1939 Sq4 sprocket finish

[RayBlack]

Hi. Can anyone tell me what the finish was on the rear sprocket for my 1939 Square Four 4G? I have seen several restored bikes and some have (black) painted sprockets, others appear to be cadmium plated. Any information would be greatly appreciated.
Ray

[roland robinson]

Rivet counting 'in extremis'. Depends on how often they have been renewed (if ever).

[nevhunter]

Plating can cause them to be brittle. Hydrogen embrittlement. I've seen one cracked. Leave it as is. Nev

[Roger Gwynn]

Only chrome will do that and then only with certain types of steel. The originals would have been Cosletized in 1939, Cadmium cam in around 1950.

[nevhunter]

Respectfully I suggest you check on that. Even stripping can cause it. There's a heat treatment process that can be done just after the plating/stripping operation. Hot oil would do it but it varies on the thickness.. Nev

[RayBlack]

Thanks Roger. I'm not exactly sure what cosletizing is, so I guess I still have some homework.
Ray

[nevhunter]

I think it's a good black undercoating process used by Sunbeam and Ariel just before the 30's. Chrome displaced Nickel in 1930. Nev

[RayBlack]

Thanks Nev. So far I have been unable to coax an answer from Google here in Canada, so at least I now have something of a lead.
Ray

[Simon.Gardiner]

Coslettizing
Producing a black, rust-resisting surface on iron and steel by boiling for some hours in water containing phosphoric acid and iron filings.
https://www.engnetglobal.com/tips/gloss ... slettizing

Commonly used on bolts, nuts and similar cycle parts. Doesn't resist rust very much which is why bikes have so many rusty nuts and bolts!
(Trying to think of an obvious example, it's the original finish on that eye-bolt that is used to draw the gearbox backwards/forwards and all the spire-bolts on the later bikes.)


HTH

SG

[RayBlack]

Thanks Simon. I guess I need look no further. Of course I'm not sure I am going to be able to reproduce that finish myself...
The online engineering dictionary appears to be a handy reference.
Ray