that's a wierd one!!
it's fairly simple to determine the capacity from the outside... of course, just check that there isn't a tag rivetted to the back of the motor
and this is assuming it's not a heavily modified motor, IE stroked, or a big bore head on a small bore or whatever, which is pretty uncommon
step 1/ is it a 'big bore' - ie 1275, 1071, 970 - you tell that by placing your thumb between the thermostat housing and the rocker cover, if it fits, then it is a big bore
step 2/ measure the stroke - get a piece of wire and poke it down one of the spark plug holes, and keep it dead vertical. Get someone turn the engine over by hand while you hold on to it, and put a mark on it (relative to the edge of the spark plug hole) where the piston is at it's highest and lowest piont. Measure between the 2 marks and there's your stroke
then refer to this page to work out which one it is:
http://en.wikipedia.org/wiki/BMC_A-Series_engine
IE a 998 will be 76.2mm (won't be a 948 as they weren't east-west)
I'm 99% sure it'll be a 998, since the plate is rivetted on, it wouldn't be an aussie motor, they didn't have many 1100's in UK, or it might be a south african motor, in which case you might be sitting on a 1071
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