Aye Steve,
Apologies, late for the update. Lion equinox weather on the Inner Hebrides and in Perthshire today. Gales, snow, marble hail, down to petit pois size, along with warm sun and horizontal rain! Be canny if considering the West Coast/Hebrides tour this year, Calmac only have 3 of their 11 major ferries operating as of this weekend, due to worn out old ships, lack of new ships, breakdowns and planned service! Bl**dy SNP and the quango that are supposed to order new boats couldn't organise a p...up in a brewery! Very sad.
Your piston clearances correspond with those given in the Yamaha SRX600 workshop manual.
Piston OD should be taken at 90° to the gudgeon pin, 5mm above the bottom of the piston skirt.
Top ring end gap (installed): 0.30 - 0.45 mm (0.012" - 0.018")
2nd ring " " " : 0.30 - 0.45 mm (0.012" - 0.018")
Oil ring " " " : 0.20 - 0.70 mm (0.008" - 0.028")
Wear limit top ring end gap: 0.60 mm (0.024")
" " 2nd " " " : 0.60 mm (0.024")
Cylinder bore should be measured 40 mm (1.57 ") from the top land of the liner.
Cylinder bore "Out of round"/Ovality limit is 0.05 mm (0.002"), between the side to side and the front to back dimensions.
If you require the piston ring Xsection sketches, along with their dimensions, side clearances (installed) and wear limits, ASK!
I also see that the majority of the times that I try to log into SteveH website SRX600.net, I receive a 503 Error, which appears inconsistently!

This too has the dimensions that I have given, but nothing of detail for the 400 engine. I would suggest that the XT400 engine dimensions would be the same as your twin shock SRX400 engine, if you can find them.
I suspect that SteveH's site provider is taking the mickey, or else the old programme, with which SteveH created the site is starting to go flaky, so will get in touch, with him.
Good health and "Slainte!", Bill