A sturdy bench and vice Compressor
A quality screw driver set
A good set of Whitworth spanners and sockets
Copper-headed mallet
Torque wrench
A 1 5/8” AF open spanner for the vertical shaft gland nuts
A hefty ring spanner that fits perfectly the engine sprocket nut
A puller for the engine mainshaft sprocket
A magneto sprocket puller
A pair of valve spring tongs
A really strong “C” spanner for removing and refitting exhaust pipe nut
Modified pliers for removing piston circlips
Burette for checking cylinder head compression Oil can
A suitable metal washing tray
Vee blocks Dial gauge
Engineers blue Fine and coarse grinding paste
Dye penetrant

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Probably the easiest way to make this is as follows:
Obtain a piece of hardwood approximately 9” in diameter and 1 1/4” thick to which two short lengths of angle iron can be attached (dexion is a little light). Drill two 5/16” holes in each length the same distance apart as the two holes in the bottom of the crankcases. The engine stand can then be bolted to the work bench is if so desired.

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The flywheels are subjected to the greatest stresses in the engine and knowledge of their condition is of paramount importance. If a fairly large mileage has been covered, or you don’t have first hand knowledge of how much work the big end has done, you simply must inspect the big end and its connecting rod. It is by far the best policy to send the assembly to a reputable specialist firm for inspection and, if necessary, fit a replacement big end. It is essential that the trueness of the wheels are beyond suspicion. However, should you wish to tackle this task yourself, you will need a quality 3/4” socket suitably ground to clear the recesses in the flywheels.

The big end should have zero up and down movement and the end float of the rod on the pin should not exceed 0.028”. Remove the drive side wheel and, if marks are found on the drive side of the rod upon disassembly, it may be turned 180° to bring the unscored side into service. Wrap the rod in cloth or similar protective material and put aside for later examination.

Examine the big end for wear. The dural cage can crack and wear unevenly. If you have any doubt about its condition, replace. Using a micrometer, measure the big end pin for ovality or uneven wearing. Any wear here over 0.001” should mean a replacement big end pin or grinding and oversize rollers fitted. The outer race in the conrod should also be checked for wear and if it is to be used again, crack tested with a dye penetrant. Provided the big end wear is within manufacturers tolerances, fit new standard size rollers. This will lengthen the ultimate life of the big end considerably. It is possible to grind outer race to +0.002” or +0.004” and use replacement oversize rollers (if available). Should a new big end be required, an INA assembly from either Molnar in the UK or McIntosh in NZ is recommended.

With the wheels apart, a very thorough inspection should be made for small cracks adjacent to the crankpin using a dye penetrant. These are often present when the engines have had extended use on cars, owing to the much greater stresses, or run on methanol. The best policy may be to send the wheels to a specialist crack testing firm.

It is now opportune to attend to the connecting rod. First check the condition of the small end bush with the gudgeon pin. The gudgeon pin should slide in very easily. Under high rpm, the small end will stretch, squeezing the gudgeon pin at the sides, so an easy fit is recommended for both performance and reliability. Too much slop or ovality will mean a new bush is needed - 0.002” should suffice. Phosphor-bronze is the usual material for small end bushes, however RR59 aluminium alloy is both lighter and longer wearing.

The rod must not have any burrs, scratches or blemishes, no matter how minute, on the polished edges. Carefully remove any from the outside of the rod and polish well. Polishing the milled inner recesses is not necessary. Shot peening is alsom recoimmended. It strengthens the rod. Do not polish after shot peening.

When fitting a new pin, ensure the oil way in the pin lines up with its supply in the timing side flywheel. Press the pin into the timing side first and, with your oil can, check to see that oil reaches the rollers. Big end to flywheel interference fit should be between 0.0025” and 0.003”

Suspended between a pair of centres, the flywheels should be trued to within 0.001” radially and 0.0015” axially on the flywheel rims. The sum of the mainshaft runout should not exceed 0.005” (you may have to reface centres to obtain an accurate reading). If vee blocks are used, slide bearings or bearing inners against flywheel face. Pack vee blocks until mainshafts are on the same plane and, using a dial gauge, measure mainshaft and wheel rim run out. Better still, have them checked by a specialist.

For final tightening of the big end nuts, a 3’ long tommy-bar welded to the modified 3/4” socket is essential. The nuts need to pulled up as tight as the 3' tommy bar will allow.

For racing, the balance factor should be between 74% to 75% and measured after pistons and rings have been finalised. If they are not, have them corrected. Andy Savage balances his racing flywheels between 70% and 71%.

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The cylinder head may now be offered up to the engine and the compression checked. Compression is determined by the use of different thickness compression plates placed under the barrel. At the same time check that the piston has adequate clearance from the head. Clearance should be a minimum of 0.040” and can be checked by placing plasticene balls on the piston crown and dropping the finished head into position. In order to check the compression it will be necessary to position the engine so the plug hole is vertical. Set the piston at TDC with the valves closed, apply grease around the valves and seats as well as the between the piston crown and barrel above the top compression ring. Fill the cylinder head with light oil from the burette (measure how much you put in the burette first) until it reaches the lowest threads of the plug hole. Rock the mainshaft very slightly until the liquid reaches its maximum height and top up until it reaches half way up the hole. Check the amount of liquid poured into the head.

Say this was 70cc and your engine is 490cc, the compression ratio would be 8:1 (engine capacity plus fluid divided by fluid). Compression is adjusted by placing different thickness shims under the barrel. A 1mm thick shim adds about 5cc (4cc for 350) to the combustion chamber volume.

