Engineering / repair guidance.

[rancidtom]

I suspect the original will be simply a medium to high carbon steel (tool steel or silver steel) heat treated after machining the profile of the sear and cutting the thread.
If you wish to attempt a repair with hard facing build up the worn areas with a hard facing alloy, I would suggest a nickel based alloy suitable for impact use rather than a super hard cutting tool re-facing alloy of cobalt/tungsten carbide. Heat treatment after welding would be advisable as you are only working on a small mass of parent metal and a very small mass of deposited metal. Cooling too quickly will inevitably affect the hardness of the facing material probably making it brittle, cooling at the correct rate will give a tough rather than brittle deposit with a good carbon/nickel dilution through the heat affected zone. Deloro 40 is a good example of such alloy.
If using oxy acetylene then simply building up with a mild steel filler rod but using a carburising flame is a reasonable method, build up, harden then temper to 230-250 deg C.
Another approach would be to case harden after building up with mild steel, the case hardening will give the hard wear resistant surface but you may need to re-finish the round section after case hardening.
If making a replacement I would try silver steel as although it is simply a high carbon steel with no fancy alloying elements it can be hardened and tempered quite easily, it is cheap to buy and easy to machine in the annealed state. I would harden then temper it to a non-brittle state, approx 230-250 deg C.

[stevegv8]

Thanks for the above, if building up with weld I had only the option of mig repair.

It seems I have a side button Improved model D of 1913 -17.

I have learned something today however, on page 68-71 of Knibbs book BSA & Lincoln Jefferies it shows a trigger block patent No 30338 which is a double sear safety.
I have removed the trigger mechanism and it is exactly this but with one sear removed and partly explains why the rod has one notch not two but not why any of this has taken place of course. Obtaining a replacement of that and a correct piston rod isn't worth even attempting plus the cost, when the gun has no original colour to the overall steel, is not viable.
So plan A being of some kind of repair is the only course of action in this instance which will achieve a working gun (hopefully) but a fairly rare model spoilt.

And what I learned today? was the pitfalls of buying something you don't know enough about but this is exactly how you learn afterall.

Now unless anyone has the parts and cheap that I need to correct it (may do a separate parts plea thread) I will consider continuing with the bodge repair.
Further advice still very welcome.

[NickG]

pound to a pinch of sh1t it will be case hardened mild steel, can you put a picture up as I may be able to help with making one .

[stevegv8]

pound to a pinch of sh1t it will be case hardened mild steel, can you put a picture up as I may be able to help with making one .

Very good of you to offer, thanks.
I do have a local guy who can make this for me if that's how it goes, the problem will be correctly making it as two notches for two sears with no pattern to copy and that's if I can correct the botched trigger.
Making one to suit the trigger now that it's single sear is more likely where this is going, still guess work but easier no doubt.
Just posting a WTB for the correct parts just in case there's something out there, as refitting the correct parts would be easiest.

[max headroom]

its just basic steel. the spring is blued.