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RENO REDD
12-12-2001, 09:05 AM
WANTED A SET OF PLUG GAUGES TO MEASURE BARREL & THROAT DIA. FOR GUNSMITH'N.
.251 -.500 RANGE. WENT TO MY FRIENDLY
CATOLOG & THEY HAVE TWO SETS IN THAT RANGE
.251-.500 "PLUS" & .251-.500 "MINUS". I
HAVE NO IDEA WHAT THE PLUS AND/OR MINUS
MEANS. THE PERSON ON THE PHONE WASN'T ANY
HELP. ANY HELP WOULD BE APPRECIATED. THANKS, KEITH

kap pullen
12-12-2001, 09:33 AM
Usually means the manufactorimg tolerance on tne pins is .0000/+.0002,or .0000/-.0002 on diameter.
Thats not significant for home shop guys but may be for a gunsmith.
My 6.5 sweed mausers are marked in tenths
by bore diameter at the factory or armory.
kapullen

C. Tate
12-12-2001, 09:11 PM
You should see that gauge pins are made in classes. I don't remember off the top of my head how the class system works but some are made to closer tolerances than others for different applications. Maybe some can post the class information my handbook is at the shop.

They reason for plus and minus is simple. The pins are made so that they do not go over(minus) or under (plus) the nominal size. I have a hole that has to be between .437 and .438. I use a plus pin on the the .437 go side and a minus pin for the .438 no go side.(They are mounted in a handle one sticking from each end). This way I know that if one goes in and the other does not I have a good part.

You can make your own if necessary all you need is a good set of mics and work carefully. Lets say you were to make one for my .437/.438 hole. You could turn the pin .4371 dia and then put a .4379 dia behind it. If the pin goes in the hole up to the second step but not past you have a good part.

kap pullen
12-12-2001, 10:20 PM
Had a job last week with pin holes toleranced .1872/.1874. I thought how the heck they gona check them?
They fooled me. Inspection had pins tapered .0005 in a 1" length. The .187/.1875 went in half way and that was it( after I laped .0001 out).
They think of everything.

Thrud
12-13-2001, 11:09 PM
Ypu might try www.clymertool.com (http://www.clymertool.com) they make some of the best chamber reamers available and can most likely supply what you need.

Pin gauges are available to grade "X, Y, Z", I believe that "X" is the highest grade available - KBC and Travers sell them.

Dave

spope14
12-14-2001, 08:05 PM
Check plug tolerances in the Machinery's Handbook, or via the MSC Catalog on the plug gauge ordering page. However, class XX is the tightest gauge pin tolerance with .00002 tol for .029 to .825 sizes, x is .00004, y .00007, Z .0001, ZZ .0002. Plus sizes show this tolerance used on the high side, minus show this tolerance used on the low side. Remember this is tolerance, so the pin may be more accurate than the stated tolerance.

Here is how I try to make plus minus selections. For determining the go pin I like a plus size, gives me that feeling that is the go does fit, the hole / slot size is not possibly below the actual lowest limit and is correct. I use minus sizes for the no go to help assure me that there is no possible oversize.

I have both + and minus in the school shop I am in, but class X and Y fits. Call my method "psyhological protection". Guarantees, probably not, but hey, I feel better