The Stag V8 was, and still is, a chunk of shit. Problems?
The camshafts were positioned in the head such that the inner head studs had to be wound in at an angle and then the nuts wound on. So immediately you have unequal clamping forces on the heads. You try even fitting the studs one the head's been skimmed! Head gaskets were always a problem.
Timing chains were notoriously weak. By 30k the tensioner travel is used up and you need to replace the chains. Nice.
Small big ends. These used crankpins that weren't really big enough and very few get past 100k without the crank knocking its tits off.
Distributor. BL used a cheap and nasty Lucas copy of a V8 Rolls Royce unit with twin points. Add a bit of spindle wear and the ignition timing is all over the place and Stags were not very fogiving of this. Cue a holed piston or two, but cured with a Lumenition kit.
Cooling system. This was a joke, period. Not just the radiator and the inadequate cooling passages in the block, but the shimmed water pump that sits in the V and needs to be shimmed to the correct height to get the required flow.
Apart from that, they weren't a bad engine.
It was an interesting design but it needed Mercedes Benz to build it. BL had enough trouble making A Series engines to a consistent standard. As it is, a Stag needs a properly fitted Rover V8 to become a reliable car. The Stag V8 has been 'sorted', but it would always be like a reformed serial killer because you could never trust it 100%.
I had an acquaintance many years ago who had a 2000 Estate with a Stag V8 in.
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Le Mans Classic 2006
Ours was the Bosch K-Jet. 