For normal pump petrol about 50cc maximum for 500s, 40cc for 350s. Having carried out this work satisfactorily, the head may now be fitted permanently making sure the thick ground washers are in place for the sleeve nuts.

Using opposite corners, draw down each sleeve nut evenly in stages to an ultimate pressure of 32 ft lbs. Once tightened down, check with a small surface block and feelers that all sleeve nuts are at a uniform height. Any discrepancy must, of course, be corrected. This check should be carried out with the cambox pillars placed in position. This is why sleeve nuts, cambox pillars and heads are often marked for assistance with reassembly.

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The lubrication system employed on these engines requires very little attention. It is designed to supply the correct amount of oil to the various working surfaces and the only adjustment that can be effected is the rear feed to the cylinder barrel, no other adjustment whatsoever is provided. The oil pressure throughout the engine is controlled by a spring loaded ball release valve which is situated at the bottom of the outer timing cover. The screw which controls the adjustment is slotted as well as having a hexagonal head. If adjustment has been inadvertently altered or you are building an engine from scratch, it will be necessary to reset as follows:

A connection should be made in the pipe which conveys oil from the crankcase to the cambox connecting a pressure gauge to a “T” junction so that oil passing to the cambox is in no way restricted. The correct pressure is 8 lbs when the oil is warm and 10 to 12 lbs when it is cold. It is preferable to set the pressure when the engine is warm. To increase the oil pressure, the hexagonal slotted screw should be screwed inwards (clockwise) and anti-clockwise to reduce pressure. The pressure screw should be locked with the small locking nut.

Should no suitable pressure gauge be available, unscrew the adjuster from its fully home position 2 1/2 to 3 turns (an approximation only).

Quality oils are recommended and the brand chosen should be rigidly adhered to. Without doubt, a castor based oil is the most suitable for racing. It is recommended that the oil tank is drained after and refilled before each race meeting. Norton does not recommend that the oil tank be flushed out with petrol or similar during an oil change unless the tank is removed from the machine and thoroughly cleaned out.

As a point of interest, never sand blast oil or petrol tanks for painting. Oily sand has a nasty way of sticking and, no matter how fastidious you are, the result will inevitably be a wrecked engine.

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With the engine assembled, it can now be transferred from the bench and dropped into the frame. On plunger bikes, this is easy; on featherbed bikes, a little juggling is required and a second pair of hands certainly helps.

On plunger bikes, it has been found desirable to drill the bottom cradle to crankcase holes from 5/16” diameter to 3/8”, there being ample metal for this modification. Engine plates should be made from 5/16 (8mm) high tensile Duralumin and extended down on both sides to pick up the lower crankcase bolts. This will secure the engine to the frame more effectively and alleviate the stresses that cause the left rear crankcase lug to fracture. Note that longer bolts will be required.

On featherbed bike, check that the engine plates (they should be 5/16” or 8mm) are flat and that their holes line up perfectly. Then check that the plates line up with the mounting holes on the frame. If engine plate holes are ovaled or oversize, bolts will inevitably work loose. The cure is to ream holes out to an appropriate oversize and machine spigoted washers to fit. Once you are satisfied with the condition of the engine plates, engineers turning marks provide a high quality finish.

Studs, bolts, nuts and washers can be examined and, if desired, lightened. If you are making new studs and bolts from scratch, use a heat treatable steel such as K.E 805. Mild steel is not really up to task and will inevitably fail under racing conditions.

Without compromising strength, studs can be machined to a diameter equal to that of the bottom of the threads, leaving an ample length for the engine and engine plates to bear on - bearing on thread crests will flatten threads and wear engine plates ultimately loosening bolts. On longer studs, it is circumspect to leave a central parallel.

Bolts can be lightened by drilling down the centre (use a lathe). Internal diameter can be up to 50% of the external diameter. Stop about 1/8” short of the threads.

Threads must be in good condition. If in doubt, restore with the appropriate tap or die. Clean up burrs on bottom faces of all bolt heads, nuts and washers. With plates and fasteners in good condition, the engine can now be placed in the frame. On featherbeds, this is achieved by removing the gearbox mounting tube and, tilting the engine back, carefully lift into position. Offer up the left side engine plate and push through a couple of bolts to hold it to the crankcase. Lift the engine, slightly tilting back (a hydraulic jack is handy) and position gearbox, then right rear engine plate and then push through the big stud at the bottom of the gearbox and attach plates to bottom rear frame mounting brackets with appropriate bolts. Lower engine, replace gearbox mounting tube, offer all engine studs, bolts, nuts and washers and finger tighten. Push home top gearbox bolt and, starting with the bolts at the back of the engine, firmly tighten. Do the same with the front engine bolts, then tighten the four bolts holding the engine plates to the rear of the frame and, finally, the front two bolts.

For racing, make sure that everything is fully tightened and that all important nuts and bolts are wire locked.

Finally, when starting from cold, always let the engine warm at slow speed. On no account rev or accelerate it violently. Untold wear and tear occurs to the engine when it is treated like this. All warming up can be done perfectly well at engine speeds not exceeding 1,200 rpm.

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Swept volume is determined by this equation:

piston radius x piston radius x stroke x 0.7857

79 x 79 x 100 x 0.7857 = 490cc (Model 30)
79.62 x 79.62 x 100 x 0.7857 = 498cc (Model 30M)
71 x 71 x 88 x 0.7857 = 348.5cc (Model 40 & Model 40M)
82 x 82 x 94.3 x 0.7857 = 498cc (Works, Senior)

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