Guiding it home
I do like an open-and-shut case. My mate Phil Clarke up in Scotland got in touch to say he’d put his rigid Ariel VG500 back on the road after a few years’ break, but was having trouble with a valve nipping up in its guide when the engine got hot. “Do you remember I had trouble with that when I first built it, having fitted new valves and guides?” he said. “But I’m sure after that we reamed them out and cured the problem; so why do you think it’s happening again?”
The clearance between a valve and its guide is automatically provided by the valve stem being ground a couple of thou undersize – so typically a 5/16in (0.312in) stem will measure 0.310in. The valve may be an easy fit in the guide in your hand, but the tight press-fit into the head is likely to shrink the bore in the middle, causing problems unless it is reamed to size after fitting. Even if the valve seems to fit OK on assembly, it’s still worth reaming because it may tighten with engine heat – like Phil’s. Because the clearance is on the stem, you just need a standard-size reamer.
Great, but if we did that, how come Phil’s valve is too tight again? I could only think of one possibility. “Did you put sealer in your tank?” I asked.
“Er, yes... why?”
Phil phoned a few days later to confirm my suspicion that the previously reliable sealer was now sticky as a result of ethanol attack. The sealant dissolved in the fuel can, then produce a similar sabotage effect to sugar in the tank, gumming up the valve in the heat. To make sure, Phil fitted an unsealed tank from another bike and it ran fine, the problem returning after refitting the old tank.