Tuned Reliant 850cc Engine Re-built (Twice) Then Blown-up Again! Jeffrey J3 at Harewood Hillclimb

The Jeffrey Mk3b historic sports racing car's 750cc engine broke its crankshaft at the Golspie Sprint last year. A standard Reliant Robin 850cc was fitted but it only had 39 BHP and was a bit slow. I rebuilt another 850cc Reliant engine but it would not run due to low compression, despite lapping in the valves. I then fitted the 'big valve' modified head, along with the 714 race camshaft, into the 850cc Robin engine and fitted that to the Jeffrey. All seemed well and off I went to the Harewood Hillclimb (Yorkshire, England). The engine pulled well (after I replaced the 1-1/4" SU carb with the Weber ICH34 single choke down-draft carb. The engine now revved all the way to 7500rpm. Saturday was very wet but Sunday was nice and dry and my times came down with each run. On the last run of the day the car started to squeal on the overrun between gear changes and when I got back to the paddock, there was a knocking sound. Initially I thought it may be a gearbox bearing but on further investigation it would appear it is a big end bearing in the engine which has failed.