A talk by Francis Beart before the Hants and Berks M.C.
At a recent meeting of the Hants and Berks Motor Club, Francis Beart gave a talk on 500-c.c. racing engines, based on his experience as a success ful motor-cycle tuner. In view of the great interest which has been shown in Class I racing since it was first suggested in Motor Sport in 1941, permission has been obtained for much of this hitherto confidential tuning “gen” to be published here.
Beart has concentrated his attention on the 500-c.c. o.h.c. Norton engine, which can be tuned to produce 45 b.h.p. on alcohol fuel without sacrifice of reliability. Engines of this type can be bought secondhand for about £80 at the present time.
The first stage in building up one of these engines is to true up the upper face of the crankcase by means of a surface plate and a mandrel through the main bearings. Attention can then be given to the built-up crank assembly.
Polishing of the flywheels and connecting rod has not been found worth while, but no tolerance should be allowed on the truth of the crankshaft. The wheels may be trued up in a scrap crankcase, a four foot tommy bar being used for the final tightening of the crankpin nuts. In balancing the assembly, Beart has balanced 50 per cent. of the reciprocating masses for road racing, reducing this item to as low a figure as 38 per cent. for track work. With the crank assembly installed in the crankcase, there should be negligible end float. The camshaft drive bevel gears should have about 5 thou. backlash.
The oil pump should deliver at 30 lb./sq. in., using castor oil. The circulation rate is approximately 40 gallons per hour at 6,000 r.p.m., and oil temperatures should be kept below 70° C. as a precaution against frothing.
The 500-c.c. Norton engine is of 79 mm. bore, and Martlet pistons (cast) or forged RR59 pistons can be used satisfactorily. A piston crown 5 mm. thick is used, three narrow-section Wellworthy piston rings (two compression and one “Superslot” scraper), having 15 thou. gaps and 20-25 lb. radial pressure. A 5/16-in. wide band protects the top piston ring. A 7/8-in. diameter gudgeon pin is a tight press fit into the piston, wire circlips providing endwise location. Clearances used with a Martlet piston are 8 thou. at the bottom of the skirt, 12 thou. below the scraper ring, and 25 thou. at the top band, virtually no running-in being necessary. Using J.A.P. fuel with 1 per cent. of castor oil added, a 14-to-1 compression ratio was used.
With moving parts as described here, a speed of 6,000 to 6,200 r.p.m. can be maintained throughout a 100-mile race. Power falls off above 6,500 r.p.m.
If petrol-benzole fuel is to be used, an alloy cylinder barrel is desirable in the interests of satisfactory cooling. With alcohol fuel the iron cylinder barrel is satisfactory, although it is, of course, heavier. Much the same considerations enter into the choice of an iron, bronze, or aluminium cylinder head, with the added point that inlet valve seat wear is rather rapid with a bronze head. The cylinder head has a ground joint with the block, and the correct torque for the holding-down bolts is 22 lb./ft.
The inlet port diameter is 1 5/32 in. at the carburetter. The inlet valve seat should be as narrow as possible, and the exhaust valve seat 1 mm. wide. Full-length bronze valve guides should be used, the exhaust guide having a chromium bronze bush and 1 1/2 thou. clearance. Standard Norton valves are satisfactory, a flatheaded exhaust valve being better than the tulip variety.
The valve collars may be lightened somewhat, and polished to prevent cracks forming. The loading of the hairpin valve springs should be adjusted until valve bounce is just prevented. Standard port shapes are satisfactory, but internal polishing does no harm and the inlet pipe can be lengthened if extreme valve overlap is to be used.
Standard Manx Norton cams are perfectly good, and have been used for motorcycles lapping at over 115 m.p.h. The timing used with a long exhaust pipe terminating in a Brooklands silencer is: inlet 35° b.t.d.c. to 68° a.b.d.c., exhaust 60° b.b.d.c. to 40° a.t.d.c. With an open exhaust system the exhaust valve can open and close 10° earlier. Using a megaphone on the usual I 3/4-in. diameter pipe, the timing which has been used is: inlet 52° b.t.d.c. to 55° a.b.d.c., exhaust 70° b.b.d.c. to 30° a.t.d.c.
Suitable tappet clearances are 0.009 in. inlet and 0.025 in. exhaust.
A needle-type Amal carburetter of the flange-fitting type should be used, having a No. 5 slide for J.A.P. or racing Ethyl fuel. Using a 14-to-1 compression ratio and 32° ignition advance, a 1,200 jet and 120 needle jet are approximately correct. If a larger main jet than 1,200 is used the needle jet must be drilled out.
Fuel consumption, in the case of a motor-cycle lapping at 110 m.p.h., is 18-20 m.p.g., with an oil consumption of 200 m.p.g. The maximum fuel flow through a single Amal float chamber, with drilled-out passages, is 5 gallons per hour.
As regards transmission, the Norton clutch and gearbox have been satisfactory with fairly heavy sidecar outfits. The close-ratio gearbox gives ratios of 1.0, 1.1, 1.33 and 1.77.
A few interesting details were given of the car which Myles Rothwell built for attacks on Class I records. This car was fitted with a 1932 T.T.-type J.A.P. engine, with 9-to-1 compression ratio and developing about 38 b.h.p. The Brooklands lap speed, with all-enclosed body, was almost exactly 90 m.p.h.
As regards other engine types, Beart obviously regarded the Norton as the safest bet for an amateur Class I car, but there are other possibilities. A factory-tuned Triumph twin at present holds the class lap record at Brooklands. A tuned Ulster Rudge with 10 1/2-to-1 compression ratio has lapped at 110 m.p.h. Overhead camshaft B.M.W. engines can improve on Norton power outputs, but not so the unblown push-rod models normally available in this country. The dirt-track J.A.P. is light and cheap and has a possible output of about 40 b.h.p. Various other single and multi-cylinder motor-cycle engines are available, but none seem to combine the performance and durability of the Norton. The latest T.T. units developed about 50 b.h.p. on petrol/benzole fuel, and the International model sold to the public was virtually the works racing job of an earlier year.