Over the weekend I needed to change the anvil on a 2"-6" micrometer. As a matter of course I checked it against my (Chinese) gauge blocks. After having done this, on a whim, I checked it against the late father's incomplete collection of gauge blocks. Those blocks are mostly Matrix, with some S.E Johansen.
The 3" blocks sere different to the eye and finger and micrometer. Checking with a comparator showed the Matrix gauge to be a whole three tenths shorter than the Chinese gauge. A combination of one and two inch blocks showed four tenths shorter than the Chinese combination. The Chinese blocks agreed with each other. A .55" S.E.J block showed three tenths shorter than the equivalent Chinese block as well.
Until this last measurement I was having severe doubts about the Chinese set, the last measurement and some later ones seem to indicate that dad's ones may have re-lapped several times during calibration but not had the certificate with them when the old man got them. I still won't be certain until I've either scrounged the use of some Grade 0 blocks and a comparator at work or paid more than the blocks cost to get them calibrated.
I'm continuing to use the Chinese blocks because they agree with themselves and most of my collection of micrometer standards and zero-able mikes, but it's been a bit of a blow to the confidence.
The moral is, don't assume that old measuring kit is accurate, or new measuring kit for that matter...
The good news is that you _can_ work to 2 tenths on diameter with an ML7, if you are careful :-)
Mark Rand RTFM