NORTON ELECTRA PROJECT
Peter Holland’s decade-long project to rebuild an electric-start lightweight twin moves onto the motor. He discovers a few flaws which escaped AMC’s original QC teams…
Peter Holland’s decade-long project to rebuild an electric-start lightweight twin moves onto the motor. He discovers a few flaws which escaped AMC’s original QC teams…
Norton lightweight twins have a unit construction engine and gearbox. It soon becomes apparent that the primary drive has been opened and then left in a damp environment. The starter motor drive chain is rusted rock solid, but the duplex chain has very little corrosion. Perhaps condensation forms in some areas of the engine and less in others. There is damage on the crankshaft drive end due to the end bolt not properly clamping the engine sprocket / alternator rotor / starter sprag. Careful investigation reveals that the partially threaded bolt runs out of thread just before clamping forces are achieved. To the casual assembler, who feels the bolt getting hard to turn, all is well. However, the plain part of the bolt is trying to drive itself down the threaded hole in the crankshaft.
The heads are removed without trauma, and two pistons, still on standard 66mm bore, reciprocate smoothly in moderately ridged cylinders. The barrel is lifted and pistons removed. Rings are seized in their grooves and the gudgeon pins are definitely worn where they rotate in the conrods. The left hand conrod has perceptible twist and play at the big end. The right hand feels OK. I make special tools and pullers where required and strip the rest of the engine with care. Oil is fed to both crankshaft journals from the timing end of the crankshaft. By making the cross-drilling vastly oversize, Norton create a centrifugal gallery inside the crankshaft for catching heavier oil contaminants. The threaded aluminium cap at the end of the cross-drilling is removed with difficulty, totally destroying the cap. Now the reason for big end failure becomes obvious. A very compacted sludge had built up in the gallery and at the far end, adjacent to the left hand big end feed, the sludge completely fills the cross drilling, blocking the feed to the journal. I recall that the Honda CB250 had a centrifugal oil filter at the end of the crankshaft. No bother with the Japanese design, where three screws and a circlip give immediate access for cleaning at every 1000 mile oil change. Be warned, all Norton lightweight owners, who think a blast of compressed air or solvent will clean the crankshaft oil galleries. It takes a determined attack with long screwdriver blades to shift the sludge build-up. What sort of service interval did Norton have in mind? Maybe ‘every three years, when the big ends or mains fail, remember to clean the sludge trap.’ I wonder why this advice never got published… Finally, the crankshaft is mounted between centres on the lathe and checked for true running. Minor damage to the drive side bearing journal, and to the radius between it and the crankshaft, is repaired. Then the crankshaft is reground at W Drake’s engineering workshop in Bradford, to minus 10 thou. A pair of relatively rustfree and unworn Navigator camshafts, the same as in the Electra, are bought on eBay to replace the badly corroded pair in the bike.
I note a dent on the otherwise flat surface of the crankcase top face. It is quite deep, near the crankcase joint, and in the region between the two spigot holes. Looking at the mating face on the underside of the cylinders I see, between the two cylinder liners, a casting flash that has remained in place, unobserved by the Norton machinist, inspectors and assembly staff for nearly half a century. It will have pushed through the
cylinder base gasket and made the dent on the crankcase face. Small wonder these bikes got a reputation for leaking oil when lumps like this were stopping face-to-face assembly. A similar fault, caused by an errant blob of weld, is noticed at the head steady. This blob also prevents clean contact and it’s obvious that this clamped joint has always vibrated, rather than steadied.
Threads in the aluminium engine casings are checked and some repaired with thread inserts. Dirt and old gasket goo is cleared from the bottom of all the tapped holes. Dowels are checked for fit and roundness. Screw lengths are checked and new screws purchased with the correct slotted head and finish.
Lumiweld, an aluminium soldering system, is used to repair some cracked bosses in the inner and outer primary chaincases. The same technique is used to restore damaged joint faces. The joint between the inner primary case and the crankcase seems particularly weak; it’s secured by only four, quarter-inch countersunk screws, three of which are set close round the crankshaft. The one countersunk screw behind the clutch is expected to support 15lb of starter motor in addition to the normal Jubilee / Navigator task of holding the oil seal securely in place and absolutely concentric to the gearbox mainshaft. It’s normal practice for vehicles to experience two-gee accelerations or more on
bumpy roads. The starter motor might weigh only 15lb, but on a bumpy road its weight is more than doubled.
