Getting it hot

[Oddbob08]

I'm going to have a go at making a new piston rod for a BSA Airsporter RB2, a slightly longer one.

I have a question, before it all goes horribly wrong. It will need some hardening, especially where it sets and is held by the sear.

I have stuff to make metal hot, water and oil are in plentiful supply, and I even have a bag of graphite powder. Or do I need something else?

Any help would be very much appreciated.

[ccdjg]

You cannot harden ordinary (mild) steel by heat treatment alone - it has to be carbon steel (usually called silver steel when you are buying rod). Then it can be hardened by taking it to a dull red heat and quenching in water. which gives you a glass-hard result, which is too brittle for a piston rod. It would need to be tempered to a hard but more flexible state by heating to about 300 oC and letting it cool naturally.

If your rod is ordinary mild steel, the only thing you can do is to produce a hard surface finish by case hardening. This would be the best way to go for a piston rod, as they are subjected to a lot of wear and tear where they contact the sear, but are not likely to be subjected go any bending forces. You will find plenty of recipes for case hardenig on the net.

Hope this helps,
Cheers
John

[Phil Russell]

Several years ago I bought a pot of stuff for case hardening ... can't remember the name but I think it was a whitish powder. You heated your (small) steel item up to ?red then dipped/dropped it in the powder ... sorry instructions are a bit vague but it was a long time ago. I used it to treat a new part I had made for a Haenel 28 and it seemed to work. It did not make it easier to cock though. But at least I had a working pistol. I guess a google search will find the powder ... it was not that expensive.
Cheers, Phil

[DrGunn]

Ive used a powder called Kasenit for case hardening in the past, it was a more dark grey than white though so may not be the one your thinking of.

[Phil Russell]

Ive used a powder called Kasenit for case hardening in the past, it was a more dark grey than white though so may not be the one your thinking of.

That's it ... remember the name. Memory on colour playing up. Thank you. It also came with instructions if I remember.

Cheers, Phil

[edbear2]

You cannot harden ordinary (mild) steel by heat treatment alone - it has to be carbon steel (usually called silver steel when you are buying rod). Then it can be hardened by taking it to a dull red heat and quenching in water. which gives you a glass-hard result, which is too brittle for a piston rod. It would need to be tempered to a hard but more flexible state by heating to about 300 oC and letting it cool naturally.

If your rod is ordinary mild steel, the only thing you can do is to produce a hard surface finish by case hardening. This would be the best way to go for a piston rod, as they are subjected to a lot of wear and tear where they contact the sear, but are not likely to be subjected go any bending forces. You will find plenty of recipes for case hardenig on the net.

Hope this helps,
Cheers
John

John has pretty much given the options avalable for most home / hobby makers of parts, but you have to be very careful with the silver steel as the tempering temperature numbers and type used can give a big difference in final hardness, even how you quench and in what changes things (you can water oil or mix quench, at school in metalwork in the late 1960's / early 70's we used agitated brine for example!)

The whole the rod then has the same properties. I personally never use the stuff, and am more a fan of other alloy steels (although Silver steel is an alloy containing chrome and manganese) , I think I used EN19 with a Nitriding treatment after some research on the couple of rods I have made, as silver steel if done wrong will snap like a carrot.

Have a read here of just a couple of forum threads of the confusion and various advice offered!

https://www.homemodelenginemachinist...r-steel.13127/

https://www.model-engineer.co.uk/for...s.asp?th=44147

I was also looking at maraging steels and EN39 after asking advice from a designer of high stress parts back then when messing around with an HW80 piston conversion I was doing.

IMHO rods ARE subject to some degree of lateral*** force as well, as the latch end when engaged in the sear is trying to "jump upwards" and disengage if you think about it, as notches are angled on many rods, hence the need for a material that has some degree of stiffness.(again IMHO).

(****If you see well used pistons, the rods can often be bent upwards slightly when rolled on glass, so forces are in play, it's not a straight pull on the rod.)

You can case harden low carbon steels in the "mild steel" spectrum, but really this is one componant I don't like to play about with, If you are looking for less swept volume, are there other options for example, ie. modified piston.

Another option is to do some research and see if there is a longer piston out there already on something else........I have also done this on older BSA rifles where luckily they made various length guns so you could use a long piston rod cut to the length you need by threading the piston end, and leaving the latch end as from the factory.

If you make one too soft, it will burr and wear and let go eventually, ........too hard and the sear engagement edge will start to crack, and it will let go eventually.

If you do more research you will find this subject comes up a lot so might be worth a Google to see what the tuners out there do that they have found to work successfully, but I cannot stress enough this is one part of an air rifle you do not mess with unless you are 100% confident with your work and materials.

Sorry it's a long post to read, but hopefully some help / ideas for you.

ATB, Ed

[cringe]

Re hardening & tempering, When I was an apprentice toolmaker [40+ years ago] we had a vat of oil in a lean to shed outside the workshop, this was used for hardening high carbon steels used in press tools. The reason it was kept outside because it stunk rancid! an older guy told me it was whale oil & had been there since before he was an apprentice! The method was heat the area of component to be hard to 'cherry red' approx' 700 degrees c? quench in oil [whilst holding your breath!]. polish the item to shiny then gently re heat from not hard end [there will be a spectrum of colours from purple to light straw], for the tooling that I made I would quench in water as a medium / light straw colour reached the part to be hard. I have done this since & old engine oil works. NOTE: try not to leave any sharp corners on recesses as this is where it will crack. hope this helps kind regards Al

[Oddbob08]

Thanks for all the replies, I knew it wasn't going to be straightforward....

The reason I need to try a longer rod is I think it has the wrong piston, but talking to someone at John Knibbs I was told BSA only used one type of piston in the RB2, and there are no spares available anyway.

The piston appears to be made from 3 pieces, and I can see no way to disassemble them to see if there is any way I can extend the rod far enough to latch onto the sear. A deeper piston head won't help here, when the piston is pushed back as far as it will go the rod is not even pushing on the sear. Having said that, it does need a deeper piston seal so that the piston can sit a bit further back when it's at rest, as the clearance between the cocking spur and the rear of the piston slot is very small and actually prevents the link between the cocking slide and the cocking lever being fitted.