Further note to above, service shots 1939 tell me that t slot pistons have a fully floating wrist pin, which should be able to be pushed in by hand at room temp, so I stand corrected.

Also for clarification, a little end bushing reamed at 0.001" Will  soon bed in to be about 0.0015". A honed 0.001" will stay pretty much at that. This is a good clearance for someone running a bike in very carefully. Racing clearances for bikes run hard would be 0.0015- 0.0025". Bonneville chiefs are recommended 0.003"

The only thing that turns the same speed as the crank is the crank itself, and very little else apart from compression at every other revolution, is resisting at TDC and BDC other than the valves open, so either the crank is crooked or some valves is sticking.

Before major disassembly, first I'd remove the plugs to feel if the engine turns heavy still, a little oil down the cylinders is good. Is the head gaskets overhanging in the cylinders and pistons touch? The engine should be easy to turn with just a hand on the kicker, but with just some resistance from the valve springs is normal. Then I'd open the valve lash maximum, turn down the screws maximum and check again if the engine turns heavy at the same piston position.

Then just loosen the cam cover screws a couple of turns to check if that free up the engine, before further disassembly. Is the trouble found behind either the cam cover, clutch cover or in the cylinders?

I'd check pinion gear lash and maybe drive gear lash to eliminate the possibility of a crooked crank or (new) off center ground pinion gear, or perhaps a too tall woodruff key that makes the pinion gear or the drive gear crooked or oval? The magneto gear lash must not be tight, a faint click-clack must be heard from the gears in any position or it will wear the mag bearings fast..

Then remove the cams to check if the resistance is because some of the valves are bent...

After that it is remove the cylinders to check if the rods are bent, with tight pin bushings or too long bushings a bent rod is more visible. Check if cylinders are straight and square.. A rough indication can be done just with the heads removed and check that the piston sideplay is equal at TDC, mid stroke and BDC.

Whatever you or anybody else do, don't force the engine try to start it despite turning heavy or making strange noise, it will cost more than it is worth in the end.. This is far from the worst that I've seen on youtube...https://www.youtube.com/watch?v=hcpU0ucwtuk

Well it just seems like it is a slightly tight engine. All parts seem to rotate as required and nothing on its own is tight. I was making the elementary mistake of turning the engine over with the rear wheel in first gear, when that should have been top gear. !!

Now back to completing the sidecar parts and am working on the body. So far have taken nearly 2 kg. of paint and filler off it !! I have decided to get a replica body made by our local metal shaper as the work involved in the old body would cost much more to fix than making new. If anyone is interested in a new sidecar body, then it may well be easier for my man to make two in one go. The body I am using for the pattern is an early Princess, around 1927 or so I believe.