Any AR builders/tuners — Lapping Q?
- Thread starter Rsadams
- Start date
I learned about thermal fit in the 1970’s as a young man, working on a Fiat automobile. I had a bearing retainer that was just an interference fit, no interlocking features. Straight shaft, straight bore in the retainer. I maxed out the hydraulic press trying to put that retainer on and it absolutely would not go, not even start. So I went to the shop manual (I know shoulda done that first). It said, heat the retainer to 450 F. So I did, and that thing just fell into place under it’s own weight, no press, no hammer. I was surprised how much difference it made.
Rocketvapor
Gold $$ Contributor
Yup,
materials usually have a known coefficient of thermal expansion.
Chrom-moly and SS are close, 6063 and 7075 are close.
I think
AR-15 upper bore diameter is spec'd @ 1.000" +/-0.002".
AR-15 extension is spec'd @ 0.998" -0.0008, + 0.
Some receivers are advertised as undersized bore (for a thermal fit with most barrels) and some extensions are advertised as oversized.
I checked one BCA side charge upper at three depths from the nose @ 0, 60,120 degrees. Got about 0.0003" under an inch.
You can buy an undersized lapping tool (for receiver face) from PTG for those really tight thermal fit uppers.
pacifictoolandgauge.com
Tool O.D. for AR-15
Mil Spec: OD= .999-.998 (Standard military specifications)
Competition: OD= .998-.997 (Competition for tight fit uppers)
An extreme thermal fit example
Dome caps for the Space Shuttle External Tank were chilled in a Liquid Nitrogen tank, placed into position into the tooling, and clamped, then welded in place.
materials usually have a known coefficient of thermal expansion.
Chrom-moly and SS are close, 6063 and 7075 are close.
I think
AR-15 upper bore diameter is spec'd @ 1.000" +/-0.002".
AR-15 extension is spec'd @ 0.998" -0.0008, + 0.
Some receivers are advertised as undersized bore (for a thermal fit with most barrels) and some extensions are advertised as oversized.
I checked one BCA side charge upper at three depths from the nose @ 0, 60,120 degrees. Got about 0.0003" under an inch.
You can buy an undersized lapping tool (for receiver face) from PTG for those really tight thermal fit uppers.
AR-15 Upper Receiver Lapping Tool - Pacific Tool and Gauge
AR-15 Upper Receiver Lapping Tool at Pacific Tool and Gauge
Mil Spec: OD= .999-.998 (Standard military specifications)
Competition: OD= .998-.997 (Competition for tight fit uppers)
An extreme thermal fit example
Dome caps for the Space Shuttle External Tank were chilled in a Liquid Nitrogen tank, placed into position into the tooling, and clamped, then welded in place.
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Rocketvapor
Gold $$ Contributor
Math looks good.
To get max tolerance stack up, is pretty unlucky.
Getting 0.001" to 0.002" of tolerance slop, and shimming for interference with 0.002" shim stock would give a solid lock up.
A X-caliber barrel I have measured 0.9991" and tapered up to 0.9999".
Combined with that .9997" receiver bore it would SLIP in MOST of the way.
Calculated what temperature I needed, spray on shim burnished for 0.002" interference @ room temperature.
Heated receiver to 240F, cooled barrel to 5F, lost some temperature handling and slipping together.
Still, this stuff is only going to matter (if at all) with heavy, long barrels, at mid or long range, getting the barrel hot shooting 3X20 round relays.
To get max tolerance stack up, is pretty unlucky.
Getting 0.001" to 0.002" of tolerance slop, and shimming for interference with 0.002" shim stock would give a solid lock up.
A X-caliber barrel I have measured 0.9991" and tapered up to 0.9999".
Combined with that .9997" receiver bore it would SLIP in MOST of the way.
Calculated what temperature I needed, spray on shim burnished for 0.002" interference @ room temperature.
Heated receiver to 240F, cooled barrel to 5F, lost some temperature handling and slipping together.
Still, this stuff is only going to matter (if at all) with heavy, long barrels, at mid or long range, getting the barrel hot shooting 3X20 round relays.
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So...I got out three unused barrel extensions and measured. I was taking notes but had to abandon my note pad in the shop. More on that later. Anyway I do recall the smallest is .9982"and the largest is .9987" the one in the middle is .9984" I think. So I got a range of .0005" for three pieces. These from various suppliers, unknown makers.
