CD, thank you for joining the fanclub of one for early Halfordesque engines. He was a fascinating chap, almost a cantankerous contemporary of Roy Fedden. Frank Halford insisted on maintaining his own design business, rather than being incorporated into Napiers or later dH. Perhaps, without the opposition of a Board, he went to far down the rabbit hole, designing exquisite internal combustion engines that were very expensive to build. When he got out of his comfort zone, with his first jets, they were simple and profitable. By the time he became confident again, there was the Gyron….Napiers was under some pressure to come up with something to equal the Lion, once RR edged ahead with the Kestrel and Fedden with his Jupiter. They brought Halford on with ‘company betting’ freedom and he came up with the Rapier, followed by the Dagger. The Napier accountants would have lost hair. There was some thought about selling these engines into the perfidious Continental market, so they were all designed in metric. The Rapier pistons are 90mm in diameter and the Dagger 100mm. Your ashtray looks like a perfidious Continental seller has run his wagon past your door…

Thank you Oracal. I don’t like to stray too far into the 50’s, too much louche morals with hip thrusting rock n’roll music and kerosine burning, swept wing, seagull splattering aircraft, so don’t have the referenced APs at hand. If anyone does, I would be very interested to confirm the Al-Zn alloy theory. 

CD, thank you for posting up the ‘mystery rocker’ but I do not think it fits Dagger. The Dagger was the world’s first engine with hydraulic tappets, invented by Frank Halford, and apart from this feature, the rockers seem to have been made, on a precautionary principle, very robust, with a pronounced offset to fit within the tight geometry. 

Top view of rocker, showing offset