5a. Barrels and Pistons

The barrels supplied when I purchased the bike were a definite plus point. Both had had brand new liners fitted and were at standard size. I think the muffs could have been new as well but was not too sure. Whatever, they were effectively new and only needed the oil feed holes drilling in them, then painting, to complete.
These oil holes are drilled to coincide with similar holes in the recesses of the crankcases, which allow oil to be fed through the barrel, lubricating the piston walls. Later engines from 1951 had this 3/32" hole slightly lower than early engines, so that the oil would exit beneath the scraper ring. There was a groove on the outside of the barrel to match up the oil galleries.
After drilling the holes I carefully ground a slot down the outside of the barrels with a small grinding attachment, to give this later arrangement.


Completed Barrel

Completed barrel after final hone and enamelling

Oil Feed Hole

This photograph shows the oil feed hole and slot

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.
They do have smaller gudgeon pins than the standard Vincent piston, which meant making new Little End's for the con-rods (see Crankshaft section). However, this was not a major problem.The only real work I carried out on the pistons was to knife-edge the skirts. This is something well worth doing with pistons going in race engines. The purpose of knife-edging is that it means less resistance as the piston is travelling down the bore, cutting through the film of oil easier.

I was not the only person to use BSA B50 pistons in a Norton, too find out more press the button below:

Knife edged piston

Knife-edging the skirt of the piston

Black Lightning Dope Piston

Black Lightning dope piston

Which Fuel To Use?
I had decided long ago that I would run the Vincent on petrol, not methanol. Having had experience of using methanol, although I liked the extra power it can deliver, I was aware how difficult it is to use. Not only does it require carburetors to be drained every time after use, but it is also very good at removing most paints. Added to which, it is hardly practical to run a bike on the road (which I might consider) if you have to keep looking for a Methanol Station!
Using a B50 piston made life very much easier from a compression ratio point of view. To take full benefit of Methanol normally requires a compression ratio in excess of 11:1, which can be quite difficult to achieve without some risk to bent valves if a gear is missed.

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.
Obviously, this piston was originally one of a pair, but I think Titch's son Roger might have used the other one in his Norton before he moved on to B50 pistons! If anyone has one of these pistons (see photograph), I would be interested in acquiring it, as I would like to have a spare pair myself in case I ever want to go over to Methanol.

Underneath of dope piston

Same piston showing original knife edging and webbing to crown. Cast writing reads :
'AT No 390801'

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.
Compression Ratio's
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.

Compression Plates

I made a selection of Vincent compression plates in various thicknesses
(ones in lower corner are for Norton though)

Fitting Comp plates

Fitting compression plates

Setting the Valve Timing
First stage in this process was to ensure that the valve timing was correct, as this dtermines the valve lift in relation to piston's position in the bore.
To do this, I made an adaptor to mount a dial gauge into a Vincent breather valve cap. This would show me exactly when the valve opened and closed. I then placed a valve timing disc onto the drive side crankshaft.
By fitting the cams according to the timing marks on the gears, and trial and error setting of the crankshaft gear (which has numerous woodruff key slots at different degree slots), I was able to find the timing for each cylinder that most closely followed the orginal factory Black Lightning valve settings. These were as follows:
Inlet opens : 56 degrees BTDC. Inlet closes : 68 degrees ABDC
Exhaust Opens : 72 degrees BBDC. Exhaust Closes 50 degrees ATDC.


Setting Valve Timing

Setting the valve timing (degree plate also fitted to crankshaft on drive side)

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.

Piston with plasticine

Marking rear piston crown with plasticine

Milliing valve cutaways

Milling a valve cutaway in the piston crown
(note : spacers between little end eyes prevent piston being distorted when clamping)

Milling the Valve Cutaways
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.

Finishing Off
Once the cutaways were complete all that was left to do with the pistons was to polish the crowns withs various grades of wet and dry, then give them a final polish with Brasso. Not sure if polishing them really makes much difference but it is traditional and makes me feel better! They were given a final (very careful) clean with degreaser and an airline before I fitted the piston rings. Fitting piston rings is one of those jobs that seems to have quite a stigma attached. In books on the subject, they show pictures of special tools being used, or the rings being fed along pieces of scrap metal, onto the pistons. I have always found the safest method by far, is to support the ring with the fingers of both hands, while prising the ring apart with your thumbs and gently lowing them over the piston. Admittedly, this hurts like hell, but I have never so far broken a ring, so I will continue to endure the pain.

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.
To achieve this seal, I first mounted the barrel (very carefully!) in a vice, then applied fine grinding paste to the inner seal, while at the same time applying course grinding paste to the outer face. 10 to 15 minutes vigorous turning of the head then ensued, resulting in a beautiful lapped finish to the inner face (liner), while a slightly rougher finish resulted on the outer face (muff). Hopefully this will prove good in service.

Finished Pistons

Finished pistons awaiting rings to be fitted

Barrel after grinding

Barrel face after using grinding paste

Finished barrel/piston

Finished Barrel and Piston during assembly.
Oil is racing synthetic