5a. Barrels and Pistons
As well as having two good barrels, the bike also came supplied with a pair of suitable pistons for racing. These were BSA B50 pistons. At the time I was racing (late 1980's), they were quite a popular piston for racing Vincent's. As well as being of the 'slipper' variety (i.e. they only had piston walls on the thrust faces, meaning less resistance in the bore), they also had a nice lumpy piston crown, meaning it was possible to get a quite high compression without too much bother.
They were not looked on as being of very high quality though, the rule
of thumb being that they should be checked regularly.
I was not the only person to use BSA B50 pistons in a Norton, too find
out more press the button below:
Which Fuel To Use?
Many years ago Titch Allen gave me an original Specialloid Black Lightning
piston (see photograph), which looks as if it would bring the compression
ration up to about 13:1. This is quite a heavy piston with a very pronounced
lump on the crown, although its seems to have a lot of its 'meat' around
the outer circumference of the crown, instead of just placing it in the
centre, as with many other racing pistons of the 1930's. If you compare
a Vincent combustion chamber with other racing combustion chambers of
the 30's and 40's you can see it is much shallower (less hemispherical),
which would explain this.
|Running on petrol, I decided that a compression ratio in the region of 9.5:1 was probably as high as I would want to go to start with, which should not be too hard to achieve with B50 Pistons.|
Having had the barrels taken to the correct bore for the pistons, I did a trial assembly complete with piston rings fitted and full valve gear in the heads (so that the compression chamber was effectively sealed). Using a syringe to pour oil through the spark plug hole I was then able to measure the compression chamber volume, compare it against the swept volume and from that, calculate the compression ratio. Without any compression plates fitted, the compression ratio was a little bit too high.
I had machined a selection of compression plates in various sizes prior to starting, so it was a relatively simple task to calculate the thickness of plate required to bring the ratio down to 9.5:1, which turned out to be 1/16".
Having inserted the compression plate, and confirmed that the compression ratio was now 9.5:1, I could carry out the next step, which was to ensure enough piston clearance for each valve.
Now this is a task I have had a lot of experience of in the past, not always with the best of results! The general rule of thumb I have always adhered to is to have a minimum of 60 thou clearance on each valve, with the valve at the highest point of lift and everything bolted down as it would be in service. This was for racing; ideally I would like closer to 80 thou on the road. That allows for the possibility of a missed gear, which normally results in over revving the engine and valve bounce occurring.
Setting the Valve Timing
|Borrow the kids Plasticine
Once, this was done, I fitted plasticine to each valve cutaway and bolted the head down. Final step was to then carefully turn the engine over, which placed indents into the plasticine, telling me how much valve clearance there was. I am always careful at this stage, especially if this is the first time I have done this with an engine, just in case there is negative clearance to start with, which results in the engine locking up because the valve head is actually touching the piston. Obviously, if you continue to turn past this point a bent valve is the result.
In the case of the Vincent, this was not a problem and the engine turned through two full revolutions without incident. Having lifted the heads I found that the plasticine was deeply indented, but inserting the depth gauge of my digital vernier into the plasticine revealed that there was around 50 thou at the shallowest point.
|Milling the Valve
50 thousandth of an inch clearance was not a major problem, the piston crowns being easily thick enough to remove 30 thou from each valve cutaway, which would give 80 thou clearance in total (the minimum 60 thou + 20 thou for luck). I have a small Cemtec 2A horizontal miller to which I mounted the piston, secured by a bar passed through the gudgeon pin hole and bolted down to the table. I fitted an adjustable fly cutter that could be adjusted to a diameter slightly larger than that of the valve. It was then simply a case of taking very light cuts into the existing cutaways, until they were the required depth (see photograph).
Once the new cutaways were complete, final job was to remove the sharp extremities of the cutaways with a swiss file, so as not to cause hotspots if left in place.
Final job with the barrels (and heads), was to grind a perfect cylinder
head seal between the head and barrel. Vincent's do not use cylinder head
gaskets, instead relying on two seperate metal to metal seals, an inner
and an outer face. The inner seal is raised approx 5mm from the outer.
As with my Norton's, the theory behind this seal is that the inner face
seats first, but as the bolts are tightened against the outer face, they
will continue to tighten for a few thou more, before the outer face touches,
thus ensuring a perfect seal. A good way to visualise this is to imagine
trying on a Top Hat, where you are pulling down on the outer rim, but
it is your head that comes into contact with the inner rim of the hat,
while you to continue to pull down until you feel it tighten.