measuring bore for determining reamer floating pilot diameter

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gunlaker

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Hi,

What is the preferred method for measuring bore diameter? I've used chamber casts and a micrometer for measuring groove diameters, but haven't ever measured a bore diameter, other than approximately, by using a chamber cast and a dial caliper. Obviously that can be improved upon. Are pin gauges the answer?

Also, when ordering a floating pilot, how is this usually sized relative to the measured bore diameter? i.e. if the bore diameter was 0.3680" what size pilot would you order?

For reference the project is a .38 cal black powder match rifle and chamber tolerances want to be as close as practically possible.

thanks,

Chris.
 
Pin gages if you can get them,would be pretty accurate. I know Harbor Freight in the States, sells pin gage sets that are Chinese but cheap enough to use for the purpose. Check the size range of the set, and the size of the increments.

Got a micrometer? Able to read one accurately to tenths of a thousandth? Make a fresh cast and follow the instructions WRT the change of size of the alloy, over time.

An expanding ball gage, as close as can be got to the size of the bore, so it does not ride between the lands too much, plus a decent micrometer will work too. Having an accurate reference to measure against, say, a .375 pin gage or a similar sized gage block, will give you a better idea of your measuring accuracy.

Asked the barrel maker what size it is supposed to be?

Order the size under the supposed bore size, and one on it, to be sure.
Or, make your own, if you have the capability.

Cheers
Trev
 
I've got a standard micrometer that is good to .0001", as well as a tubing micrometer but neither will allow me to measure the bore diameter on my chamber casting as the grooves get in the way. I'm reasonably confident of my measuring abilities, although I'm certainly not a machinist.

I do have a couple of reamer pilots from reamers I've ordered in the past. I've generally gone with the barrels published numbers an gone 0.0005" under but I'd like to measure it to be certain. I've found that bore diameters can vary by a thousandth or so from the published values. ( I know this as I shoot a lot of bore diameter paper patched bullets which usually work best when they are a very light friction fit in the bore. I have a number of Badger barrels with published specs of .450" bore diameter and it's very clear that there is a difference between he bore diameters, although the grooves seem to have a tighter tolerance ).

Maybe I'll see if I can dig up some pin gauges.

Chris.
 
I think pin gage would be my first choice. I've got my set from Shars.com; reasonable prices and shipping charges...

In case you haven't used them before, they come in Plus or Minus sets; I usually measure od of the pin gage that fits, rather then going by the listed diameter.
 
Pin gauges work but if you indicate the bore at both ends then you need to check both ends as they are often different. If you figure out what size you think you need buy one larger and one smaller to be sure. If I need a bushing that I don't have I use gauge pins and then make the bushing in brass if I don't want to wait to order one in.

Forgot to add, Shars is a good source and CDCO is also good.
 
I have made up a series of tapered plug or pin guages for my own use. Just a 6 or 8" length of material that goes up in increments of .002" in segments about 3/8 - 1/2" lengths. I try to make them to measure a range of around .015 - .020" over their total length. I find them useful in measuring the bore diameter at the muzzle particularly for british barrels with an odd number of rifling grooves. Slide them in until they stop then measure the last one to fit with a micrometer. The length of each segment has to be enough to extend past wear at the muzzle from cleaning rods etc.

cheers mooncoon
 
gunlaker said:
For reference the project is a .38 cal black powder match rifle and chamber tolerances want to be as close as practically possible.
Click to expand...

If you have any doubts as to the actual measurement, pour a cast with Cerrosafe or other fixturing alloy, then send that to your reamer maker. They can measure, then make the correct tooling. Be sure to let them know the type/brand of fixturing alloy and date poured.
 
"...chamber casts and a micrometer for measuring groove diameters..." Chamber casts are just that. Nothing to do with the bore or groove diameter. Hammer a cast bullet through the barrel, preferably from the chamber end, then measure it with your micrometer. The OD of that is the grooves. The bore diameter is the diameter of the hole that was drilled before the rifling was done. Measure the grooves on the cast bullet for that.
 
I've used an expanding ball gage in the past. I had a really tight fit with a 338 Edge reamer and a Ron Smith barrel. Had to make my own pilot so the reamer wouldnt get stuck in the bore.
 
Lodi said:
If you have any doubts as to the actual measurement, pour a cast with Cerrosafe or other fixturing alloy, then send that to your reamer maker. They can measure, then make the correct tooling. Be sure to let them know the type/brand of fixturing alloy and date poured.
Click to expand...

Need to keep this in mind with cerrosafe though;

"Cerrosafe shrinks during the first 30 minutes of cooling and then at the end of an hour, is EXACTLY chamber size. At the end of 200 hours it will have expanded approximately .0025". from Brownells...
 
aakoksal said:
Need to keep this in mind with cerrosafe though;

"Cerrosafe shrinks during the first 30 minutes of cooling and then at the end of an hour, is EXACTLY chamber size. At the end of 200 hours it will have expanded approximately .0025". from Brownells...
Click to expand...

Which is why the date it was poured is important. Like he said in his post.And I said earlier.

Heck, if there's material to spare on the barrel, send them a section parted off it, and deburred. Ask them to test fit the pilot before sending. Or is everyone using all 30 inches of tube these days?
:)


Cheers
Trev
 
trevj said:
Which is why the date it was poured is important. Like he said in his post.And I said earlier.

Heck, if there's material to spare on the barrel, send them a section parted off it, and deburred. Ask them to test fit the pilot before sending. Or is everyone using all 30 inches of tube these days?
:)


Cheers
Trev
Click to expand...

I wonder how much dimensions vary with temperature changes in addition to the known expansion... i.e. cast at 15C and measure at 30C a week after... Or does it vary at all...?
 
aakoksal said:
I wonder how much dimensions vary with temperature changes in addition to the known expansion... i.e. cast at 15C and measure at 30C a week after... Or does it vary at all...?
Click to expand...

Yeah, it'll vary, but if you are able to measure that small an amount, you would already be working in a temperature controlled room and handling all your 'work' with tweezers or gloves.

Look up the coefficient of expansion for bismuth (cerrosafe= bismuth, tin, lead, but mostly bismuth IIRC. Oh yeah, some cadmium) which wont be exact the same as the CofE of the alloy, but it will be representative... It'll have a whole bunch of zeros after the decimal point, which will be the expansion in (unit of choice) per degree (unit of choice) per inch of diameter.

So, times that number by 15 (degrees C, or convert to Deg F equivalent, yo will have to convert units somewhere as like as not you were measuring in inches and you may as well not have to convert midway), multiply the result by the diameter you originally read at 15C (for sake of plugging in a number, say it was .3015, ie: roughly 1/3)

So still following?

Coefficient of expansion, X Number of degrees temperature change x the measured diameter at the original temperature.

Longhand, show your work! :D

15C is 59F
CofE is 7.2 micro inches per inch (.0000072 inches), per Deg F if the net is to be trusted at all. :)http://www.engineeringtoolbox.com/thermal-expansion-metals-d_859.html

So .0000072 x 59 = .0004248 x .3015 = .00128, so.... almost one and a third tenths of a thou.

But wait, you have to calculate the temperature change f all the other metals involved!

Can you see why most folks don't worry about it that much? If you can work to tenths, you won't have to ask how to measure it, I think.

Cheers
Trev
 
The cerrosafe expansion with time is pretty predictable. When I do chamber casts ( which I usually do for all of my black powder single shots ) I always get cast the first inch or so of the rifling. When I compare it to a soft lead slug driven into the rifling it's easy to see that the correction factors come out pretty close. If I remember correctly the expansion pretty much stops after a couple of days.

Chris.
 
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