If the cap fits...
WILL TRIED TO sound calm on the phone. “Erm, just went to change the oil on the Tiger 100...” he said, “and the sump plug nearly fell out. It’s really loose in the hole – you don’t think it could be the threads in the crankcase, do you?”
The drain plug on a unit-construction 500cc Triumph is the same part as a pre-unit model rocker-box cap. It’s made of zinc alloy and very unlikely to damage the thread in the crankcase, but I remembered the original plug in Will’s engine had a pretty chewed-up head and I had given him one of my spares. I couldn’t imagine I’d given him one with a stripped thread.
“That’s what’s worrying me,” said Will. “The thread on the plug looks good – if a bit tapered at the end.”
I suggested he come round; I’ve got a few spares and could find him another one. He was right – the thread looked absolutely fine, but did seem to taper at the end. It was a 1960s pattern part, and in my box I found another that had an original Triumph part number inside. The two looked identical; it was only when I decided to measure the threads with a micrometer that I found the trouble. The pattern cap’s thread was a whacking 35 thou smaller than the genuine one – that’s almost 1mm! But in fairness, checking later, all my spares measured up with different-size threads, whether pattern or OE. Maybe that explains why Triumphs are so prone to shaking them off on the road!
Will phoned back later to say the larger cap had solved his problem, thankfully. But it’s obviously something worth watching out for...