xs850 pistons, rings... same as XS1100?

Seeing lowish compression on my 81 XS850, avg 90psi.

I'm looking at the service manual for both xs850 and xs1100...

XS1100H calls for piston 71.5x68.6 x qty4 #3H5-11631-00-46 (std),

XS850SH calls for piston 71.5x68.6 x qty3 #3J2-11631-00-46 (std).

Both diagrams call for the exact same wrist pin(2H7-11633-01-00), and clip(90468-12069-00)

How dissimilar are the 850 and 1100 pistons really? Can they be interchanged?

I called Wesco and they do not specifically offer a piston for the xs850, only the xs1100

I did some searching and couldn't find anything specifically discussing xs850/1100 piston crossover, except I talked to Scramble Cycles on the phone and they said the rings /pistons for the 850 and 1100 are the same.

Any thoughts?

Chacal, can you get xs850 pistons and rings if need be?

Side note, the rental gauge from oriley could be giving spurious readings (when hooked to my compressor the oriley gauge read about 10psi below what the compressor gauge reads)... Also my mistake, I tested compression with the throttle closed and carb intakes taped shut... prbably need to test again to verify.

I posted on other forums as well and will report back if i learn anything concrete