6" flywheel is about right for a 1×2 cylinder assuming it is spoked, just look at all the Stuarts that use that cylinder size and they all have 7" flywheels.

I've enlarged and reduced quite a few engines in my time and every part gets enlarges or reduced by the same amount, not had one that did not run well. Only time were a reduction of mass may be worth doing is where you want the engine to pick up or loose speed quickly but that will be at the cost to smooth slow speed running.

J

PS New capacity will be 1.57 cu in or if double acting 3.14cu in

The difference is that as engines get larger, they generally run slower so they need a proportionately heavier flywheel.

Your double size engine probably should not run as fast as the little one.

Care needs to be taken not to dangerously overspeed flywheels on mid-sized engines. If speed is what's needed a small diameter solid flywheel is safest.

Neil

You could always make a solid flywheel, relieve the portion between the rim & the hub then drill a series of holes to make it look more like a spoked wheel. Easy enough to do on the lathe & mill.

regards

If you need to lighten a flywheel the more you take off the outer diameter the more the effect.

When I couldn't turn the 7" flywheel for my Stuart Beam Engine in the Myford Speed 10 I had at the time, Reeves sold me the flywheel from a 'Lady Stephanie', which is about 5 1/2" diameter.

I added a thick brass rim to bring it up to 6", plus a broad and heavy brass pulley wheel on the far end of the shaft.

This runs a bit faster than I'd like, but works quite well.

Thinking in the opposite direction (as usual) if the proportions of the model were taken from a typical full size original (not an unreasonable assumption) and both the big one and the little one worked then any scale in between should work too. In practice ,in my limited expirience, rim thicknesses tend to be proportionately thinner on full size engines (particularly noticeable on traction engines). I suspect actually they have been thickened up on models to achive smother running. So proportionate flywheel and thin the rim a little so it it looks right if you really want too.

regards Martin