NORTON'S UNBUILT TWIN REBUILT
From time to time, tales surface concerning the great What-Ifs of the old industry. One such is the story of the unit-construction Norton heavy twin. Anthony Curzon has a couple of the engines, and Richard Negus rebuilt one. This is his story…
The first cylinder supplied to me, when examined closely, turned out to have been fitted with a thin steel liner and, more worrying, appeared to have a fault in its casting. Some of the cooling fins had sections that were not actually joined to the cylinder section and would not have been very effective in dissipating heat. I couldn’t imagine any logical reason for this arrangement, except an error either by the pattern-maker or by the foundry personnel.
After a brief discussion with Anthony, another cylinder arrived, and was obviously of an improved design and carried a later casting part number. This cylinder too had been fitted with a steel liner, leading me to believe that these had been salvaged when original machining revealed porosity in one or both main bores. The ‘good’ cylinder was Aquablasted and powder coated gloss black.
The cylinder employs standard Norton twin cam followers and guide plates. Slight machining errors in the guide plate locations of this prototype cylinder required the plate profiles to be modified to suit, fortunately without affecting their function. The pistons were fitted to the connecting rods and supported at TDC with two hardwood strips while the rings were fitted.
With ring gaps equally spaced around each piston, the rings were then compressed with ring clamps, a generous squirt of Torco MPZ assembly lubricant on everything, and then the cylinder placed over the pistons. With the ring clamps removed the cylinder skirts were allowed to rest one of the wooden strips while the upper face of the base gasket was coated with Wellseal sealant. Washers and nuts were then fitted to the cylinder base studs, tightened evenly, and excess sealant wiped away.
Unusually, the pistons protrude from the cylinder at TDC but a check against the arrangement drawing confirmed that this is correct.
Drive to the camshaft is via a conventional 3/8” pitch chain with 15 and 30 teeth sprockets. The latter were made from off-theshelf sprocket blanks with the keyway slots filed by hand. Without any documentation to suggest the intended cam timing for this engine, I believe the ndcam timing and profiles of the newly manufactured cam are the same as a standard Commando; inlet and exhaust profiles are identical with a duration of 304 degrees. The engine was set up with a degree disc on the drive side mainshaft and an extended dial gauge onto the cam followers.
In theory, opening and closing angles should be measured at the cam follower after an initial lift of 0.013”. As lift in this part of the cam profile is relatively gentle, I opted to measure at the cam peak. Using this method, inlet cam peak is calculated at 102 degrees after TDC and exhaust peak at 110 degrees before TDC. A key slot was laboriously filed in the cam sprocket and the final figures of 94 degrees and 107 degrees were far closer than anticipated.
Installation timing, with the pistons at TDC, is alignment of a stamped arrow ( " ) on the large sprocket with a stamped ( ! ) on the adjacent crankcase casting. Without an intermediate gear and only one key slot in each sprocket, this drive is very simple to install correctly.
Repair work on the cylinder head, to replace broken fins, had already been completed but, in my opinion, the fins between the exhaust ports were a significant restriction to air flow. As an aid to increasing air flow to the centre rear of the head, the upper four front fins were machined away and dressed with a file to look as though they were ‘as-cast’. The complete head was then Aquablasted to give a uniform finish and
thoroughly washed in solvent.
With valves lightly ground to their seats, they were installed with standard valve springs and, learning from later Commando developments, oil seals fitted to the inlet guides. The head is secured to the cylinder by a total of ten fasteners, eight of which are 3/8” x 26 TPI bolts and the remainder 3/8” studs, as used on the Atlas and Commando. With a very light smear of Three Bond sealant around the pushrod tunnels, the copper head gasket was placed in position and the cylinder head fitted. The bolts and nuts were sequentially tightened to 30 lbs.ft. With the rockerbox as a separate assembly, there were none of the usual Norton pushrod alignment worries.
Attempting to assemble rockers into the rockerbox revealed that machining of the casting was incomplete. The rocker spindle bores were finish machined but the internal faces were still rough-cast. Even worse, access to these areas is very restricted and impossible to machine using conventional tools. As would have been done for every Dominator-type engine, tooling with demountable cutters on an arbor through the rocker spindle bores was designed and made by my local machinist.
In the belief that the rockerbox was now ready for assembly, it was placed on the cylinder head – to reveal another challenge. There was insufficient nd clearance inside the rockerbox to accommodate the inlet valve spring collars. Working with a rotary file, metal was cautiously removed to provide sufficient clearance without breaking through to the outside of the casting.
Having rectified that defect, it was then noted that another machining feature inside the rockerbox had broken through into one of its fixing bolt holes. As this could be a potential source for an oil leak, the area was thoroughly degreased and filled with ‘Devcon’ epoxy/aluminium putty.
With that challenge overcome, the next was not unexpected. The rockers, individually made by Malcolm from EN24T bar and handfinished, needed minor ‘tweaking’ to align each adjuster with its valve stem. This was done by a little ‘percussive engineering’ using a copper-faced mallet with each rocker firmly clamped in the machine vice. Three rockers have near-perfect alignment with their valves, but the right-hand inlet rocker was well out of alignment and needed re-manufacturing as the distance between spindle and tappet adjuster bore was 3.00mm too short. With this new extended rocker, alignment with the inlet valve was corrected. Both exhaust rockers, after adjustment, were spot-on. Oil feed to the rockers is via a single central union and internal drill-ways in the rockerbox to the inner end of each rocker spindle.
The arrangement drawing for this engine shows the primary drive having a Dominatorstyle clutch with a 3/8” pitch duplex chain. Neither engine sprocket nor clutch drum was available, so the decision was made to revert to standard Dominator parts and ½” pitch simplex chain.
