Lapping uppers

[justpete]

Finished a couple of Denny's CMT rifle uppers this evening. Used Slip2000 as the lube and Brownells 600 grit followed by 800 grit compounds. Lapped the entire face this time and after degreasing with 725 and hot water used Aluminum Black to refinish the lapped surface.

The first one was a really tight fit in the barrel extension area but went smoothly past that. Very little asymmetry of wear on the inside surfaces although all the dry lube is gone from the barrel extension area.

The second one was similar but wore the inside down to bare metal along the top edges of the bore at the ejection port and the other side of the CH/key channel. The dry lube was missing from the barrel extension area and the anodization was worn off to bare metal right at the breech end along the top. There were also spirals cut in the metal where the barrel extension goes so something got trapped in there but it's not deep, just cosmetic as far as I can tell.

In both cases the drill vibrated as if it were rotating something off center, didn't do that before. Couldn't feel it touching the pilot through the back of the receiver but the bench shook a bit. Doesn't make a lot of sense as there's no evidence of waviness in the receiver face after lapping. What's up with that?

I don't think the wearing to bare metal is an issue, just that particular receiver wasn't as perfectly circular there as it should've been. The receivers were lightly clamped between polyurethane caps, ejection port up and door open obviously, and tilted slightly down toward the front so any oil dripped into the ejection port opening for lube during running would wash lapping compound out the front instead of allowing it to weep back into the pilot area.

So the question is whether those who do this often scrap a receiver that shows wear from the lapping process like that described above or is this normal and not a worry when working up a precision rifle? Isn't the dry lube there for temporary use until the parts run-in? It's not supposed to remain in place for the life of the rifle is it?

Thanks for your time, Pete

[lw8]

The way I always understood the lapping process for ARs was that the goal was to lap the face of the receiver and true it to the bore...not lap the bore itself.

The way it was demonstrated to me was that the part of the lap that inserts into the bore should be lubed and it is used as a guide so that the lap can true to face of the receiver where the lip of the barrel extension mattes up to the receiver.

I would think that removing the anodizing from the interior of the receiver would be bad...soft aluminum is not a durable setup.

[navyretired1]

I've always understand it to be receiver face only too. I've also got m4 upper that a bolt won't move easily on the inside and haven't decided what to do to fix it ,I'm fairly sure it's been painted with a very tough epoxy paint. stripper don't even start to remove it. I don't want to glass bead blast as I'm afraid anodizing would be damaged. any suggestions?

[justpete]

The way I always understood the lapping process for ARs was that the goal was to lap the face of the receiver and true it to the bore...not lap the bore itself.

The way it was demonstrated to me was that the part of the lap that inserts into the bore should be lubed and it is used as a guide so that the lap can true to face of the receiver where the lip of the barrel extension mattes up to the receiver.

I would think that removing the anodizing from the interior of the receiver would be bad...soft aluminum is not a durable setup.

Yep that's the way I understood it as well. I've yet to have one upper come through the process without removing at least the dry lube somewhere or along one side. This one is a first though, removing anodization. Apparently not enough to allow Aluminum Black to refinish the surface though but enough to make it look like shiny metal instead of flat black. And there was oil dripping out the front, floating on the pilot in the open ejection port, not like there wasn't enough lube.

Thing is the internal bore would have to be perfectly in line with that of the barrel extension part and both would have to be just a tiny bit larger in diameter and dead constant along the length of the receiver for the lapping tool's pilot to ride on a film of lube and not damage the internal surfaces. So far, no joy with four receivers from two sources.

I'm pretty sure I didn't tighten the vise too much and I was using polyurethane caps so it's hard to imagine deforming the receiver enough for this to happen since it was done right after the first one. It's a remarkably difficult process managing the drill and lapping tool without applying too much stress to the receiver, keeping it lubed, not running it too fast or pressing too hard, or going too long without replenishing the lapping compound, and keeping the compound out of the body of the receiver.

Could be by lapping out the surface until all the anodization was gone and then lapping again with a finer grit I spent too much time at it, the key being to spin the pilot in the receiver bore for as little time as possible to minimize the damage likely to occur, dunno, can't find any books or stuff on the process. Hopefully in the end they'll still shoot well.

But you're right, unanodized aluminum is a definite wear point that's possibly a problem unless the receiver was damage along the way to make the bore too tight in which case the anodization would've been worn off along with the fnish on the BCG if it'd run well at all. Weird that one was fine and the other, not. There was plenty of time for the pilot to cool to room temp between receivers as the first one had to be degreased, Aluminum Black applied to the receiver face and rinsed and buffed and repeated, then degreased again and rinsed in hot water, dried with a hair dryer until almost too hot to touch, soaked in Slip2000 and the excess wiped off before the second one could be started.

Maybe the success rate is never 100% and you have to have a bunch of uppers on hand to get one perfectly lapped undamaged one out of the process. That'd suck but it wouldn't surprise me. One thing's for sure, I'm not touching my Daniel Defense uppers....

[larry0071]

Im just a dumb redneck, but what about this for a thought on the BC being tight in your upper....

Paint some valve grinding compound on the exterior of the BC and stroke it back and forth. Valve grind compound is an abrasive grease like paste.... would that do it without removing anything excessive?

I never tried it, and don't know... I'm just throwing up in front of you so you can ponder it.

(Thoughts of a pure ameture!)

[justpete]

I've always understand it to be receiver face only too. I've also got m4 upper that a bolt won't move easily on the inside and haven't decided what to do to fix it ,I'm fairly sure it's been painted with a very tough epoxy paint. stripper don't even start to remove it. I don't want to glass bead blast as I'm afraid anodizing would be damaged. any suggestions?

The first two I followed the instructions that came with the tool from Brownells to face only about 80% but still had excessive wear to the bore but only to the dry film, not the anodization. This time though I must've overdone it. Follow directions? Nah. Ooops.

The finish on your receiver should be a Type III hardcoat anodization that's as tough as, well, aluminum oxide - the lapping compound for aluminum or the bead blasting material for stainless so it's nowhere near soft enough to remove easily. The dry film lube is relatively soft and easy to scratch off but it's not that thick. If it's there and it has to be if the receiver meets spec the BCG should be scraping it off or dulling it where it's running tight, do you see any places like that?

Glass bead blasting would really muck things up I'm sure. I'm no 'smith so I couldn't tell you how to deal with a tight running BCG. Maybe the only thing you can do is try another receiver or BCG, or let the BCG and receiver run themselves in over time shooting it. I'm assuming from what you say the BCG will at least move without having to force it, right?

The two uppers I just either a) lapped correctly or b) mucked up beyond repair are getting Y/M NM BCGs which are supposed to be built exactly to spec so I'm hoping they're not a tight fit. I won't know if they would've been now that material's been removed though. Hopefully the hard chrome finish will slide more easily, as they claim, and the reduced anodization won't become a wear issue over the near term. At least it's an easily replacable item on the rifle so the more heavily worn one is going on the CMP build since the barrel'll go soon enough anyway.

[scottryan]

You guys are wasting your time with this.

[justpete]

You guys are wasting your time with this.

Possibly but it's fun trying.

[markm]

I'm back on TOS.

[navyretired1]

I'm not having much fun I really want to use this M4 upper for a close tolerance bull barrel target weapon. I've tried every bolt I've got and all are too tight the barrel socket is normal with approx 99% contact and shouldn't need lapping. What would you do scotty? Throw the upper away? Hate to do that but but might have to, I guess I could do what someone else did when they sold it to me on the EE.

Possibly but it's fun trying.