Thanks to John and Jimmie for their help here.
I have a feeling that the cocking arm might come too far and touch the coils. It may be a case of getting a few thou taken of the face of the piston that the sear engages on, so the sear will engage a fraction earlier and the cocking arm will not have to pivot quite as far.
Any thoughts to this John and how close the arm comes to the coils of the spring?