If the trigger is pinned through the plate, install just like you would the double. If it is pinned through the wood drill the pivot pin hole into the lock mortice, slide the trigger into place in its recess and use a small center puch to locate where the hole should be drilled in the trigger itself. Make certain that the distance from the trigger pivot is no less that 3/8" which gives a soft pull with a lot of creep, and no more that about 5/8" which gives a crisper, but heavier pull. I usually install the trigger before I try any lock tuning beacause they need to work together as a system.
