Lock and key
Jeremy Hurd had a question at the NMM. On his 1970 650 Triumph, the alternator drive key had loosened it its slot, damaging the crank. He had the crank repaired, but when he put it back together with a new key, the alternator rotor was still loose. “What can I do?” he asked. “I don’t suppose I could just Loctite the key in the crank...?”
I think not, Jeremy. Locking fluids are useful; they expand as they set, taking up clearance, but they need airtight conditions to set – like in a thread or around a bearing – but not a key in a slot. And although they can work very well restoring the fit of components, they’re not really OK with impact loads like this.
The trouble with the Triumph alternator key is that while keys usually locate a taper or interference fit, here the key acts as a driving dog and if it’s a loose fit trouble will inevitably follow. The rotor is heavy and subject to a pulsing magnetic force as it passes from pole to pole, so slack in the keyway will encourage it to hammer to and fro, flattening the key and stretching its slot until the key breaks.
My solution would be to hand-file a new key that’s a snug fit in the slot – maybe stepped to match both crank and rotor slots tightly. Use quality steel – old, damaged engine bolts are a good source. I don’t rate tab washers here; the flexibility that enables you to bend the tab can allow crushing in service, so the nut ends up loose despite the folded tab. A large, flat washer to sandwich the rotor centre is better, with a drop of Loctite on the nut – but use sparingly, too much can make the nut so tight it damages the thread when you undo it.