Bent and broken

After a rebuild last year, Keith Livingstone’s ’74 Triumph T120V came to a sudden halt with a broken pushrod. The cause appeared to be a faulty Allenheaded tappet adjuster which had broken, creating enough slack for the pushrod to jump out. Replacing the necessary bits, Keith tried again but had a similar problem after 200 miles; this time both pushrods on the right cylinder failed (one bent, one broken). “Could both valves have seized briefly?” asks Keith, “They look fine now. The bike has seemed noisy since rebuild from the camshaft area and Paul at Shropshire Classics suggested that having covered 45,000 miles it may have worn tappets/guides; could that have anything to do with it?”

This is a curious problem. The valves would need to have seized shut to damage the rods without bending by hitting the piston, which seems unlikely they’re hottest when open, losing heat by conduction when closed, and there would certainly be seizure marks on the stems. I don’t think worn tappet blocks will cause much trouble, either – I’ve heard they can be noisy, although they are immersed in oil the whole time and I suspect that much more noise comes from worn timing gears.

It’s worth checking the tappet blocks are straight; if not, the cam lobe can just clip the other follower and I once saw an engine with so much cam endfloat that it could open both valves at once!

But what’s most likely to cause this? I would check the rocker oil supply – if you remove the domed nuts and start the engine, oil should squirt across the workshop; if it just dribbles, there’s something wrong with the pump or pipework. But it’s also very easy to get the pushrods out of place when assembling the engine so that they rest on, rather than settle into, their cups (see previous page). You might think that would show up immediately, but I did it once rebuilding a friend’s Tiger 100C and it covered about 50 miles before the pushrod jumped out. The usual evidence this has happened is a crescent-shaped indent in the ball of the rocker or tappet.

‘START THE ENGINE AND OIL SHOULD SQUIRT ACROSS THE WORKSHOP’