Modern Sporting Rifle Evolution
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JohnOD

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I purchased this Pacific Tool & Gauge AR-15 Upper Receiver Lapping Tool http://pacifictoolandgauge.com/ar-tools/1139-ar-15-upper-receiver-lapping-tool.html to lap my new Young Mfg upper receiver

The problem I'm running into is the lapping tool mandrel only slips into the upper about 3/4" before it comes to a dead stop[confused]. I'm not going to force it and end up with a bigger problem thats for sure!

The tool and upper are both new, cleaned and lubricated

According to my digital calipers the mandrel on the tool measures .9995" (I know my cheapo digital calipers are not dependable to 5/10,000)

Bummer

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Dvalin

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Reply with quote  #2 
I'm pretty sure your lapping tool is out of spec. The exact numbers aren't coming to mind but I think it should be closer to .995
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Rifter

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Reply with quote  #3 
Quote:
Originally Posted by JohnOD
I purchased this Pacific Tool & Gauge AR-15 Upper Receiver Lapping Tool http://pacifictoolandgauge.com/ar-tools/1139-ar-15-upper-receiver-lapping-tool.html to lap my new Young Mfg upper receiver

The problem I'm running into is the lapping tool mandrel only slips into the upper about 3/4" before it comes to a dead stop[confused]. I'm not going to force it and end up with a bigger problem thats for sure!

The tool and upper are both new, cleaned and lubricated

According to my digital calipers the mandrel on the tool measures .9995" (I know my cheapo digital calipers are not dependable to 5/10,000)

Bummer


The tool I bought from Brownell's several years ago was such a tight fit in the upper receivers I tried to use it in that I had to go to a precision machine shop and have it shaved in the lathe.  They took off about .6 of a thousandth.  After that it worked perfectly, and I've used it on quite a few uppers with perfect results.  The problem is probably not the tool itself as much as it is the receiver casting itself and the boring bar they used to finish it.  If you mount the upper vertically so the tool drops in from the top, you'll get better results after you have it shaved.  Be sure to take the upper you are trying to use it on with it to the machine shop, so they can try it and only take off the minimum amount necessary.


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BobT

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Reply with quote  #4 
I have had issues with PT&G tooling in the past, I would look there first. I don't know if you are anywhere close to me (Missouri) but if so I would be happy to fix you up. I'm a machinist by trade and have access to about any machine tool you can think of.
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JohnOD

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Reply with quote  #5 
I have an email into PT&G asking what the spec is on the mandrel... I'll let you guys know what they say

My PG&T large frame AR lapping tool worked fine for the one upper I used it on. I just used some valve grinding compound and clamped it vertically like you said RIFTER and did it by hand... it didn't take much effort at all

BOBT thanks for the generous offer but since I'm in Oregon unfortunately the shipping would be a killer


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pnw

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Reply with quote  #6 
Make sure the dust cover/port door is removed before trying to insert the tool.
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JohnOD

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Reply with quote  #7 
10/4 PWN

This receiver doesn't have a dust cover
https://youngmanufacturing.net/product/side-charge-upper-receiver/

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JohnOD

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Reply with quote  #8 
Success!

I used my hillbilly home machinist skills and chucked the lapping tool in a 3/8" drill and started with 120# emery cloth and took about .002" in diameter off the mandrel until it fit into the upper receiver by hand.
Then I went over it with 400 grit wet dry sandpaper and finished with 600 grit. It took a while checking & rechecking the fit but I got it done.  

Once I got everything cleaned up I lapped the face of the upper receiver by hand (no drill) using lapping compound until it just broke through the anodizing. As it turns out the face of the receiver was darn near perfect and in the overall scheme of things probably didn't even need to be lapped... of well it kept me busy[biggrin]

In hindsight I think the PG&T tool was in speck and the problem is in the upper itself. The upper was undersized in the front about .001" tighter yet in the rear. The carrier is snug in the upper but not unduly so.

Now once my 1 1/4" crowfoot wrench shows up I'll get everything back together.
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Cabob

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Reply with quote  #9 
I just used a PT&G lapping tool today on an Aero Precision upper. First time use. It worked great. 10 minutes by hand with some lapping compound and job complete. I'm almost certain your upper was a little out of spec.
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lapriester

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Reply with quote  #10 
Could it be hanging up on the forward assist as you insert it.
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JohnOD

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Reply with quote  #11 
No forward assist on this upper LAPRIESTER. The Youngs Mfg upper/carrier I used have a reputation for being tight. I got it worked out by slightly reducing the diameter of the mandrel.
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