With a standard 41 tooth clutch drum, 19 tooth engine sprocket, and a 64 pitch chain, tension was perfect, and any wear would be accommodated by the tensioner. Further, alignment of the engine sprocket and clutch sprocket was also perfect, leading me to believe that it was originally designed with a simplex chain before being replaced with the duplex.
Unfortunately, fitting the aluminium inner chaincase proved this not to be the case as the clutch backplate fouled that casting. Looking more closely at the arrangement drawing revealed that the clutch backplate had ample clearance from the inner chaincase because the gearbox mainshaft was some 3/8” longer than standard Dominator. Using a Commando mainshaft and a little ‘engineering’ in the clutch hub provided the requisite clearance but, obviously, now the clutch sprocket did not align with the engine sprocket by 3.15mm. With the engine sprocket fitted on a taper, the only solution was a new, non-standard, engine sprocket.
Using fine grinding paste, the new engine sprocket was lapped onto the crankshaft taper and the clutch hub shimmed on the gearbox mainshaft to align the sprockets. With the chain and generator rotor now installed, final assembly of the primary drive was completed.
To allow for stroboscopic timing of an electronic ignition, timing marks were scribed on the generator stator every 10 degrees up to 40 degrees BTDC. There is no provision in the primary cover for checking the ignition
timing with a stroboscope. Any such check will require the cover to be removed.
With the oil pump now fitted, a goodly smear of Three Bond was applied to the timing cover joint face, the gasket placed in position, another smear of sealant applied to the gasket outer face, and the cover placed in position. After checking that the nd camshaft oil seal had not inverted, the fixing screws were fitted and then tightened evenly. Excess sealant was wiped off the outside of the joint face.
The gearbox selector plate was also fitted, with a generous amount of sealant, and operation of the gearchange ratchet mechanism checked before this had time to cure. Initially stiff, the gearchange action improved after a good squirt of oil over the moving parts.
No information is available regarding actual ignition timing for this engine, so the Pazon was set to 31 degrees BTDC as recommended for an 850 Commando. This seems reasonable in view of the similar combustion chamber shapes and compression ratios.
Missing from the gearbox is a feature which is standard on almost all Nortons – the gear position indicator. The gear selector mechanism components are mostly standard Norton and it would have been simple enough to incorporate the indicator. Progress perhaps?
An obvious design omission in this engine is that, unlike the Commando, there is no provision for an integral tunnel for the contact breaker cable. For this engine, the cable has to pass over the outside of the timing cover, something which, I’m sure, would have been corrected had the engine reached production.
One of the things that puzzle about this engine is that neither side of the crankcase makes me think ‘Norton’. The primary side quite clearly looks like a BSA, but the timing side is an amorphous lump quite lacking any recognisable design features. However, add a balancer shaft in the usual Norton camshaft position at the front of the engine, driven by an inverted-tooth chain, and you have a unit construction engine with strong design clues to its predecessors. For confirmation, take the example of the modern Norton 961 engine
which, despite being a ‘clean-sheet-of-paper design’, is instantly recognisable as a Norton Commando by virtue of its forward-leaning cylinder and the shape of its timing cover. A balancer shaft would reduce the perennial problem common to all rigidly-mounted parallel twins – that of vibration experienced to a greater or lesser degree by the rider.
To place the transmission close to the engine, it was necessary to rotate the gears and selector set by 45 degrees. One perhaps unwanted result of this is that the kickstart shaft is now higher than before, higher than the crankshaft even, and may be difficult for the average rider to operate. This will become clear once the engine is installed in a chassis.
Finally, it is said that the original prototype engine seized when running on the test bed and seized again when being ridden on the road by Fred Swift. Why that should happen is not recorded, but overheating may have been the cause. Certainly both cylinder and cylinder head of this engine have had considerable work done to improve the flow of cooling air through the fins.
I see several positive engineering features in this engine and, had it progressed successfully through development and had some design input, it could have been the basis for a new family of Norton motorcycles. But that wasn’t the decision taken 55 years ago at a time when I was still at school, studying for GCE O-level exams. Now back in the real world, and the engine assembled as far as possible, thoughts turned to the cycle parts and, particularly, what might the complete motorcycle have looked like if the project had been carried through. None of the project’s senior design staff now survive so we will perhaps never know what they had in mind. Cue optimistic me again, with a crystal ball, soft pencil, and those empty fag packets.
To be continued…
An alternative barrel was supplied by Antony Curzon, and was refurbished appropriately
The first cylinder barrels had serious casting faults The barrel uses cam followers in the usual Norton twin way, and required only gentle repair work in this area
Pistons attached to their rods and ready for the barrels. Which fit, carefully, in the conventional way
One timing chain tensioner You can work out where Richard removed metal from the front top four fins to aid airflow over the combustion chambers. The complete engine shot shows the original arrangement
It’s unusual for pistons to protrude above their cylinders, but this is how the engine was designed. The second shot shows a dial gauge being used to work out the valve timing by measuring the cam lifts. The timing marks are now the arrows shown in the third pic. Just in case the engine ever needs rebuilding… The head, from the rear, with valves and springs The four rockers were hand made by Malcolm Saggers, and are true works of art If you look closely at the inlet rockers, you can see that a little judicious ‘percussive engineering’ was needed to align rockers and valve stems The right-hand inlet rocker needed to be remanufactured to fit accurately
The primary side showing the arrangement. Unusual for a Norton twin, and nothing like a Dominator! Timing marks were made onto the alternator stator to match up with the rotor The timing side. Oil pump fitted (skew drive from the end of the crank) and ready for the cover The primary drive, partially assembled. The detail shows the slipper tensioner The points housing has no obvious provision for a lead, so Richard produced a temporary development for the Pazon unit used instead of contact breakers. Neat enough, but not entirely elegant. It does, however, work to spark the engine