Tuning a Crosman Trail NP XL 725

[hd833]

I think I read somewhere on this forum about enlarging the transfer port on this weapon to increase air volume to the pellet. Thus increasing speed. Like a dummy I didn't bookmark the message and now I can't find it. I have a small machine shop so mechanical ability is no problem for me. I guess what I'm getting at is are there any more mods to be done on this weapon besides the washer upgrade and cleaning. I'm not a big fan of honing any thing as far as piston travel is concerned (tolerances are too tight to try and enlarge any chambers thus destroying seal to chamber fit). But I'm still open to suggestions for increasing power for a .25 cal. Yeah I know if I want more power I should go with an Mrod. Been there done that. Too darn expensive. I'm a disabled retired vet (23 yrs. Army) and have to live within my budget. That's why I self tune all my weapons. This a great site and gave me alot of insight to all the little quirks about this particular weapon.

[SAADE]

Did you do a search in the GTA Library?

A lot of the transfer port talk is more centered on PCP and co2 guns but, you may find some good info in the Library anyway.

One caution, transfer port mods on spring guns can be very touchy based on diameter and shape. Basically, we are dealing with a very small volume of rapidly expanding/heated air when a spring gun fires so, hopefully, the maker designed the port for best air flow with enough restriction to make use of that small volume of air converted into the highest pressure possible. That's the the theory anyway! Think of it like the Harley guys who put short drag pipes on their big cubic-inch bikes and then have to set the idle up at 2000 rpms for it to run. Those drag pipes are called drag pipes for a reason, low restriciton and high flow for drag racing, they are horrible for around town low speed cruizin.

[Nathan]

The problem with doing any modifications to the transfer port is a little can be too much. In theory (and hopefully) the factory has figured out the optimum shape and size for max power and shoot ability (within the confines of mass production). Any changes you make may alter that balance and loose power and or smoothness. Plus if you remove too much metal it’s very hard to put it back. You could easily ruin the compression tube. The upside is that a replacement compression tube can be purchased from Crosman. If you’re real careful, you could smooth out the transfer port of any rough edges. That MAY help with smoothing airflow and give you a little power. You’ll probably see more benefit (and power gains) from honing and cross hatching the compression tube and properly fitting a seal.

Nathan

[mtsheron70]

I cannot respond to your post other than I was directed to a similar request by the fine folks here to:

http://www.airgunweb.com/?s=benjamin+trail+NP&search=Search

This should be conducive to a XL as well.  Good luck!

[hd833]

Took my np xl 725 apart today. Horrible machining! Breech face is not square causing one side of the o ring not to seal correctly all the way around as evidenced by the wear marks on the compression tube face. Transfer port on the compression tube face is oblong by either a bad drill bit or drilling too fast. A very slight chamfer on the compression tube orifice, off centered and horrible. Piston seal is nicked. The machining reminds me of when I was an apprentice tool and die maker! Very shoddy! So to remedy some of these problems I'll try to crown the transfer port orifice of the exterior of the compression tube, then polish with valve grinding compond the length of the orifice itself to reduce friction of the compressed air flowing through (idea from a fluid dynamic engineer) and NOT enlarging the orifice itself. Put a slight chamfer on the breech to help seat the pellets. Square up the breech face and compression tube face with a file for a better seal and recrown the barrel. Replace the breech o ring and the hinge washers. Am I on the right track?

[microsalmo]

how does it shoot right now?

[mtsheron70]

Pics. would be nice.  I am going to upload my breakdown pictures once I get the gun up and running again.  You sound like your on the right track but like I said, a picture says a thousand things.

Here is what I did to my Trail NP-

Prior to breaking the gun down to see what numbers I could get on the chrony the only mod was a GRTIII trigger.  Since I have base line numbers now to the breakdown to hopefully increase those performance numbers.

