Driving the demons out of an old English motorcycle seems to be a circular process: just when you think you’ve got them all, another one pops up. Of course, any piece of machinery built 40 years ago from a 70-year-old design is going to have a few quirks compared to a modern appliance; and higher maintenance is one of them.Take the clutch for example. The primary drive in a Triumph Twin was drawn up in the mid-1930s and designed to transmit around 25 hp.

By the late sixties, the Bonneville engine was producing a solid 46 hp and probably twice the torque of the first Speed Twin. And though the primary got beefed up over the years, it still needs to be in top shape to transmit power reliably and predictably to the cog box.I’d rebuilt the clutch in my 1970 Bonneville a few years back when I replaced the 19-tooth countershaft sprocket with a 20-tooth for more relaxed highway riding.

But recently it had started to slip, so much so that I couldn’t use more than about a quarter throttle in fourth gear. And the clutch had a tendency to stick if I didn’t ride the bike every couple of weeks. I’d usually check it was separating okay before I hit first gear, but occasionally I forgot, leading to a violent stall as I toed the shifter. Checking the cable and pushrod adjustment for the correct amount of slack showed nothing amiss.

Time for a teardown, pretty routine, I thought.But when I pulled off the cover, there was quite a mess inside. Chunks of the insulation “potting” from the alternator stator lay in a pool of oily sludge, which also revealed another surprise: half of a roller from the primary chain. Where the other half went, who knows?I guess I should own up to a small oversight here. A few months back, I garaged the Bonnie without checking that the fuel cocks were off. Next time I tried to start her, it had hydraulic lock; the crankcase was full of gas. It seems that as well as one of the petcocks being on, at least one of the carburetor float valves wasn’t doing its job. I drained the engine and changed the oil, of course, but I guess the internal carnage was more than I anticipated. The gas must have swollen and disintegrated the alternator stator’s coating. I put the broken chain roller down to one of my inadvertent cold-start clutchless shifts.

Whatever, I needed a new timing chain.Dismantling the primary unit on a Triumph unit is straightforward, but you do need the special puller to remove the clutch hub. You’ll also need a puller for the engine sprocket—I used a GM harmonic balancer puller. There’s also a special tool to slacken off the primary chain tensioner, but a cut-down screwdriver works just as well.First, though, you need to remove the alternator stator by undoing the retaining nuts. The stator should slide off, though mine was a little sticky because of the swollen insulation.

The rotor is retained by a large nut and lockwasher. I find that with the lockwasher folded back, the nut usually succumbs to a suitable wrench whacked with a rubber mallet. Inertia beats brute force most times …Unwinding the three nuts that hold the clutch springs allows you to remove the springs, the cover plate, the clutch pushrod and the driven and friction plates, at which point the two pullers mentioned earlier will remove the clutch hub/basket assembly and the engine sprocket. There’s a set of cush rubbers in the clutch hub, and if there’s any free play in the hub drive (between the hub casing and the hub centre), you’ll need to dismantle the hub (two screws hold it together) to replace the cushes. Mine was fine.

With all the pieces on the bench, I could assess their prospects. Everything looked fine except some glazing on the friction plates. I roughed them up a little with a wire brush and washed them thoroughly in Varsol.Checking the friction plate thickness and the spring length showed everything to be within tolerance, so I started reassembly with a new duplex chain. I used blue Loctite on the clutch hub retaining nut and a new lockwasher on the engine sprocket/alternator rotor. The only tricky part in reassembly is making sure the rotor’s Woodruff key is snug in its slot, and that there’s at least 0.008” of rotor/stator clearance all round.

I like to screw the clutch springs down so that the springs just bottom with the clutch lever fully squeezed, then back off a turn. That ensures the springs provide as much force as possible but can’t bottom out. Similarly with the clutch pushrod, after making sure the cable adjustment is appropriate, I use the screw adjuster to take up all the slack, then back off a half turn. With this done, a few squeezes of the clutch lever will show if the clutch cover is moving freely and evenly. If it isn’t, the clutch assembly needs to come apart to find the cause—a weak spring, for example. Either way, don’t forget to readjust the primary chain.Nineteen-seventy and later Bonnevilles share engine oil between the primary and the engine, so I did a full oil change and refilled with fresh V-Twin grade (20W50) synthetic. The result? I now have a clutch that doesn’t slip and doesn’t stick.

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