I had three upper receivers out but didn't get to measure them. because;
I got home two days ago from surgery and I've been sitting around since then. I went to the shop to get these parts out and the surgical site began leaking badly and required immediate attention so no more measuring in the shop today! All good now, it's just not ready for activity yet.
I think, with a 20 inch heavy barrel, or a 24 or greater, and a free float hand guard, even with a secure barrel to upper joint, that's a lot of downward pressure exerted by the barrel on the upper with it's thin walls and only two pins. The whole set up seems to be not real rigid.
I had three upper receivers out but didn't get to measure them. because;
I got home two days ago from surgery and I've been sitting around since then. I went to the shop to get these parts out and the surgical site began leaking badly and required immediate attention so no more measuring in the shop today! All good now, it's just not ready for activity yet.
I think, with a 20 inch heavy barrel, or a 24 or greater, and a free float hand guard, even with a secure barrel to upper joint, that's a lot of downward pressure exerted by the barrel on the upper with it's thin walls and only two pins. The whole set up seems to be not real rigid.
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You are very correct , thank you.... now I am having some other problems that if they don't get better I will probably be going to the emergency room tomorrow..... I thought I was out of the woods now something else... Hopefully it's just a reaction to the medicine from the first problem.....
Years in industry on the technical side of things, when dealing with production parts where fit or mating of parts is an issue sometimes we've had to develop methods of measuring all parts, assigning them into groups based on actual measurements, and then selectively fitting various parts together. For example, the depth of the pocket in a hydraulic pump body matched to the thickness of the rotor and gear.
In production if you can't have near-zero tolerances, which you can't, in some cases having a varying range of parts within a wider tolerance is actually a good thing.
I don't want to invest in enough AR parts to allow me to do that so...shims and ovens for me!
I think in the AR world manufactures probably stay on the "wide" side of things so all parts will always assemble and never have parts that won't fit together. ?
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Going back over the entire thread, it seems like Rsadams has come to the conclusion that lapping the receiver face is not what he’s going to do right now, but maybe later when he goes to a 20” barrel and a free float hand guard.
I agree with this approach. In my opinion lapping the receiver face may help to improve accuracy in some cases and a top match shooter who is a serious contender for the win certainly will want to eliminate that variable. For any other use I think while it wouldn’t hurt, it probably won’t make a difference that is noticeable. That’s from an accuracy standpoint.
If the concern is where the barrel is pointing, OMG I am absolutely OCD about having my rear sight centered and I will jump through hoops to make sure my rifle zeros with the rear aperture centered perfectly...which usually turns out to be a waste of time because then the wind blows or I change ammo or...anyway here’s an interesting story.
A friend of mine bought an A1 parts kit which was everything except the lower receiver and it's internals. He put it all together on commercial lower. He couldn’t zero the gun. At 50 yards he couldn’t lower the front sight post enough. It bottomed out and he was still way low on the target. He asked me about it and I said I don’t know...first guess I’d take is try a different barrel. He wanted to keep it original so he bought another A1 surplus barrel. I put that on for him. Now, he couldn’t RAISE the front sight post enough. It ran out of thread and he was still high on target.
We fixed it with a plus .060 height sight post from Wyndham. But the point is, if the barrel is not pointing where it should there’s a lot of things which could be at fault. The receiver face is only one.
While trying to solve that issue I talked with a guy who was a USMC armorer when the A1 was current issue. He told me if they had a rifle that wouldn’t zero within the range of sight adjustment they’d bend the barrel! Not sure if that was the approved procedure but according to him, that’s what they did.
I agree with this approach. In my opinion lapping the receiver face may help to improve accuracy in some cases and a top match shooter who is a serious contender for the win certainly will want to eliminate that variable. For any other use I think while it wouldn’t hurt, it probably won’t make a difference that is noticeable. That’s from an accuracy standpoint.
If the concern is where the barrel is pointing, OMG I am absolutely OCD about having my rear sight centered and I will jump through hoops to make sure my rifle zeros with the rear aperture centered perfectly...which usually turns out to be a waste of time because then the wind blows or I change ammo or...anyway here’s an interesting story.
