This item is the distinctive long cylindrical exhaust lifter assembly plunger box, that screws into the front of the Douglas 2 3/4hp models and retains the screw in Exhaust Lifter cable adjuster and cable. Inside the cylinder is the spring that holds the exhaust valve lifter assembly in tension. This item listing is for the plunger box bolt (CNC manufactured in stainless steel) and spring.
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 also replicated from the original Douglas spring dimensions - this being of a stronger gauge than most springs this size and barreled at both ends - which makes a difference when fitted, the exhaust lever on the handlebars requires a nice pressure to operate. We have had a batch of these springs made by our trusted UK spring manufacturer and they are also available seperately (Item 0992)
Finally, for anyone restoring one of these models, and not having the full assembly to refer to - I have included a few of the photos of how the Exhaust Lifter cable and adjuster mechanism was assembled, including a picture of the distinctive bent shaft with barrel shape at one end, that is designed to slide inside the sleeve bolt, acting against the spring inside it. The other end of this rod has a sideways pin, that attachs to the exhaust lifter lever assembly in the timing case. The sideways pin on this special rod initially looks like it is too long to allow the rod to be fed through the threaded hole of the timing case (once it has been assembled onto the exhaust lifter cable and assembly - but if you look carefully in one of the forward facing photos of the crankcase, you will notice there is small notch placed in the Douglas exhaust lifter hole in the crankcase for this purpose.
At the 'barrel' end of the rod is hole that is threaded internally (with a 3/16" BSW thread I believe), so it can accept a lightweight cable, which has an unusual threaded nipple. As the photographs show, I manufactured one of these from a small 3/16" BSW bolt, drilling it approximately 55 thou so the lightweight cable inner could be fed through the threaded area and belled out at the end and soldered. I hope this may help for anyone restoring or overhauling this assembly in the future.
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