- Broke gun down by taking out the gas ram and piston cylinder.
- Cleaned the internal chamber the piston cylinder fits into.
- Wiped down the gas ram to check for any issues but really noted none other than a scratch on the ram main housing.
- Cleaned out the piston cylinder and used a Dremel to polish the entire piston cylinder and round up the rough edges.
- I then used the Dremel to polish and round the main internal chamber the piston cylinder fits into.
- Ordered a James Maccari Tesla  piston seal to re-install and his moly paste to lube the cylinder and contact points.
- Did the brass washer exchange on the pivot point of the break barrel as many here have stated a necessity.
- Polished the trigger sear and took off the very slight burrs on same.

Now my new seal and moly paste should be here tomorrow to reassemble.  I am anxiously awaiting it to arrive to get this back together.

I will put back together; put about 200 pellets through her then chrony again to compare results!  Hopefully only good things to report or I will consider myself a total tuner failure!!!!

[uncle paulie]

An angled breach face is normal, I had originally thought the breach face should be parallel to the receiver tube to allow the breach to close on the seal and the seal would hold everything straight. But that would require the seals to be less resilient than they are and not seal as well. When the breach closes, the actual stopping mechanism is the metal to metal contact on the top of the breach, which locates the barrel in the same place every time. I wouldn't file the breach face, could be a can of worms. if the seal isn't leaking, its doing its job.

pv

[john]

Am I on the right track?

Not entirely. Your first mistake would be to "square up the breech face."
http://www.gatewaytoairguns.org/GTA/index.php/topic,17738.0.html

Regarding smoothing or shaping the transfer port, your engineer is referring to establishing laminar flow. The airgun firing cycle is not laminar; it is turbulent because it is short lived and because it is flow across a restriction; no amount of shaping or smoothing is going to change that. The air flow thru the port is over long before anything laminar has a chance to happen...the air that follows the pellet out the muzzle might be approaching laminar flow. imo.

You are planning multiple changes to your gun. How are you going to know which if any of those changes improve accuracy IF you don't do them one at a time?

Welcome to GTA. Don't let me discourage you...i'm a pessimist. 

[microsalmo]

Im nervous about one turning out.

[hd833]

Paul you brought up a valid point about the breech face and something I wasn't aware of. My concern is if you face the breech the o ring is flattened on the right side of the port and the left side is normal i.e. raised slightly above the metal thus ensuring a good seal. I guess I should have taken pix b4 I took it apart. Plus I guess as an old tool and die maker I expect perfection. But I'll try to to get some macro shots of the transfer port on the compression tube. Now my engineer friend told me that you want to try to achieve something like laminar flow and not turbulence. Vortices are created and cause friction which we all know is an impedance even though it happens in micro seconds but I'm open to suggestions. I've only fired 2 shots through it just to make sure it worked. I'll worry about the fps and fpe once I get the accuracy. I do that too when I reload for my powder burners. Speed and power are no good if you can't hit your target! This is fun aint it?

[hd833]

After reading the post about squaring the face of the compression tube I understand the difference in the o ring from top to bottom I found the seal pinched from left to right. Now on the 725 parts breakdown I noticed an "adjust washer". So after Crosman gave me a hard time I had to pay for the parts; piston seal,breach seal and the adjust washer for a total of $4.50. I'm assuming the adjust washer is just a shim. Time will tell if I need it or not. If I can post decent pix of the transfer port I'll post them, I don't care for those macro shots that are out of focus and if it aint sharp I won't post it. So we''ll see.

[uncle paulie]

Left-right variation would indicate to me that the breach is not aligned correctly and the rifle should have a poi which is way off to the side, it could be a badly machined breach or the bore of the pivot pin could be skewed. How does the alignment look when you sight down the receiver and barrel?

pv

[hd833]

Gonna put a radius and polish the tranfer port on the compression tube and also on the breach first then check the groove for the breach seal for depth all around the groove. Will also check the live jam and make sure it's correct. But all that has to wait ...getting ready to watch the 500!