I first degrease nuts and bolts, and any steel parts that need some rust-proofing, then a fine grit or glass bead blasting. Bolts are put in wood blocks to protect the thread. Then a 5 to 10% phosphoric acid bath, till they are dark grey, dry and either apply a clear rattle can varnish (for automotive use) or WD40. Not as good as professionally applied Parkerizing, but sufficient for me
email (option): viaconsu [at] planet [dot] nl
The only trouble I've found with the Birchwood Casey Perma Blue is that it also blacks your fingers very nicley too! Just be careful or you'll have black hands for about three days. Also, remember to shake the tube well before each coating as it tends to settle and separate and you can end up with a streaky finish if you've not given it a damn good rattle first. 
You can also do oil blacking which is very easy to do. I don't know how permanent it is but the finish is exactly the same as the original finish on my Norton.
I wrapped copper wire around the bolt and heated it up with a blow lamp until red hot. Then quench it in a tub of old engine oil. Lots of smoke and flame so do it outside. As the bolt comes out of the oil I keep the lamp on it and burn off the oil. It looks really good and identical to the original parts, some of which I left as they were.
I took the grease off the new old stock head nuts, only to find they were bare metal. That is not paint on them but oil blacking.
email (option): horror@blueyonder.co.uk
Yes, old fashioned oil blackening is another option and it's cheaper and it's great fun into the bargain! The first time I did it, it burst into flames and I nearly burned the shed down, needless to say, I now do it out on the back lawn when the wife is out 
I oil blued the ends of the fork spindles of my Enfield WD/L by heating till red and plunging in the old engine oil from our 7 1/2 ton tipper which was black black! Repeated three times for a lovely dark colour. Ron
email (option): ronpier@talk21.com
No Dave I posted after you. But just confirmed with a picture. Ron
email (option): ronpier@talk21.com
The M20 didn't have all that many 'blacked' fasteners. Remember this was a cheap and very temporary treatment and you can expect it to rust fairly rapidly once the bike is in use...and very rapidly if you use it on salted roads in the Winter or early Spring.
Personally I wouldn't use it unless I were embarked on a 'concours' restoration or the bike was going to be unused in a collection...but then I don't like rusty fasteners...Ian
email (option): ian@wright52.plus.com
Some of the parts I oil blacked I'd already sent to the platers to be dull chromed. I only realised they should be black after.
But the head nuts etc were bare metal and missed being sent off. When I looked at the WD motorcycle training film it showed the Norton with black head nuts, so black it was. If it starts to rust I'll spray them.
email (option): horror@blueyonder.co.uk
I've got one of these home plating kits for nickle. It's terrific for small parts like nuts, bolts, levers and so on, well worth the money, but I've never had much success with anything bigger than that. I never seem to get an consistant finnish on big things, even with new nickle and fresh salts. The most annoying thing I attempted was the nickle plated hubs on my WW1 Douglas. They looked great at the time, but after just 8 months sat in a dry garage, the nickle has started to peel away, so its now either paint them black or tear both wheels to bits again and get them professionally nickled? 
Quite...you can't just do what you feel like to high carbon or heat treated steels without some consequences. The exact results will depend on the original state (at a molecular level) of the material, the temperature applied and the oil type and oil temperature that the part is quenched in. With unknown material and treatment specifications and an uncontrolled heating/ quenching process it's very much a game of 'russian roulette'..I would be more inclined to use gun black to achieve the finish which does not carry the same potential risks... ...Ian
email (option): ian@wright52.plus.com
There isn't any high strength (low alloy) steel on an M20 as it was not developed until the 1960s..However, this term should not he confused with high carbon steels which have a higher strength than mild steel but are completely different to HS steels.... What there is on an M20 is mild steel and high carbon steel.
Mild steel cannot be heat treated to alter characteristics such as hardness for example and subjecting such steels to 'blacking' by heating and quenching will have no effect but to alter the colour.
High carbon steels (which have varying amounts of carbon and other elements) can be subjected to a wide variety of heat treatment processes to alter the characteristics of the steel for different applications. Surface hardness, yield strength, ductility etc. can all be altered in this way.
High carbon steels can found in abundance on an M20..the example of the clutch pushrod being just one example. Many engine components and cycle parts are manufactured from heat treated high carbon steels...in fact anything you can find on the bike that has been hardened and other components where the effects might not be so obvious..for example springs, or where the material has been treated to improve surface wear characteristics or tensile strength etc. (as opposed to hardness).
Any high carbon steel if heated and rapidly quenched will become harder and more brittle and if a tempering process is not carried out will remain in that state...If heated and cooled slowly the reverse is likely to be the case...it will get softer and weaker.
Regarding hydrogen embrittlement during plating and other processes high carbon steels are particularly susceptible to this condition...thus the snapping cylinder head bolts I mentioned in my previous post, and such components should be treated after plating to reverse the process..
I'll say it again...if you are unsure whether a part is mild steel or high carbon steel it would be best to err on the side of safety and use a blacking solution such as those used for guns or a professionally applied chemical blacking process suited to the material rather than applying an uncontrolled quenching process which is likely to either seriously weaken such components or change their characteristics from those intended by the manufacturer..Ian
email (option): ian@wright52.plus.com
I should have been more specific and said mild steel cannot be hardened by heating and quenching processes..It can have it's surface hardness altered by the introduction of additional carbon at the surface...carburising..which is a heat treatment process but not based on heating and quenching to harden and temper the material... .Ian
email (option): ian@wright52.plus.com
Exactly my point John and I think your caution is sensible...particularly as in the context of this discussion there are other ways of achieving the same visual effect (blacking) that don't carry the same risks...Ian
email (option): ian@wright52.plus.com
But the head nuts etc were bare metal and missed being sent off. When I looked at the WD motorcycle training film it showed the Norton with black head nuts, so black it was. If it starts to rust I'll spray them.
Hi Horror
I think it depended on the year, I had a lot of NOS Norton head nuts (all gone now) which were plated.
Rob.
email (option): robmiller11@yahoo.co.uk