A friend of mine bought an A1 parts kit which was everything except the lower receiver and it's internals. He put it all together on commercial lower. He couldn’t zero the gun. At 50 yards he couldn’t lower the front sight post enough. It bottomed out and he was still way low on the target. He asked me about it and I said I don’t know...first guess I’d take is try a different barrel. He wanted to keep it original so he bought another A1 surplus barrel. I put that on for him. Now, he couldn’t RAISE the front sight post enough. It ran out of thread and he was still high on target.
We fixed it with a plus .060 height sight post from Wyndham. But the point is, if the barrel is not pointing where it should there’s a lot of things which could be at fault. The receiver face is only one.
While trying to solve that issue I talked with a guy who was a USMC armorer when the A1 was current issue. He told me if they had a rifle that wouldn’t zero within the range of sight adjustment they’d bend the barrel! Not sure if that was the approved procedure but according to him, that’s what they did.
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Wanted to add: I also use permatex sleeve retainer when assembling my gas tube to my gas block. Id almost bet you could leave the roll pin out and it would hold. (Dont try that) I only use clamp style gas blocks and have never had one move or leak so I dont use anything between the block and journal. I do use a dab of medium on the gas block screws.
Some helpful videos:
Criterion has a 4 part series going over ar15 assembly. They go over bedding and lapping the receiver. Here is part 3 but i suggest watching all parts
This channel has some great ar15 info. He also goes over lapping and bedding/shimming.
Some helpful videos:
Criterion has a 4 part series going over ar15 assembly. They go over bedding and lapping the receiver. Here is part 3 but i suggest watching all parts
This channel has some great ar15 info. He also goes over lapping and bedding/shimming.
I like a drop of blue or purple just to make it seal at the gas block more positively. But like you, a clamp style is usually pretty good without anything. But it doesn't hurt either.
LVLAaron
Gold $$ Contributor
A couple boxes of cheap/dirty ammo will seal it up real nice!
Probably so. I do hate that leak stain around a gas block though. It's just nice when it never happens, from the get go.
I've never had any real leakage between the barrel and a gas block. My experience has been the leaks are more common between the gas tube and the block. But then I usually run AGBs so it doesn't matter.
Rocketvapor
Gold $$ Contributor
I run a No-Gas AR, no port, no block
Wife has gas. Set with minimum to get lock back and it dribbles out cases (slow fire, single load). Get carbon with 60 shots but wipes right off.
Wife has gas. Set with minimum to get lock back and it dribbles out cases (slow fire, single load). Get carbon with 60 shots but wipes right off.
Rocketvapor
Gold $$ Contributor
Started the prep work for the NEW upper.
Checked the bore and using the borescope to watch, blended the edges of the gas port with a small pointed diamond burr. Then a little bore polish and clean up.
Measured extension and receiver.
Extension has a taper and will required a little taper with the spray on shim.
Once the paint cures I'll measure and buff down a little. Gonna take heat and cool to slip together.
Measured receiver nose I.D.
Lapping the receiver face with a Wheeler tool.
Coated tool with a little spray on shim for a tighter fit, oiled and held receiver nose up while I turned by hand.
Checked the bore and using the borescope to watch, blended the edges of the gas port with a small pointed diamond burr. Then a little bore polish and clean up.
Measured extension and receiver.
Extension has a taper and will required a little taper with the spray on shim.
Once the paint cures I'll measure and buff down a little. Gonna take heat and cool to slip together.
Measured receiver nose I.D.
Lapping the receiver face with a Wheeler tool.
Coated tool with a little spray on shim for a tighter fit, oiled and held receiver nose up while I turned by hand.
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xr650rRider
Silver $$ Contributor
Rocketvapor
Gold $$ Contributor
Well, I probably wasted some Bar-B-Q paint on my latest one tonight.
Got a shimmed O.D. of about 1.0015".
Barrel/extension would probably tapped on in with little effort, maybe warmed with a hair dryer, without the paint.
Heated receiver in oven, barrel in freezer, slipped together.
With a 5+ pound barrel, I just feel more comfortable with a good tight fit.
Got a shimmed O.D. of about 1.0015".
Barrel/extension would probably tapped on in with little effort, maybe warmed with a hair dryer, without the paint.
Heated receiver in oven, barrel in freezer, slipped together.
With a 5+ pound barrel, I just feel more comfortable with a good tight fit.
LVLAaron
Gold $$ Contributor
I think I lost some IQ points.
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