Summary of development work
Rather than trying to visually compare inlet ports, we found it best to compare by measuring capacity with valve fitted in cc's using a burette, several engines that were down for power were found to be smaller capacity; 136cc instead of 148cc on an A7 twin, a B34 GS port 117cc extension 128 total induction 245cc.
All bench tests were carried out in uniform manner starting at lowest practical RPM and going up in increments of 250 RPM with no pause. Flash readings were never reported. We did try running power curves in reverse order starting at maximum revs and coming down but decided this gave false high readings.
Sandilands discovered almost by accident that a venturi shape after the carburettor can be 85% of the area of the carburettor body area with no loss of power, in fact it gave a small increase. We did most of our power curves with a flow meter hooked up between fuel tank and carburettor, this gives a valuable check on efficiency and also on correct carburettor settings.
When using two into one exhaust system it is desirable to have each pipe from the cylinder head to the junction of equal length if not carburation may be drastically affected at some point in the range. The way to avoid this problem is to ensure that each pipe has the same length to area ratio, in other words the longer piece will be larger diameter. In our experiments with the short stroke 500cc A7 twin we also found the angle of the two pipes at their intersection was important. I was at MIRA once waiting to use the timing strip whilst a group of Royal Enfield people tried to get the carburation right on one of the 700cc twins fitted with a two into one exhaust system, I noticed they were using the same diameter exhaust pipes of different lengths before the junction. Strangely enough this same rule holds good for induction pipes as found by Guzzi on their later single cylinder racers.
The A7 and A10 connecting rod bolts were originally BSF threads, we changed this to 26 threads per inch cycle threads as we thought larger area at the root of the thread would make a stronger bolt, we had to reduce torque when tightening the big end nuts proportionally, unfortunately someone in the drawing office forgot to tell the engine assembly and several bolts were broken before the error was discovered. Even more strange was the fact the broken bolts only happened to one of the several mechanics assembling the crank assemblies. I was asked to investigate, so sent Arthur Butler up to the engine assembly to watch how they did this torquing up. He returned a little later laughing with a small tobacco tin in his hand, It had plumbers tallow in it, apparently only one mechanic was putting tallow on the threads before tightening, with the results that nuts tightened appreciably more before the torque wrench recorded the correct torque figures. We accordingly tested some bolts with various lubricants on the thread and decided that important bolts such as connecting rod bolts should be assembled clean.
Oil Pumps.The BSA gear type pump is very simple and reliable, at one time we did some tests by simply running a pump on a drill press in a can of oil and measuring the temperature of the oil, we were surprised at how quickly the temperature rose especially as there was not resistance to the flow. On examination of the pump we decided that some oil was being compressed between the two gears, accordingly a small bypass was cut in the cover plate allowing oil to feed back to the inlet side of the pump. Further testing showed practically no heat build up in the oil. Some tests were carried out on A7 (using various viscosity oils, starting with 50 wt and coming down to 40 wt to 30 wt, 20 wt SAE 10 and finally 5 wt. We discovered that normal oil pressure was maintained on the SAE 10 50 wt, 40 wt and 30 wt. At SAE 20 viscosity pressure tended to drop particularly when hot. The engine was stripped at this point to see if the low pressure caused any bearing problems, everything looked pretty good. At SAE 10 wt pressure seemed very low in fact when hot was nil a further examination of the bearings and pistons showed no sign of trouble but the cam followers did not look too happy, starting to score.
A final run was made with SAE 5 wt pressure almost nil when cold, showed nil when hot especially when engine was revved up, finally the timing side main bearing failed. On stripping the engine we found the white metal had melted and run, but no trace of it in the oil or crankcase, when the sealing plugs were removed from the crankshaft the white metal, was found to be inside the oil passages of the crankshaft, the connecting rod bearings appeared to be in good condition. Apparently the light 5 wt oil was thrown outwards to the big end bearings by centrifugal force enough to lubricate the big end bearings adequately but in such quantity that it robbed oil from the main bearings faster than the oil pump could replace it.
One interesting test was when we set up a A10 engine and gearbox unit on the test bed driving the dyno by a short chain, the object was to try and measure the power loss in the gearbox in each gear. We could not measure the power in 1st gear as the dyno was turning too slowly to absorb the considerable torque due to the low gear ratio. As we expected the most efficient gear was 4th, the direct gear.
