Big 4 barrel

I don't believe that Bracebridge Street ever painted an iron barrel silver. Black is better for heat dissipation.

I've never seen an alloy barrelled side valve either.

Silver painted barrels were an attempt to make the Commando look more modern for a while.

Not sure there's any evidence that black is better at heat dissipation than silver is there?!

Heat dissipation is a function of the emissivity of the surface (amongst other things). Emissivity is a function of the surface finish more than the colour. So a dull finish will have a high emissivity whilst a polished finish will have a low emissivity. Usually silver fishes tend to be polished (timing case etc) so will have a lower emissivity than the dull black pot. George

Yep, I agree, so a Matt silver should be pretty much the same as a Matt black?

No, in theory matt black will radiate heat better than Matt silver. However, in practice, the difference is probably negligible. As a teenager, I painted my B31 barrel, and head, Matt silver, it didn't go any faster, or weigh any less!

Checking a sales brochure, it looks black. http://www.nortonsingles.se/broschyrer/att/1951.pdf

About heat dissipation, I have read a lengthy thread in Swedish. As most heat radiation is in the infrared part of the spectrum, what colour we see is not what really matters. And that radiation is just a part of heat dissipation. The conclusion after some pages was that nobody really knows.

Mike

Re: Black or Silver.....

In terms of radiating heat... not a great deal of difference. Reason irrespective of colour, the fins are stacked on top of each other so whatever heat they radiate they radiate the heat back into the fins either side so back into themselves. From the side, the profile of the fins is quite small (if you ignore the inner barrel core) so not much radiation from the side either.

Most cooling is done by conduction.

As the air passes over the fins to which it is in physical contact, the air is heated and removes (lowers) the fin temperature and cools the barrel. Heat is conducted from the hot inner side of the barrel (or head) to the outside of the fins so there is a temperature gradient across them. Now Aluminium is a better conductor than cast iron so the temperature gradient is more shallow and heat transfer to the air will be better simply because the aluminium can conduct the heat more quickly to all parts of the fins and their surface as they are being cooled by contact with cool air. In effect the aluminium is pumping more energy to its surface per second and therefore heats the moving air more quickly too and consequently loses more heat from the inside of the engine. This explains why engines with aluminium heads or barrels run cooler than cast iron versions for any given power output.