Stefano
Sorry to hear of your problems. Here are some thoughts re bonnet compatibility and overheating in general
1) May I suggest contacting Dr Stefan Cembrowicz, the Chairman of the Bristol Owners Heritage Trust ,who played a significant part in the rescue of various body bucks when Bristol cars assets were sold during the final liquidation, with a view to discovering if he knows whether separate bucks were created for 407, 408, 409 and 410 or an existing buck modified. If for example an entirely new buck was created during this series comparability would probably be lost, but if there was only one buck that was modified (forward of the bonnet) compatibility seems likely.
2) Graeme Payne at the Classic Bristol Parts Company may also know know about compatibility even if he doesn’t actually have a second hand bonnet in stock. Bristol cars services tended not to throw anything away and anything potentially useful was salvaged and put aside
3) Some panel beating may still be necessary to ensure a good fit, that’s how it was done originally
4) It seems likely that most of the extraneous heat under the bonnet actually emanates from those horrible cast iron exhaust manifolds rather than the block itself. The former are tucked up right up under the cylinder heads and there is simply nowhere for the vast amount of heat they generate to go when you’re stuck in traffic, even though the block itself is still cooled by the radiator. I went for broke £’s wise when I owned PEU186F having had a lot of other trouble with them and went for SLJs magnificent tubular manifolds. I perceived (and these are only my perceptions, they’re not scientifically proven) a significant drop in under bonnet temperature alongside a worthwhile improvement in performance - it felt as though the engine had previously been stifled by the cast iron manifolds. It might well be possible to wrap SLJs manifolds with insulation as they are fitted which should further improve matters as far as temperatures are concerned.
5) if you still have the original Carter carburettor also be aware that the float needle valves wear and leak over time which results in the carburettor bowls overfilling, particularly when you’re idling in traffic, resulting in an over rich mixture and a stall. The hotter the air temperature the richer the mixture will be. Suspect this as a potential issue if the idle needle valves have to be screwed at all far in (from memory once I’d fixed this issue the I found to my surprise that the engine idled most happily about three turns out) or if there is a tendency to fail pick up cleanly from idle.
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