750 piston direction

From the photo I can't see a lot of difference in the cutouts to be honest. But yes, the inlet cutouts are usually deeper and wider and the exhaust cutout is usually closer to the edge of the piston.

Still, it doesn't look like anything's been hitting, so you could leave it as is. Maybe check the clearance with some plasticine while it's apart, and try the pistons the other way round to see which is best.

What I do see is a mark that looks like an L on the left-hand piston. And it looks a lot like the JCC pistons here:https://www.classicbritishspares.com/products/norton-commando-750-pistons

Original Hepolite 750 pistons were stamped with EX next to the exhaust cutout.

Surely the inlet cut-aways should be in line with the inlet valves?

I believe that your pistons are reversed....as you suspect.
You will notice that one valve cutaway is next to the edge of the piston.
This is the exhaust side of the piston.
The other slightly larger piston cutaway has a small area of metal between it and the piston edge.
This corresponds with and should match the large crescent of metal inside the cylinder head on the inlet side.
Both being positioned at the inlet valve side of the piston and head.
See photo below.

If the have been happy where they are and no sign of valves hitting pistons, wouldn't it be safest to leave them where they have already run in happily?  You won't be disturbing circlips either.  Last time I did that I tried to follow the book and fit new ones...but failed to find replacements anywhere...

Quick job to remove. Good chance to clean the carbon off and check the ring gaps too

Eddie

The L and R stamps on the crowns show that the pistons are fitted on the wrong sides.

 Last time I did that I tried to follow the book and fit new ones...but failed to find replacements anywhere...Said David Cooper.
I too have tripped over this issue, it turns out the issue is these old engines are IMPERIAL and most of the circlips you find are Metric!!
Now as with most of our nuts, bolts and washers once you define one dimension the other dimensions all 'line up' due to standardisation, the same happens with circlips. Once you define the 'bore' it goes into then the rest of the numbers line up. Happy hunting.

AKA snap rings, perhaps someone like Springmasters of Redditch might be able to help?  

This may help:-
https://www.youtube.com/watch?v=fydQTylBUjA

Another couple of observations on Ashley's YouTube.  I found two different diameters for the wire type clips.  The spares I had (and ones I bought on line) were different from the ones I took out.

And he advises (with the 850 where valves don't interfere with pistons) to always replace them where they cam from.

Also in respect of a, b, c  and d grade pistons.  In the old days before modern NC machines with feedback systems ensure exact sizes every time, factory engines were bored to a specified size at the upper end of the tolerance range.  Then, as the boring machine was used, its cutters would slowly wear and not cut quite such a big hole.  So the production bore size would steadily get smaller until it went outside specification and the cutter was refurbished,  So 'standard bore' cylinders were graded in the works as a, b, c and d, according to the actual hole size.  And pistons were selected and fitted to suit.  So there might be several sizes of 'standard' pistons.  (I'm fairly sure BL cars had 4 sizes: a,b,c,d)

On re-boring, the machine shop would always aim for the correct size for each setup, since they wouldn't be mass producing over size bores.  So oversize pistons each came in only the one size. And of course if we sleeve back, the same applies (which is what Ashley says).

Hi Adam, OK I'll ask, what have you found? Terry.

Hi Adam, sorry to hear about your further issues with the Commando, media blasting is a great way of cleaning components but a lot of care must be taken both before and after, I personally would not blast an oil tank, to difficult to clean properly beforehand. I think you should add main bearings to your list of new parts. Best of luck (think you're owed some), Terry.

Blasting grit always gets into an oil reservoir and you never get it all cleaned out.

That's my point of view anyway.

Every time i hear the term 'Just restored' i inwardly groan. It very often means that the thing has been thrown together using whatever was available, usually by incompetant people.
Golden rule 'Just restored' means walk away...Fast!