An update at last on this project, I have started a new thread as it has been a few weeks since it dried up, link to original below if a catch up is needed.

http://www.airgunbbs.com/showthread....rigger-problem

To recap I needed a piston and double sear unit for this project as the gun came with a botched piston rod and double sear.

The end notch on the rod had been removed and the front lobe(?) of the sear had been cut right down no doubt to become single sear but was the one that should have been left alone hence no latching as no surfaces mate up.

I searched the normal known parts outlets, and this forum for the parts and a sear showed up on here.
I had the chance to borrow a piston of a double sear type for measuring how to cut a new rod to fit to my piston.
The borrowed piston was from a shorter cylinder gun but the rod pattern was all we needed. Careful measurements taken to work out where the rod notches need to be for latching at the correct point of full cocking.

Then it turned out the new sear was also worn and filed such that it would not be able to latch on the new rod and the answer was to build up the profile on each sear with weld, re shape to match the rod notches and case hardened afterwards.

New pics show the parts & trial assembly

https://imgur.com/gallery/tsBo4R7

Next concerns are:
When setting the trigger guard screws nice and tight the trigger locks up but is Ok if they are not made tight??

It lacks power so original spring and leather washer in question. Power is such that it won't fire Ely Wasps but will fire (at low velocity) Milbro Caledonian.
The Wasps are tight through the loading tap (when trialled out on the bench) at 5.56 & the Caledonians are not so tight at 5.55, I think I recall wasps are good for BSA's but maybe not early guns? Especially if power is low.
So I have a working gun at this pint but will knock over feathers more than cans