So you bought the parts kit, you got your tabs, but how to you put them back on?
First things first. You need to clean up your trunnion and get the torch cuts cleaned up and ready to reweld. Start by setting your parts up in a vice. Thread your barrel retaining nut on all the way to cover the threads on the trunnion, and protect them from damage. If your barrel nut has slag or grit in the threads, make sure you clean them out first. You do not want to damage the threads.
Once you’ve got your parts set up in the vice, break out the 4.5″ angle grinder with a thin cut-off wheel. I grind the spot welds down by using the edge of the wheel, coming in like I’m going to slice vertically through the weld, rather than grind it down with the face of the wheel. This gives you much better visibility of when you’ve cut through the weld, and far less damage to the sheetmetal surrounding it.
You’ll want to grind out both of these welds here-there is one flat weld near the back of the torch cut. That’s the weld that holds the front of the internal receiver rail in place. The raised welds are what is holding the front trunnion in place. All these welds will need to be cut.
You can see in the images a slight circle in the middle of the ground areas-this is the edge of the hole that was punched in the receiver prior to welding. You can stop grinding when you can see a complete circle-do not cut all the way through the sheet metal or you risk damaging your front trunnion.
Now, all the spot welds on the ejection port side are ground out far enough to remove the torched sheetmetal. Next is to remove the weld at the bottom of the trunnion, a single MIG weld that holds the two folded sheetmetal tabs that wrap around the bottom.
Here, you want to carefully grind away the weld but leave the trunnion in tact. Make sure you’re grinding away the sheet metal and the weld only-you will see the sheet metal turn blue when it starts to get thin, that’s your cue to stop grinding.
Flip your trunnion over, and grind out the welds on the opposite side. Now all the welds holding the sheetmetal shell on have been cut away far enough to remove it and start cleaning up your trunnion.
Set your flat-nosed punch against the sheetmetal edge, and knock the sheetmetal loose from the trunnion here on the bottom. Fold up the tabs, but keep your fingers clear. The thin edges of the sheet metal, and any weld that is left on the trunnion, will be razor sharp and will shred your fingers if you try to do this with your hands. Use a punch, and wear gloves.
Once you’ve got the front tabs folded up, set your punch right here, behind the remnants of the front takedown pin tabs, and knock the lower section of sheetmetal loose. You can start folding this part back and it will rip clean of the welds.
Rotate your trunnion around in the vise, and continue using the punch and hammer to roll the sheetmetal off. Don’t worry about the weld bumps, you can clean them off after you remove the sheetmetal. Do not gouge the trunnion with the grinder, as you’ll need as tight a fit-up as possible when you go to re-weld your trunnion into your new upper sheetmetal.
Continue until your trunnion is free of it’s sheetmetal prison. At this point, you can grind and clean up the spot weld dots remaining, and clean off any slag left on the takedown pin tabs. You need to make sure there is no slag or scale left here, and you’ll want to cut the tabs clean and square to make fit up of your AR70/90 Front Trunnion Tab Repair Kit as easy as possible.