Scars inside the inner chaincase and a stripped thread hint that this grossly overloaded countersunk screw gave up the struggle, even though it was supposed to be centre-punched at the head for security. I solder a lump of aluminium to the inner chaincase so that it rests on the left-hand crankcase and takes the weight of the starter. During assembly the suspect screw is renewed, treated to a nice new thread insert, thread locked and centre punched.
Once the inner primary chaincase is removed, the gearbox sprocket is revealed. As expected, it’s worn past reasonable use. Unexpectedly, the large nut securing the sprocket is MIG welded, down one of the flats, to the sprocket. Either PO had no faith in the Norton tab washer, or he didn’t have one. Probably the latter. Careful grinding and work with a file breaks the weld and saves the nut, not easily replaced, for future use. A new tab washer, complete with internal spline form, is filed from sheet metal.
The gearbox internals have survived fairly well. New bronze bushes are made for the layshaft and gearchange shaft. The selector forks and the scrolled cam that drives them are a bit worn. In an ideal world, I’d replace them, but the best I can do is to reverse the pegs that sit in the cam grooves so that the unworn end of the peg rides in the worn grooves and drives the worn selector forks. Drive dogs on some gears are a bit rounded. I grind the
driving faces back by a small amount, careful to maintain the geometry. The kickstart assembly, which comes off with the gearbox cover, is put to side for later inspection.
The time comes to start putting everything back together. I make a stand for the engine and commence the rebuild on a steel table. Before fitting the new main bearings into the crankcase, I take a pair of used bearings and with a belt sander reduce the outside diameters until they fit easily into the crankcases, but no more. Using emery cloth I do the same for the bearing inner diameters until they fit smoothly and easily over the crankshaft journals. It is now easy to check the crankshaft end float. I decide to omit the gasket between the crankcase halves in favour of Blue Hylomar, which won’t settle with use. A new aluminium cap goes on the crankshaft oil gallery, the mains and conrods are fitted, and the crankcase halves joined. I lightly nip the barrels onto the crankcases without a cylinder base gasket to ensure the best chance of a flat surface when the crankcase halves are finally bolted together.
I decide to use the original pistons, even though they are a little loose in the bore. A friend has blown up his Norton 99 and by some coincidence the gudgeon pins, or wrist pins to you colonials, are the same diameter and length. And they appear unworn. I ignore the very, very, slight detail differences in the through hole, and in they go. New 11/16” circlips and piston rings follow. The ridges at the top of the barrels are scraped down, to line up with the worn bores, and then the barrels honed. I measure all 24 worn cam follower bushes I’ve got, and fit the best four into the barrels. This technique, which will bring pain to the purist readership, is repeated throughout the build. Please remember when accusing me of being a useless spendthrift that nobody knows if the bike will run – yet.
EBay throws up a starter sprag assembly in the USA. I win this and then feel I’ve won again when an alternator rotor, that I don’t need, comes included. I check out the Wipac alternator for resistance and insulation and restore the missing insulation from one of the windings.
The clutch centre is a good fit in the mainshaft spline. Filth in the bottom of the blind splined hole of the clutch centre, probably old Loctite remedies, stops the clutch centre from bolting up tightly onto the mainshaft. The pile of lightweight spares is scoured and a better clutch drum found. I’d prefer a tighter fit in the roller bearing race supporting the clutch drum. Despite fitting new rollers, things feel a little sloppy. The clutch rubbers are judged OK. I ferret through a small mountain of used plain and friction plates to make up a ‘best’ assembly.
A new primary chain and tensioner block arrive from the NOC spares scheme. Clutch and gearbox sprocket are fitted together with the endless duplex chain. A starter motor chain tensioner is devised. This departs from the original design in that it will tension the slack side of the drive and prevent the slack chain machining its way through the internal fixing bosses of the primary chaincase. The starter sprags get new drive posts made from roll pins. Being hard, these should be fine running in the slotted wave spring. Finally, a gasket is cut for the primary chaincase and the cover carefully screwed in place.
Some of the valve rockers are not central to the valve tops. Not by a long way. I take more care than AMC to shim the rocker end float and stone the rocker surfaces for proper rolling contact. The heads are fitted as a pair, with the inlet manifold and head steady lightly nipped up to ensure flat face alignment as torque is applied. Finally, the repaired carburettor with its new chrome bellmouth is fitted onto the complete engine – except for the kickstart assembly which I leave for later.