It might be Aluminium-bronze or similar, which does look more brassy than bronzed. A metal suited particularly to marine equipment.

Before you go anywhere near a lathe with this gear , suggest you undertake a spark test on it first in order to give you some indication of workability . Trevor

Try it with a magnet first. Obviously if it really is bronze or brass there won't be any suck, however this looks a bit like a couple of gears I have in a drawer. They were spare primary drive gears for some sort of two stroke motorcycle engine that I picked up at a surplus shop. Mine are steel, but as is sometimes done they had been copper plated. This is done selectively to allow nitriding only the surfaces that need to be hard. So the teeth would not have been plated, and would get the hardlayer, while the rest of the material would remain in its normal state. Which is still likely to be fairly tough. The plating typically seems to end up going black, but apparently is enough to stop the nitrogen reaching the steel underneath.

I've seen connecting rods treated the same way, so the eye where the rollers ran on the crankshaft would be hardened while the body of the rod would not.

John

The large bore compared to the OD means you would have to turn away the hub and fit something less bulky and with a smaller bore if you wanted it to look reasonable as a small flywheel but it could be put to that use. If not worried about looks then a simple bush to reduce the bore would work.

Leave the teeth on to one day drive something or be driven by a concealed electric motor for display. Put a bush in the hole, You are hardly going to be making a precision replica of some old engine with it. Leaving the gear itself unmolested means if necessary you can use it as a gear in something else if the planned engine doesn't work out or you find a more appropriate bit of scrap to make a replacement with.

I think for the work and electricity involved trying to modify a gear of unknown metal and machineability, you'd be far better off making the item from a blank of new, or at least known, mild-steel or cast-iron.

You could keep it in case you need a gear of that size etc., but there is a limit to "come-in-handy" collections, and as NDIY says, you'be be better measuring it and offering it to anyone who can use it.

Nice bit of stock for something, rotary table project springs to mind, nice lump for the worm wheel.