Go with the flow
Peter Newton is concerned about the ‘tell-tale’ button on the oil pressure relief valve of his 1955 Triumph Tiger 110. The button pings out eagerly when cold, but much less when the engine’s hot – despite the engine being fully rebuilt, including a new sealing bush in the timing cover. He says: “A recent 40-mile ride caused me some consternation – but no actual problem. The bike has a new oil pump and valve and I am using SAE 40W oil.”
Oil pressure is a bit of a red herring. Good lubrication is about flow. Pressure is a measure of pump output versus restriction – the close fit of the big-end shells. Shell bearings rely on a perfect fit, cushioned by oil. There needs to be just enough clearance in the bearing for this cushion and a limited oil flow. The back pressure created thus provides a measurable guide to engine health; pressure drop suggests worn feed (pump) or restriction (big-end bearings). However, hot oil is thinner than cold and flows more easily, reducing pressure – that’s partly why there’s a release valve; it opens a bypass, reducing strain on components when cold oil raises the pressure artificially high.
Triumph’s tell-tale button is operated by the release valve piston; it doesn’t indicate flow, only that the valve is bypassing excess-pressure lubricant back into the sump. They’re surprisingly sensitive, even showing a difference before and after an oil change. I use 20/50 oil in Triumphs and multigrades are supposed to maintain viscosity across the heat range better than straight oils, so maybe Peter’s variation is exaggerated by using Straight 40 oil. Triumph originally fitted oil pressure gauges and allegedly only stopped because so many customers complained about low hot-oil pressure, so Peter probably has nothing to worry about.