Crosman 1377HPA Step-by-Step



Introduction: Crosman 1377HPA Step-by-Step

This is a guide of how to easily convert a generic 1377 into a PCP, without the need for a precision lathe, the need to mill out specific sized shapes. There is no need to get someone to machine custom parts for you (at additional cost) If you don't have steady hands then a drill press is the most expensive tool you will need.

The tools and fittings you will need are relatively inexpensive and are an industry standard.

The tools needed are as follows:

A drill (drill press is advised)
9mm Drill Bit
1/8" NPT Tap
Teflon Tape (PTFE)
Wet & Dry Paper
A Vice

Edit (See Step 2):

5mm Drill Bit
5mm Machine Screw Tap
5mm Machine Screw

The Fittings needed are as follows:
#1 1/8" NPT (Male) to Quick Disconnect (Female)
#1 Quick Disconnect (Male) to 1/8" NPT (Male)
#1 1/8" NPT Female) to 1/8" BSP (Male)*
#1 1/8" BSP (Female) to 1/8" BSP (Male)*$
#1 1/8" BSP (Female) swivel elbow to 1/8" BSP (Male)*
#1 1/8" BSP (Female) to 1/8" NPT (Male)*
#1 Remote line paintball adapter

#1 1000psi Pressure Gauge
#1 Extra Quick Disconnect (Male) to 1/8" NPT (Male)

* Quick note!

*Depending on what you can find you can stick to NPT.
*$ You may need more than one depending on how far forward you want the HPA tank to be.

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Step 1: The 1377 Modification

Start by removing all internals of the main tube of your 1377 except the Valve.
Now you have two options!

1 You cut the main tube enough so that there is around 1cm space between the left over main tube and the face of the valve. You can cut it flush with the valve face but it's better have some excess.

2 You get the additional BSP Male to Female fittings so that they come past the opening that lets air into the main tube where you open the pump handle fully. You will need to widen the notch that the pump lever goes in by 3mm on each side to allow the swivel elbow to slide through. Please note that if you go back to making it a pump it will be very wobbly!

Step 2: The Valve Modification

The Valve face has a cone on it. You will need to cut it off with a hack saw so that the surface is flush.
If you have steady hands you can use a 9mm drill bit to drill the hole. The main aim is to make sure that the hole is drilled in parallel to the Valve. Conveniently the hole that the air originally entered into is already centered.

You can now use the 1/8 NPT tap on the 9mm hole.

Keep the valve spring and the Nylon seal that originally pressed up against the inside of the valve air inlet. It will seal up against the 1/8" NPT (Male) end of the Female QDC. That way you can get consistent FPS and save air if you turn the remote line adapter on until the valve is filled with 850PSI (about 1 sec) and then close it.

Edit: When you drill through the center hole you might come across a small problem. The screw hole on the valve may be drilled in too deep so when you drill it out with the 9mm drill bit, it also drills out a hole on the small screw hole. So when you tap it to 1/8" NPT, put Teflon tape on the Male NPT and screw it in, the hole catches some of the Teflon, ruining the seal. The way around this is to cut and sand just enough so that you don't see the threads anymore. [See the second illustration to know what i mean)

The front of the grip frame will need a larger screw than the original. You will need a 5mm machine head screw and a tap to go with it. You wont need to drill out the grip frame hole any bigger. Tap the hole with the 5mm machine screw tap, tapping from the top (don't bother tapping from the bottom as the trigger guard will get in your way!)

The main tube front grip hole will be needed to drilled out to 5mm. Now you can slide the valve in (make sure the valve output is facing the right direction. When you screw the 5mm machine screw in, the threads of the screw will make threads on the valve. the Screw will make contact with the HPA fitting.

The advantage to cutting off a bit of the valve is that it can handle higher pressure. The original valve parts could not be screwed down tight as the small screw hole and the output hole would not be in line. You had to loosen the parts so that the two holes were parallel. Because of this when using high pressure the gap between the two parts is enough to force the O-ring out, losing pressure. By now being able to to tightly screw the two parts together the O-ring has no way of escaping, keeping pressure.

Step 3: The Fittings

Now if you look at the illustration of the fittings needed, this should make a little more sense.

The Valve has to be 1/8" NPT because all generic Paintball fittings use NPT. The Female Quick Disconnect (QDC) is best to be used for safety. If a very high pressure (lets say over 2000psi) of HPA was to go in to the fittings, instead of the steel being ruptured and causing injury the O-ring in the female QDC will fail saving you a trip to the A&E and new fittings. The only thing you will lose is the O-ring.

If you aren't going to cut the main tube the only way you can remove the QDC is to slide the valve with the fittings out the front of the main tube.

After you will need a male QDC to male NPT. The male NPT will go onto a female NPT to male BSP. Here I changed from NPT to BSP because I could only find a swivel elbow in BSP. If you notice on the Illustration there is one fitting that is colored in green. This indicates the additional fittings you may need if you wanted to make the HPA tank to stick out more.

The reason that I decided to use a Swivel elbow is because I can tighten down the elbow semi-tight and adjust the HPA tank until it is parallel to the barrel and then fully tighten it down.

You then need a female BSP to male NPT because the Remote line is standard NPT.
When you buy the remote line there will be a coiled line that has to be removed. You can get a remote line that has an additional pressure gauge. You can swap it out for either a 1000PSI pressure gauge or another QDC male to 1/8"NPT male. So that you can read the pressure in the fittings and valve. And fill the fittings and valve if you are not in the mood to pump the HPA tank up to 200 BAR.

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