BSA L29 350 SV parts - and advice!

[Rex]

I'm sure they tried that as steam engines used copper for gaskets.
Surely the problem was that SVs have a very large hot-spot over to one side of the cylinder/head joint causing unequal expansion and distortion in that area, so a one-piece casting was one less failure point to worry about?

[R]

It was the 7 or 8 or 9 bolts around the perimeter of the bolt-on (sidevalve) heads that made the difference.

Previous versions had mostly used a clamp-down style of head - which was OK for compression ratios of 3:1 or so, but any more was tough to keep in there.
This was dependent on the cylinder/head design though, some were less prone to distortion / leaking than others.

Ohv heads ditto, although 3 or 4 bolts were sufficient, since the distortion is less localised.

[R]

Well if i could go back in time, i would've suggested a piece of flat copper might do the job!!

Well yes, but for that to work you need cylinders and heads that don't distort.
Or 7 or 8 or 9 headbolts, not just a simple clamp-down arrangement - which may work for 3:1 or 4:1 compression, but not so well for more compression.
Took quite a while to figure that out, for most.