My inability to measure

I don't get why I fail at measuring. I do find it a bit tricky holding the bore gauge steady in the micrometer, but I persevere until it's zeroed. Yet, my measurements are always wrong. Just all over the place. I always end up Plasti-gaging it which is consistent and as expected every time.

So what am I doing wrong? No good linking some video, I have watched them all, and follow what they do exactly.

There is definitely a feel associated with using a micrometer. I have found it helpful when starting a measuring session, to repeat the first measurement multiple times, and continue to repeat until I can get two or three consistent measurements (+/- the last digit) in a row.

Now move onto the other measurements, and try to use the same feel until repeating more that 2 times is a waste of time, you are able to get the measurement right on the first one, and confirm with the second.

Do your micrometers have clutches on the wind in?

Are you avoiding expanding components with body heat or other sources?

Try to avoid "muscling" things. May be worth trying different handing/observational angles while measuring or even getting someone to watch you.

Clutches? Do you mean the ratchet? Yes. My bore gauge and my piston size micrometer are Wright & Moore and my journal size micrometer is Mitutoyo. Those should all be ok.

I have just tried again. Really, really careful. When I measure my rod journals, did one yesterday and another today, I get 2.0095" on both. Left, right, middle and at 0 and 90 degrees. All twelve measurements 2.0095". Just looked up the crank (Scat 383 Stroker) and it's listed as 2.1". So it's close, but definitely consistent.

I also tried holding the bore gauge horizontally when zeroing, this time, getting down to its level. It was definitely easier. But when I then measure the rod inside bore, I get 2.0123" (yesterday) and 2.0126" (today). That is 0.0031" clearance (today). That's too much. It's even bigger going at an angle.

I then Plasti-gage it in two places, opposite the rod and at about 60deg off that. Did this both today and yesterday and three Plasti-gage measurements were 0.002" and one was 0.0017". Both consistent and correct.

I just don't get where it goes wrong. I can only assume it is when zeroing the bore gauge but I'm a bit blank.

Funny thing is, I have meticulously noted down measurements during this process, and my bore gauge measurements are consistently around 0.0015" too big, so whatever I do wrong, I'm very precise at it :-D

Have you calibrated your gauges with a standard? Micrometers will usually include a standard that will be marked with it's size, you should measure that exactly, or have a way to calibrate the tool to make that the correct reading.

For a bore gauge, the standard is called a ring gauge. Something like this:

https://www.judgetool.com/products/mitutoyo-setting-rings-for-bore-gages/

As David said, and something else to be aware of is that it doesn't need much temperature variation to affect the measurements as the tool and/or test piece is measured. Avoid leaving parts or tools in the sunlight, and most will have a ceramic piece where you're supposed to hold them, so the heat of one's hand doesn't warm the tool.

I have calibrated the micrometer using the standard, yes, however, I don't need the micrometer to actually measure anything, it's just a reference to set the bore gauge by.

And bore gauges only measure a delta, not an absolute value. Those rings are for zeroing to a specific value, that won't help me.

These tools are on the same shelf in my garage, or on the work bench next to it. The garage is on the north facing part of the house (attached) so get very little sun light. The only sun light would come through the door at the end and hit the opposite wall, away from my workbench.

I do have to touch the measuring end of the bore gauge to hold it inside the micrometer jaw. Not sure how else to do it (unless I by a stack of those extremely expensive zeroing rings) and people on Youtube do the same. I don't think I excessively hold the micrometer either but maybe it's something to think about.

Not really sure what to say (or do). Maybe I do need to video it an post it here and you can critique. I don't know if my micrometer reading of 2.0095" instead of 2.01" but it still doesn't account for my readings being 0.0015" out.

I tried again yesterday, carefully. Same result.

This time, I back-tracked, so the locked micrometer used to zero the bore gauge, I took back to the journal and it was correct.

I also went back and forth between the rod bearing and micrometer and it was correct.

I don't get it. I thought for a minute it might be due to the rod not being on the journal also the measurements for straight up are different to at a 60 degree angle where Plasti-gage measures the same. I've just never heard anyone mention that as a problem.