has been fitted and tightened down. This last
operation has only been done to a point of being semi-tight, knowing
I need to set valve timing and valve/piston clearance before the final
|Although there are various valve timings for SOHC Norton’s, below I have detailed the most commonly seen timings. All timings are shown in degrees:|
The timings shown should be obtained with a 4 thou clearance on the tappets set (normal running clearances are 10 thou inlet and 20 thou exhaust). I have found the best way to measure when the valves open is to use a dial gauge, held in place using one of the cambox bolts. However, if you don’t have a suitable dial gauge to hand, a more basic method is to use a cigarette paper between valve tappet and the valve itself. When the cigarette paper goes tight, the point of opening is reached.
This picture shows the main crownwheel bevel, with its 12 holes, and the camshaft whose corresponding shoulder has 11 holes. Above it can just be seen the spacer, that on the roadgoing cambox feeds oil to the cams via the slot along it. On the central fed racing camshaft this acts purely as a spacer
|Setting the Exhaust Valve
So Having fitted the cambox in place, and ensured the top bevel gears meet up with the punch marks on both vertical bevel and camshaft crown wheel, next step is to set the exhaust cam closing point correctly. As it is the exhaust cam that is keyed to the camshaft, it is this cam that must be timed correctly first. The inlet cam is pegged to the exhaust cam, so there is no point trying to time on the inlet until the exhaust is correct.
By the way, I know this is pretty obvious, but don’t be tempted to rush and only fit the cambox bolts finger tight. They do need to be tight enough to ensure tolerances are correct, and if not fully tight there is a slight risk of breaking off one of the back cambox legs (it is not uncommon to find magnesium cambox’s in this predicament).
The main crownwheel bevel gear has 12 holes around its inner circumference. The camshaft shoulder behind it has 11 holes; so there is a few degrees difference between each. A peg fits through both of these holes to secure the two together. In turn this peg is held in position by the camshaft retaining nut, which is lipped for this purpose.
Therefore, placing the peg through a different pair of holes will result in a slight variation in the exhaust cam lobe position in relation to the bevel gears. In this manner it is possible to adjust the exhaust cam timing slightly, to account for any slight variations in the opening/closing position of the exhaust cam lobe in relation to its key slot on the camshaft.
It is very important to ensure the peg is a good slide fit, with no wear, as this can result in erroneous valve settings (and even lead to shearing). I stock them myself if you need a new one.
|So, to initially set the correct
position of the exhaust cam, having ensured the engine is at TDC and the
top bevels are both lined up with their punch marks, revolve the crankshaft
forward until it is 42.5 degrees beyond top dead centre. Please note that
while doing this, the camshaft should not be pegged to the crownwheel
(and therefore the lipped nut does not need to be fitted), so it is not
rotating while the crankshaft is being set.
Now, while ensuring the bevel gears do not move, gently rotate the camshaft clockwise ( the camshaft runs backward, so this is in effect finding the exhaust lobe closing point) until the exhaust rocker comes into contact with the valve. When this point is reached, look for the point around the crownwheel where two holes are in line and push in the peg. You may need to gently tap this in, using a soft punch. You can now fit the lipped nut, semi-tight, and rotate the engine to confirm the opening and closing points of the exhaust cam are as per the valve timings above. Repeat process multiple times until you get it right!
If you find you cannot get the valve timings exactly right because there is a couple of degrees difference between the actual cam and the book settings (I have found they are rarely spot on), then I tend to even it out between the opening and closing settings. By the way, if you find your initial timings are close, but not spot on, remove peg and gently turn crankshaft a couple of degrees until the next hole on the crownwheel lines up. If this has moved the timing further out, go back to the previous position and repeat the process in the opposite direction. Using this method, you should find it possible to adjust the timing a few degrees at a time.
This process takes some time to carry out, but in the case of this engine there were no real problems and I soon had the exhaust timing pretty much spot on.
Setting the Inlet Valve Timing
I did not mention earlier, but the inlet cam uses a similar method
to the exhaust cam for varying its timing, a vernier system, that pegs
it to the exhaust cam. The inlet cam is numbered from 1 onwards in a
clockwise rotation, with number 1 being in the 12 o’clock position.
The exhaust cam starts in the same position, but the numbers rotate
in an anti-clockwise rotation, therefore number 5 is approximately in
the 8 o’clock position.
Anyway, the normal starting
point for setting the inlet cam is to locate the roller in position
1 on the inlet cam and position 5 on the exhaust cam. If adjustment
is necessary you should then try 2 and 6 and so forth. Like the exhaust
cam pegged to the crownwheel, this will have the affect of varying the
inlet cam by a few degrees in relation to the exhaust cam, allowing
you to subtly adjust the timing.
All in all, setting the timing on one of these cambox’s takes
a considerable amount of time to get right, but it is not an altogether
unpleasant process and you cannot help but admire the subtle logic of
the design. I am sure it must have been considered cutting edge when
it was first designed back in the early 1930’s and far in advance
of other designs at the time.
Valve to Piston Clearance