(4) _Spring retainer_, a brass, hollow bushing which screws into the rear opening of the valve body, and which is threaded internally to receive the plug. The retainer has a hexagonal head to take a 1-3/8-inch wrench. As its name implies, the retainer holds the valve spring in position. (5) _Plug_, a brass part, resembling a cap screw, which fits into the spring retainer, closing off the rear end of the gun. It permits adjustment of the needle (see _d_ below) without removing the valve spring and spring retainer. (6) _Valve spring_, a coil spring located in the valve body between the spring retainer and the yoke block. The spring keeps the needle seated in the nozzle until compression of the grip safety and valve lever forces back the yoke block, spring, and needle. (7) _Yoke block_, a steel piece, 1 inch long, which fits into the arms of the yoke Y. It is secured by an internal thread to the valve needle. Movement of the yoke in turn moves the yoke block and the valve needle. (8) _Lock nut_, on the valve-needle thread at the rear of the yoke block, which locks the block on the needle. (9) _Valve-needle_, a pointed rod, which extends through the inside of the barrel from the yoke block to the nozzle. The valve needle is seated in the nozzle except when firing. It controls the ejection of fuel from the nozzle. Two sets of three fins each, known as needle guides, are mounted on the front and rear of the needle, respectively. These guides keep the needle centered in the barrel. The rear end of the valve needle is threaded to hold the yoke block and permit adjustment of the needle by means of the lock nut which screws on the threads. (See _d_ below.) (10) _Barrel_ (Figs 47 and 54), which carries the fuel to the ignition head. It also supports or contains other components of the gun. The barrel assembly is replaced as a unit with the needle. It consists of a tube, made of thin metal, with a threaded fitting at the back end, and a nozzle brazed into the front end of the tube. The nozzle ejects the fuel from the barrel through the ignition head. The fuel emerges from two holes in the nozzle: (_a_) _Atomizer hole_, a small opening which sprays a fine, readily ignited mist of fuel. This helps ignite the main stream of fuel. (_b_) _Main hole_, which is tapered inside, and which conveys the main stream of fuel from the barrel. When the gun is not being fired, the valve needle is seated in the main hole of the nozzle. When the gun is being fired, the needle is withdrawn from the nozzle seat, permitting the fuel to be forced from the gun. _b. Removal of barrel and valve-body assembly._ If gun group and tank group are connected, release any pressure in the fuel tanks by compressing the valve lever and the grip safety. Then disassemble as follows: