PISTON BLOKE
Richard Wyatt has come back to ask about pistons for the late ’20s Blackburne engine he’s building out in the States. A Ford Escort piston looks suitable, although shorter on the skirt. He asks if it will do the job.
There are issues with using modern pistons, especially from water-cooled engines, in vintage air-cooled motors.
The design and material of a piston designed for a modern engine running with perfect fuelling at a relatively consistent temperature is unlikely to be ideal for a nearly 100-year-old air cooled sidevalve. But you could also argue that a lot has been learned about making pistons over time, and the original probably wasn’t great either. Commissioning a bespoke piston from a modern manufacturer is the ideal, but otherwise probably the
answer is to allow a generous bore clearance. Like two-strokes, sidevalve cylinders tend to change shape with heat, and running Ford Escort clearance would be asking for trouble. I modified a Honda FT500 piston to fit my Sunbeam 9A, and the reborer worked to Honda clearance, reasoning that it matched the expansion rate of the piston material, but the Sunbeam ran hotter than a Honda and partial seizures demanded extra honing to increase the clearance.
A short skirt may encourage noise from ‘piston slap’ but I can’t see it causing any other problems at the sort of revs the Blackburne delivers.
One thing, though – being a total-loss oiling system, you need to leave off the oil control ring. These engines need to burn and leak some of their oil to leave room for the drip-feed supply.