Hi Rob, This is the interesting video / sound clip. There are 2 things. Does the noise actually disappear when you rev; the engine or is it an artifact of the audio automatic-level-control on the recording device.
It also doesn't seem to follow engine revs; in frequency. I would expect the noise to go higher in frequency with increased engine revs. To my ears it sounds a constant pitch. Wierd.
Even more so is when you re-start the engine after stopping it. Each time the noise doesn't start for a good 1/2 second after the starter has finished cranking. This odd noise is not there when the engine is cranking over ! Is it ?
So can you try something else please when you get a mo. Start with a well charged battery. Remove the spark plugs and disconnect the L.T. lead from the coil. ( Stops the sparking) . The idea is to get the engine turning over as fast as possible on the starter.
Now is the noise still there when cold ?
If not, I'm thinking of the starter cog just
being in mesh with the flywheel when the noise is there but taking a good 1/2 second to retract. It doesn't make a noise when turning over as the starter cog is fully meshed. As to why it occurs on 2 starters I don't know.
But it occurred to me the flywheel may be sitting further back due to worn thrust washers. The 1850's are not known for thrust washer wear, but could you make an estimate of the fore/ aft movement of the crankshaft please ?
Again I would have thought pressing the clutch would have modified the noise.
It would be really nice to have that starter out again and try starting it. Perhaps with a ratcheting rattle gun on the crank pulley ?