The resident gunsmith here at GD tells me that a lot of people show up with their Ruger Standard Auto pistol in pieces, in a box. If that describes your Ruger pistol assembly situation, we're here to help.
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Apparently, the source of the trouble in reassembly is the mainspring housing.
Here are a couple of tips to help you put your gun back together. These should work for the Standard Auto, as well as the Mark I and Mark II models.
1. Just as it is sometimes necessary to tap the barrel unit forward during disassembly, the barrel may need a tap toward the rear during reassembly to set the unit on the frame hook. To gauge whether the unit is in position, check the hole for the bolt stop pin at the rear to be sure that the unit is properly aligned with the frame. (see Figure 1)
2. After reinsertion of the bolt stop pin, be sure the hammer is in the fired position and tilt the pistol as shown to drop the hammer strut against the inside of the mainspring housing (see Figure 2). This will insure that the strut seats against the mainspring plunger as the housing is swung back into the frame. If the housing is locked in place, but the bolt cannot be retracted, you will know that the strut has missed and you’ll have to reopen the housing and try again. Although this operation is not difficult, it is the one point that often baffles those who take the Ruger apart for the first time.
One last note: The mainspring housing can be disassembled by drifting out the takedown lever pin at the lower end of the housing, but this sequence has been intentionally omitted for two reasons. First, the mainspring is quite strong and has a ball bearing at its lower end, making reassembly extremely difficult. Second, the pins in the housing, retaining the lever and the bolt-stop pin at the top, are riveted in place at the factory, and are usually difficult to remove and replace. If disassembly for repair is necessary, it would be wise to return this unit to the factory.