Surprisingly, 2nd gear was more efficient than 3rd gear due presumably to the fact that 2nd gear wheels were better supported than 3rd. The final part of the test was when we removed the gearbox and clutch and tried to run a fairly long chain from the engine to the dyno direct to see what difference was with gearbox out of action. To our surprise it could not be done, this rather long chain just whipped and banged and would not transmit power. We had to move the engine back on the block and used a short chain. Incidentally the power loss in 3rd gear was about 10% with about 8% loss in 2nd gear as compared with 4th gear with only 2%. One of the lesser known advantages of the eccentric rocker spindles is the ability to adjust the valves whilst the engine is running.
I suggested this to Reg Wilkes one day, however, he did not seem very keen to stand astride a high revving engine with a wrench in either hand so I appointed myself as the rocker manipulator. We started off on the 500 Gold Star race engine at about 5000 rpm with me firmly astride and adjusted each rocker until the maximum power reading was obtained. The engine was then blown cool with the fan and clearance checked, the inlet figure was something like .009" and exhaust a little more. Then the engine was run at 6000rpm and again rockers adjusted for maximum power and cooled down to room temperature and clearances checked, this time the inlet was .006" and exhaust .008".
Next a reading was taken at 6500 rpm and again clearances checked when cool. Now the inlet clearance was down to about .003" and exhaust .006", subsequent checks were made at 7000 rpm and 7500, with some pointed remarks made about what happened to me if the engine shuld blow up while I am astride it. The results at 7000 rpm were clearances Inlet .001" Exhaust .004" at 7500 rpm inlet nil, exhaust clearances .003". These were not exact figures (my memory is not that good) but the general trend was there at the higher revolutions the rockers and push rods were bending and whipping slightly.
Prior to these tests we had always set valve clearances with a cold engine, with inlet push rod just free to rotate and .003" clearance on the exhaust. The normal practice with regard to rocker angle in relation to valve stem has been to arrange for the rocker to be at right angles to the valve stem at half lift, the object being to minimise side thrust and cut down on valve guide wear. In the course of experiments with both the C12 and the A7 engine better running at high revolutions could be obtained when the rocker was at right angles to the valve stem at about l/32"to 1/16" lift. It was thought that this arrangement aligned the valve seat with the valve seat in the head just as the valve was closing, whereas with the common setting at half lift, the rocker side thrust will tip the valve slightly in the guide when seating and will have to realign itself as it seats. At very high revolutions there is not enough time for this and some efficiency will be lost. On the C12 this was visible even by turning the engine by hand, by inserting some packing between the rocker mounts and the head we were able to see the improved action. It is quite possible that most overhead valve engines using rockers could benefit from this rearrangement of rocker angles.
One unexpected results when trying a much larger and stiffer crankpin with plain bearing was considerable difficulty in lining up the flywheel assembly, presumably the wider shoulders made the assembly more rigid. Engine gaskets between rocker box and cylinder head usually made of paper were replaced with metal corrugated gaskets, these could be used over and over again and sealed better.
During the five years I was at BSA doing engine development I was fortunate in attracting some very bright and enthusiastic mechanics and apprentices, they all contributed to the progress we made with development of the BSA range. There was tendency to specialise but most could handle anything that came in the shop. Jimmy Gibbard did most of the odd machinery jobs, Jack Turner did the intricate inlet port arrangements and exploited the extra down draft angles and straight ports. Reg Wilkes, Bert Hole, Arthur Butler, Arthur Bridgewood, Bill Bently and Alan Sandilands could handle any aspect of engine building , the apprentices who spent more than average time with us were Gordon Smith, Robert Trigg, Ray Beech, David Harris and John Taft. Albert Dyde, ??? Harrison Ward and Hill came into the shop later. We also had some colonials like Barry Stormont and Cohn Mather during the winter months. I learned a great deal and was able to try out many ideas that I could never have done in the ordinary way, for this I am grateful to Mr Hopwood and BSA for this opportunity.
One of the chief abilities necessary in this type of work is to be able to 'sell' ideas to the Management, something I was not very good at. Thanks to BSA sending me to the USA on business trips I decided to emigrate and have had an interesting life in USA becoming a citizen in 1982, finally having my own business and a and a decent workshop including a dynomometer. I have not lost interest in motorcycles and at 75 own a 250cc Kawasaki 'Ninja', a remarkable little machine with a performance in road trim better than my old racers. We still enjoy trips to Road Atlanta to watch the motor cycle road each year.