How related were the Seven, Big Seven and Eight Engines?

[Ron Hayhurst]

Hello

And lastly is it known whether the Big Seven or Eight engines were  recipients of an OHV conversion along similar lines to the LAP Head for the original Seven engine (possibly by Alta, etc)?

I would think not- there have only been two types  of Austin 7 OHV conversions I have heard of, both very heavy and I would say unlikely that the Big 7 and 8 were ever so fitted.

Tony.

For those pedantic - visually I don't know of any physical appearance differences in the Big 7 and Eight engines, apart perhaps from the paint colour.  The only difference I'm aware of is in the timing and the carburetters.  Big 7 = Ignition timing TDC.  Valve timing TDC.  Carb. Choke 23, Main 90, Compens. 50, Slow 60.                                                                                                          Eight =  Ign Tg  TDC.      Valve tmg 5 deg before TDC (7/16" on flywheel). Carb 19, 70, 60, 45.
Perhaps the above specs could be applied to improve your Big 7?  In the postwar Eight, above details were different again.  Maybe this is of help to somebody.   Cheers,  Bill in Oz.

The momoblock of the eight is about three-quarters of an inch longer than the Big Seven, also I seem to remember the gearbox is also longer and possibly not interchangeable between the engines. This makes the rear mounting of the gearbox extend beyond the mounting point on the cross-member. It is definitely not a simple swap. 
Ron Hayhurst