TRUING AN OUT OF SPEC UPPER

    Before you use information on this page we recommend you read our page on squaring face of upper receiver. The same tool was used in this instance but a much more aggressive technique was required. The upper is a Bushmaster with Cerro Forge mark as initial forging company buat after forging all receivers must be final machined to proper dimensions and someone missed it ba on this unit and quality control did not catch it. Owner was going to trash is but we were able to diagnose the problem and fix with a standard lapping tool and a lot of patient effort.

BOLT OR LAP WILL NOT INSERT FROM EITHER END

    With the upper firmly viced, picture one shows where bolt carriers were attempted to be inserted in past if look in evection port with picture expanded to full size. Picture three shows how far was able to get a bolt in tapping with rubber mallet and picture four shows how far could get lap in front tapping with rubber hammer. (Wheeler Delta Series Upper Receiver Lapping Tool) There were multiple contact points and a lot of metal needed to be removed to allow bolt to travel full length and freely enough to function.

SLOWLY WORKING THE OUT OF SPEC HIGH SPOTS

  Upper two pictures show slowly working the lap into upper and with it being so out of spec the trick is to move slowly, give tool and upper plenty of breaks to cool so neither overheat. First bottom picture shows result of first pass was able to get lap almost to rear of ejection port then final picture shows upper after many passes and final pass from fron using 250 grit lapping compound. With time given for part and upper to cool we are 45 minutes of work into a sixty dollar part minus the hyro-dip.  

MAKING REAL PROGRESS

  Have finally gotten lap travelling to almost full depth (do not want to engage face of barrel threads yet) and now is time to make a pass or two with 400 grit compound to clean up some of the roughness the lap and 250 grit compound left. Alsays wash parts well when changing grit or the heavier grit will contaminate the finer unless your wanting to slowly transition from cutting to polishing and trying to do a bit of both. Before washing noticed a couple lines of burrs where lap transitioned from contact with wall to areas that were milled out so cleaned these rough areas up with a hand file.

SO FAR OUT ONCE FRONT END CLOSE HAVE TO WORK FROM BACK

  

  Even though have lap almost touching face of receiver and smoothed up some of the rougher lapping marks from almost violent use of a lap with heavy grit the bolt will still not go in the receiver and this is the first time have HAD to go in from back to get this upper and bolt carrier meshing. Trick is to not have lap come in from new angle which dould cause a different issue or ruin the upper. Used a dry lubricant to work lap in till almost all the way in and then coated the lap with 320 grit compound from ejection port multiple times as worked lap out the rear. As it freed up was able to move a little faster and in a few minutes had lap moving freely back and forth so turned it around and made a few additional passed from  front, finally made contact to square upper tace with bolt raceway which took very little work to square the face and we are almost done.

NASA WE HAVE BOLT TRAVEL

  

  Bolt is moving o.k. in clean and dry upper with a little light tapping using a rubber hammer. Claened it well again, make passes from front and rear using 400 grit then cleaned and switched to aluminum polish made for aluminum diamond-plate truck tool boxes. Cleaned again, inserted bolt and tapped it back and forth a dozen times and then cleaned upper very well and inspected for any other issues and went to final step.

CLEANED & HAND WORKING BOLT WITH OIL LUBRICANT

 

 

  Now that have finished with lap, lightly oiled a new black phosphate coated bolt carrier and manipulated bolt back and forth full distance of raceway by hand. At first would occasionally get stuck just enough had to tap bolt carrier with my little brass hammer but kept working it till could move it fully by hand without any areas could not move by hand albiet is still pretty snug and my guess is when built on a rifle will need to run full gas first hundred rounds or so till the two parts mesh and then will be able to adjust gas and select best buffer for build then run another hundred rounds and it will be totlly happy and the lower that was dipped to match will finally have its upper mated and its owner a happy person as a few years ago had abandoned this receiver for building on. This was a favor for a friend as if a smith job labor would have exceeded value of part. This is why I say anouone that builds more than a couple AR 15's have some of the more important specicific tools necessary and an upper lap is a required tool as all uppers should be squared before building but few will ever need this much effort.

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