The 313 was a Siamese bore 318 produced by the Canadians and sols on the British Empire markets to over come an old archaic way of taxing new engines. Basically the English system was base on a weird bore/stroke calculation to estimate an engines HP and thus be taxed accordingly. The calculation was slanted towards long stroke small bore engines - the bigger the piston dia the more HP so More tax to collect. What this did was create a market where by early English engines ran ridiculously long stroke engines way longer than anything comparable to the USA market. So a 313 Polyspherical Plymouth/Dodge engine in a Canadian or Export model would have a 3.875" nominal bore instead of a 3.91" like the yankee engine and also had mechanical lifters instead of hydraulics. Nearly all other components are the same - block, heads, valves, pumps etc etc. I've had to live with this anomaly for all my Mopar life!.