This item is the barrel shaped spring that was fitted to the Douglas 2 3/4hp exhaust lifter assembly, where it feeds into the plunger bolt. We also sell the plunger bolt as a kit (Item 0975) - but this listing is for spring only
An original exhaust lifter plunger box and spring was used as the template for manufacturing these (these were fitted to a spare 1922'ish Douglas engine I have) and was very useful as all fittings in this engine looked original. The dimensions of the plunger box bolt have been copied exactly from the original, including the correct thread to allow the unit to be screwed into the crankcase. The plunger bolt has an internal 1/4" x 26 tpi thread, to allow a cable with standard cable adjuster and locknut - which is the primary means of adjusting the exhaust lifter assembly tension when not acting against the cam followers.
It is worth noting that the original Douglas plunger unit bolt that screwed into my spare engine crankcase did not have any locknut, and neither does the original Spare Parts List show a locknut - instead it just screwing into the crankcase timing chest, until the correct position attained. The bore of the plunger box bolt holds the spring, but then the original exhaust lifter mechanism inside the timing case has a plunger (which the exhaust lifter cable screws into, with a special theaded ferrule) which also fits into the bored plunger body - the mechanism then all being held in line inside the plunger body.
I noticed that on the original Douglas plunger bolt fitted to this spare engine, that the plunger body - being very thin bore by design, was completely worn through on one side, with the spring visible. Although this wear would be inevitable after heavy use, and would still allow operation unless a large amount of the 'tube' bore had worn away, it may be worth checking your original plunger body if you have this model, to check how badly worn your original is.
The plunger box bolt is CNC manufactured in stainless steel and had the correct thread and Imperial hex spanner size as the original item. The spring is included and UK manufactured by my main spring manufacturer to an original Douglas pattern