I am looking to buy a new set of rings for this '86 XJ6 engine rebuild. I'm planning to reuse the stock pistons. I notice there are "deep" and "shallow" oil ring groove sets. Is there an easy way to tell which set I need? The difference in measurement seems so small! Deep is defined as "0.190 plus" and shallow is "0.170 or less".

Thanks!

do you have digital verniers?

I do. I will make an attempt at measuring this. I was thinking that perhaps there is a simple answer though (e.g. certain years or engine numbers, etc.) which would guard against me making an imprecise measurement, if the difference between them is close.

So, I'm not 100% sure I'm measuring this correctly, but here's what I did:
- Using the digital verniers measured diameter of the piston from inside the oil ring groove, at the bottom, and zeroed it here
- Measured piston diameter near the groove but outside of it
- Difference was 0.355, divided by two is 0.175".

Great. Neither "0.170 or less" nor "0.190 or more" !

Is my method flawed? Is the "shallow" one the right one because it's closest?

This issue is moot now. I thought this engine was completely stock, but after measuring the pistons themselves they just seemed too big, I cleaning them up and sure enough they are 0.030" oversize - this block was bored at some point in the past! That explains why the internals look to be in such great shape despite 150k miles. I feel kind of dumb now not to have realized this sooner. Anyway the pistons are AE 21380 so that identified the right ring kit. I ended up buying new bearings, gasket sets, timing chains/guides, rings, etc. with Moss Motors. Hope to get started putting this thing back together this weekend!

good to hear you've established what the motor is all about....you'll have to leave the word > dumb < out of your vocab/ you did everything that would be done in a workshop.......good luck