I haven't been here in a few days and come back to find you guys have let me down. I saw the discussion about RR vs Packard and couldn't wait to find the TRUTH about the two versions of the Merlin.
1) The US version used imperial measurements while the Brit version used metric.
2) The RR had all the bolts reverse threaded compared to the Packards, so they had to be turned the "wrong way" to be installed.
3) Because all the bolts were backward, the propeller shaft spun the other way, so planes with the RR engines had to have their propellers put on backward.
4) Planes with the RR Merlins had to drive on the wrong side of the runway.
(Sorry, that's all I could think of on such short notice. I'm sure some of you guys will help me out with the rest.)
-Irish