Step L7: Receiver Extension Installation
Objective: In Step U5 of upper receiver assembly, we will install the receiver extension (including the castle nut and end plate, as applicable) onto the lower receiver.
Tools Required
1: Only required if installing a carbine/pistol receiver extension.
2: Required if removing/installing a carbine/pistol receiver extension or if removing an existing rifle receiver extension.
3: Only required if installing a rifle receiver extension.
4: Only required if installing a carbine/pistol receiver extension. When using a rifle receiver extension, there is no castle nut; therefore, you do not need to stake it.
Components Required
| Quantity | Component |
|---|---|
| 1 | Lower Receiver |
| 1 | Receiver Extension |
| 1 | Buffer Retainer Pin |
| 1 | Buffer Retainer Spring |
| 1 | Receiver Extension Castle Nut1 |
| 1 | Receiver Extension End Plate1 |
1: Only required if installing a carbine/pistol receiver extension.
Instructions
Secure the Lower
Secure the lower receiver in the torquing vice block.
Step L7.0: Remove Existing Receiver Extension, Castle Nut, and End Plate (replacement/upgrade only)
If you are replacing your existing receiver extension assembly (or need to remove them for any other reason), first ensure the firearm is unloaded.
Before you continue, you will need to first remove the pistol grip, remove the buttstock, and remove the buffer system.
Secure the lower receiver in the torquing vice block.
For carbine/pistol receiver extensions, you will need to loosen the receiver extension castle nut using an appropriate castle nut wrench and breaker bar (DO NOT use a torque wrench to loosen the castle nut). Note that this may take some force if the receiver extension end plate/castle nut is properly staked. Thread the castle nut toward the rear of the receiver extension.
For rifle receiver extensions, you will need to loosen the receiver extension using an appropriate A2 receiver extension wrench and breaker bar (DO NOT use a torque wrench to loosen the receiver extension).
For all AR-15s and AR-9s, the pivot/takedown spring and detent are typically held in place by the buttstock (rifle) or end plate (carbine/pistol). These may fall free when the buttstock (rifle) or end plate (carbine/pistol) are removed. Use caution so as not to damage or lose these parts.
Step L7.1: Thread the Castle Nut Onto Receiver Extension (carbine/pistol only)
If installing a carbine/pistol receiver extension, fully thread (i.e. as far as it will go) the receiver extension castle nut onto the receiver extension, castle profile first (i.e. the side with the 4 large notches facing the closed end of the receiver extension).
Step L7.2: Add the End Plate to the Receiver Extension (carbine/pistol only)
If installing a carbine/pistol receiver extension, slide the receiver extension end plate onto the receiver extension. The indexing “button” on the receiver extension end plate should face forward (toward the threaded/open end of the receiver extension). If equipped, the alignment tab on the receiver extension end plate will slide into a groove on the bottom of the receiver extension.
Step L7.3: Grease the Receiver Extension Threads
DO NOT USE THREAD LOCKER.
Apply grease to the threads of the receiver extension. We find that 5 pea-sized dabs evenly spread onto the threads is the perfect amount.
Step L7.4: Thread the Receiver Extension into the Lower Receiver
Begin to thread the receiver extension into the lower receiver by rotating clockwise. Thread 2-3 turns and stop.
Step L7.5: Install the Buffer Retaining Pin and Spring
Install the buffer retainer spring into the buffer retainer cavity in the lower receiver.
Install the buffer retainer pin into the buffer retainer cavity in the lower receiver, over the buffer retainer spring (with the open end facing down/in).
Step L7.6: Finish Threading the Receiver Extension
DO NOT USE THREAD LOCKER.
NOTE: If you are building an AR-15 or AR-9, the takedown spring should be protruding from the rear of the lower receiver. Take care not to damage the spring.
While depressing the buffer retainer pin and spring, continue to rotate the receiver extension clockwise until 1) the buffer retainer pin is held in place by the receiver extension but can move freely up and down, and 2) the receiver extension is oriented properly (for carbine/pistol only).
Step L7.7: Tighten Castle Nut (carbine/pistol only)
DO NOT USE THREAD LOCKER.
Carefully slide the receiver extension end plate along the receiver extension until it is against the lower receiver. If you are building an AR-15 or AR-9, doing so will compress the pivot/takedown spring protruding from the lower receiver. Be sure that the pivot/takedown spring compresses into the lower receiver and is not pinched/kinked between the receiver extension end plate and lower receiver.
Hold the receiver extension end plate in place against the lower receiver. If there is no visible grease on the threads of the receiver extension between the end plate and castle nut, apply a pea-sized dab ahead of the castle nut. Thread the castle nut forward on the receiver extension by rotating it clockwise, until it is seated against the receiver extension end plate. Hand tighten.
Step L7.8: Pre-Torquing Checks
Before you torque the receiver extension, you should verify these 3 conditions to ALL be true:
- Make sure the receiver extension does not protrude into your lower receiver far enough to interfere with closing the upper and lower together. Verify by installing the upper onto the lower receiver, closing the pivot pin, and pivoting the upper open and closed in the lower. If the upper receiver strikes the receiver extension or will not close, back the receiver extension out one full turn.
- Make sure the receiver extension is screwed in far enough to retain the buffer retainer pin, but not far enough to bind the buffer retainer pin (i.e. the pin should be securely held in place and free to move up and down). If the buffer retainer is not securely held in place, screw the receiver extension one full turn farther into the receiver. If the receiver extension is pressing against the pin of the buffer retainer and preventing it from operating, back the receiver extension out one full turn.
- Make sure the receiver extension is screwed in far enough to maintain sufficient gap between the back of the gas key and the lower receiver. To verify, install the buffer spring and buffer into the receiver extension (see Step L10). Using a bolt carrier assembly, push the buffer into the receiver extension until it bottoms out against the back of the receiver extension. The gap between the back of the gas key and the lower receiver should be at least 0.130″ (about the width of 1 quarter and 1 dime; we shoot for 2 quarters). If the gap is less, try to rotate the receiver extension into the receiver (clockwise) one full turn and recheck all 3 conditions. If doing so creates other issues (receiver extension protrudes into the lower receiver or binds buffer retainer pin), rotate the receiver extension back out (counterclockwise) a full turn; you will need to shim the inside of the receiver extension by dropping a shim, washer, or quarter into the tube before installing the buffer spring and buffer.
Once you confirm these conditions to be true, it’s time to tighten and torque the castle nut.
Step L7.9a: Torque the Castle Nut (carbine/pistol only)
Install the castle nut wrench on the breaker bar. Exercise the castle nut 3-5 times by applying moderate force (about 15-20 ft. lbs.) to tighten it and then loosen it. This will exercise the threads and distribute the grease.
Set your torque wrench to 40 ft. lbs. (38-42 ft. lbs.) and install the castle nut wrench. If you are using an AR armorer’s tool, be sure to adjust the torque setting appropriately.
Torque the receiver extension castle nut until the torque limit is reached.
Step L7.9b: Torque the Receiver Extension (rifle only)
Set your torque wrench to 37 ft. lbs. (35-39 ft. lbs.) and install the 5/8″ A2 receiver extension wrench. If you are using an AR armorer’s tool, be sure to adjust the torque setting appropriately.
Torque the receiver extension until the torque limit is reached.
NOTE: If building an AR-15 rifle, the pivot/takedown spring for the takedown pin will not be secured at this point. We recommend that you proceed to Step L9 (installation of the buttstock) next.
Step L7.10: Stake Castle Nut (carbine/pistol only)
Stake the castle nut. We have a dedicated tutorial for castle nut staking.
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