pin gage diameter tolerance

[40-82 hiker]

I want to order a few pin gages to measure chamber throat diameters in revolvers. They are not very expensive, and I only need a few.

Do I want to order positive tolerance (+.0002,.0000), or negative tolerance (.0000,-.0002) pin gages.

I feel like I am over thinking this, but I am in uncharted territory. Ouch!

Thanks.

[M-Tecs]

Either one will work for you. Most use the minus tolerance (.0000,-.0002) pins. The tolerance selection is mostly to ensure the pin tolerance doesn't allow a bad part. Example it a hole with 1.000" max and a .999" min. You would select a minus pin, however, for the lower end of the tolerance you would select a plus pin.

http://www.aajansson.com/pdf/understanding-fixed-limit-gages.pdf

[40-82 hiker]

Thank you. That was very good information, and I seem to understand the points of both articles.

[Wheelguns 1961]

You want the minus gauges

[uscra112]

I order Grade Z. For gun work this is tight enough tolerances.

Order a range of sizes. Buy both + and - around your nominal, and a few sizes bigger and smaller. Otherwise all you have is a go/no-go gage, which doesn't tell you much. The pins are cheap enough.

My set for .38/357 cylinder throats.
.3560 +
.3565 -
.3565 +
.3570 -
.3570 +
.3575 -
.3575 +
.3580 -
.3580 +
.3585 -
.3585 +
.3590 -

[contender1]

While it seems expensive at first,, buy a complete set of the pins. I have a set, .250-.500 that I gave less than $80 for. (It's a minus set.) And when I got mine,, I didn't THINK I'd need more than a "few" for my guns. Looking in the box I now have about 1/3 of them unwrapped & used for different projects. If I had bought them individually,, it would have cost me more.

[EDG]

A pin will not fit in an exact same size hole except as a press fitted pin.
Usually a set of minus pins is used to gage a hole diameter with the -.0002 giving you room to get the pin in the exact size hole.
The current Chinese pins sets offered at low prices are good enough for gun stuff.
When you get older the most important feature is nice dark high contrast marking so they are easy to read without using a mike.

[DougGuy]

Definitely minus .0002" and DO get the half thou increments.

[M-Tecs]

One more point on the tolerancing. Normally manufactures tend to shoot for the middle of the tolerance. With gage pins they tend to be very close to the plus or minus tolerance. A .0002 plus will be very close to .0002" over and a .0002" under will be very close to .0002" under. Not sure if this is true with the cheap Chinese stuff but it applies to better quality pins. This is important if you are using pins to calibrate or using the tolerances for your benefit. Example would be a plus and minus .358" pin set. One will be very close to .3578" and the other will be very close to .3582"

[Gtek]

Used as go/no-go's and it makes me feel better when I think I have it lay it in a real micrometer for just cause. That old measure twice thing I guess stuck!