Combustion engine injector

Another idea which someone may have time and resources to try out before I can:
One of the barriers to building an injector is the problem of having a plunger which fits perfectly inside the barrel. This is difficult to achieve without a lathe. However, a single-piston combustion engine such as a motorbike engine has both a barrel and a plunger perfectly milled to incredible precision. Maybe we can take the spark plug out and put a nozzle instead? A heating element  could be wrapped around the engine block, between the cooling fins, and the cylinder filled with plastic, melted and plunged with the piston.
Maybe I’ll get to try this, maybe not.

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Good thought. You would want to find a small bore, long stroke to keep the injection force reasonable.
I think the injector piston may not need to be that precise. If you can cut an aluminum disk slightly oversized with a hole saw and then attach it to a rod, and chuck it up in a drill press, you can file or sand it to fit. Mine is about 40mm diameter and 10mm thick, pushed by a 12mm rod.
Things like HDPE are fairly viscous so the fit does not need to be that tight.