The reason that Dave Swarbrick has such a great reputation I think is because his pipes are just so nicely made and he has been supplying race pipes for years for a huge range of bikes.
I have used Swarbricks pipes on a couple of TZ motors and an RD400 and I like them. I have no idea who works out the dimensions for him, but I think Dave is the fabricator and someone else works out the sizes, but I could be wrong about that.
BTW, there's no such thing as "just a megaphone" ;-)
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1965 Honda CP77 — Ulsan Metro City, Rep. Korea
Restored with original parts — Owner: J. Doe