compare 803cc piston

[andrew34ruby]

To compare the seven piston to the 803cc A-series piston is a bit odd. Both engines are the same stroke, and the bore sizes are almost the same, so why are the pistons so very different? 

The 803 is a much taller piston and a much bigger gudgeon pin.  But why?

       

[Bob Culver]

The BMC engines with siamesed bores were prone to distortion and 3 comp rings were considered desirable.
Until about the mid 50s most engines wore notably; nearly all cars were rebored at some stage and also received new rings sets .
Long pistons reduced the degree of piston tilt with associated noise, poor oil control etc. Split skirt pistons rapidly closed up worsening the tilt. Near all designs incorporated long pistons to moderate the effects..
In the 50s robust temp compensated pistons without full slots appeared. In moderns due thermostats and fancy oils ring and cyl wear is now near eliminated . The 3 section oil ring is effective and consistent. Full skirt contact is not necesary to moderate consumption. .010 taper was common; the rebore point for moderns  is now often given as about .004 and few reach that. Clearances remain small  and consistent so short pistons can be used and are again. In the early 1920s huge oil consumption was considered normal. 
Cars with small gudgeons were noted for wear of (Vauxhall LIP ). Max pressure goes up with cr and pinging, which many engnes did a lot of in early 50s.
The BMC A and B engines were developments of the A40 from about 1948 and retained many features of the time, including long strokes. The 3 bearing engines could better tolerate heavy pistons (although not esp well in the case of the A40 and A30!) Pistons exert several hundred times own weight at revs.
When cars were still evolving fast, the designers of the  original Seven would have little imagined that many cars would soldier on to 200,000 miles and more. 
The Napier Lion aero engine had similar proportioned pistons but would not have been kept in service worn.