Gamo Big Cat hometune question

[xschug3]

I will attempting my very first hometune today flying solo and would like to confirm some things before I begin. I made a great spring compressor that is safe and will do the job. It's similar to Charlie's design, just not as neat looking  

I purchased the tuneup kit based on Andy Wong aka Larspawn's recommendation, http://www.gatewaytoairguns.com/airguns/forums/thread-view.asp?tid=17946&mid=145453.
Thanks Andy! Very big help!

I found what seems to be a great 4 part video on youtube of someone in detail, disassembling a Crosman Phantom. I will be following his basic outline the whole way, step by step. I don't own, nor have shot a Phantom but by the looks of the gun, the components seem identical to my Big Cat.

Can anyone confirm whether or not the Crosman Phantom is similar to the Big Cat mechanically and if it's safe that I follow this video step by step in order to disassemble my BC?
The video is very detailed and has good closeup shots and explanations. I just don't want to get into something different when I take my old spring out and there be different components in the compression chamber than what the Phantom has. I'm just trying to prepare fully in advance before I start taking things apart. Dont want to screw up my gun.

Here is the link to the video: http://www.youtube.com/watch?v=TTr0jfVbC4c&feature=related.

Someone please advise. Thanks in advance!

-Trey

[longislandhunter]

Just got finished watching the 4 parts of the video,,,,,  seems like a pretty straight forward video.

To answer your question,,,, the internals of the Big Cat are just what you see on the video, pretty basic.  You're not going to run into anything that will throw you off course

Good luck with the tune, I'm sure it'll work out great  

Jeff

[xschug3]

Jeff, REALLY appreciate the insight man. I've been sitting around here waiting for a veteran like you to respond before I begin I just wanted confirmation, the video seems legit and well done. I don't expect to run into any problems, but I  know if I do I can count on you guys here for help and advice. Thanks a ton!

I will be updating soon on my results

[gamo2hammerli]

Don't own a Phantom....but do own the G1 Extreme (Basically the same rifle)....and the innards are the same as a Big Cat (I've lubed and changed springs on both).  I can't watch the youtube vid because my home computer is too slow....If Jeff says the video shows the same parts....it's good to go...Jeff's DA MAN.

If you have time, check out Pyramid Air Gun's blog...written by Tom Gaylord/B.B Pelletier......a few years back he did a 13 part series on how to take apart and lube tube a springer.  I read that 13 parter 2 times to memorize them and took apart my Crosman Sierra Pro....not too diffucult....just becareful of the VERY SHARP EDGES on the receiver where the cocking linkage goes into.  Btw, you need to file down (Dull) the inside of that slit....that's where most piston seals will get cut up/damaged when pushing the piston back in.  Good luck....it shouldn't be too difficult.

[xschug3]

Ok, thanks guys! Things are going great so far! Spring compressor did its job, spring is out and its warped and kinked in two different spots. This new spring will do wonders. I've polished the tophat, as it was real rough and "edgy". I'm curious, I don't have moly but I do have heavy tar, is that applied to everything in the compression chamber? I know I will apply that to the spring itself and the ends of the spring. But what about the piston? Do I need to apply that to the piston and seal as well? My original seal had a large gouge taken out of it. I can't wait to put this thing back together.

Gamo2, thanks for that advice. I will file down the reiceiver as well. I will take some time and look at that blog too. That will help me.

[xschug3]

Also, how in the heck do I get the new seal over the end of the piston? I've wrestled with it for 30 mins, it seems it should be more flexible in order to fit over the end.

[JimL911]

Spring tar is sticky. Only put it on the sides of the spring, a string at 12,3,6,and 9 o'clock position is good. It will spread around. You really need moly for metal to metal contact(spring ends,piston). As for the piston seal place it in boiling water for a few seconds or wrap in a damp paper towel and microwave for about 10 seconds. This will make it more pliable and it should slip on easily. Remember to twist it around a couple of times to fully seat it once it is on.

[longislandhunter]

Don't apply heavy tar inside the piston chamber or on the seal.  You should really use moly and only apply a small amount of it on the piston behind the piston seal.  It will lubricate the piston chamber walls at the piston moves up and down inside it.  Order some moly, hold off until you receive it and then lube it correctly, you'll be glad you did.  

As for the piston seal.... have you tried  heating it up a bit using hot water ?  That helps sometimes.  

Jeff

[xschug3]

ok thanks guys, finally got the seal on the piston. Now, the spring with the spring guide is sticking out approximately 5 inches outside of the compression chamber. I'm certain that the piston is in the chamber as far as it's supposed to go. Is this spring and guide supposed to stick out this far? I'm hesitant to compress the spring back into the chamber. Is this normal? The spring that I purchased was the E3650 ultra spring, so I know it's the correct one.

[JimL911]

If you didn't order it set it will be longer. Be careful closing it up to avoid kinks. Once the action is cocked it will be "set" to the operational length and will be fine.

[gamo2hammerli]

What Jim said....and make sure the spring is seated correctly on the top hat....sometimes the spring gets caught on the "Long thinner part" of the top hat. Had that happen to me a few times....I had to wiggle the spring around abit to make it seat all the way in.

[xschug3]

Finished! I completed the hometune last night. I worked on it off and on most of the day yesterday. I had a few hangups but got the tune done and it is now a tack driver with the new spring and seal! It sounds solid and has authority. No more hollow twang.

My biggest hangups weren't with my gun, they were with my deisgn of my compressor. I had the C clamp start moving slightly whenever I was torquing on it to compress the spring. I had to tear apart my compressor and rebuild it and reinforce it. While rebuilding it, I had the wood split on me so I had to go to Home Depot and pick up another piece of wood for the base. The good thing is, now I have a very trusty compressor that I don't have to worry about.

The other problem I had was putting the cocking foot back into the groove of the compression chamber. I had to do quite a bit of sanding on the foot so that it would snap back into place. That took some time.

When pushing the piston back into the compression chamber with the seal on the end of it, some of the seal got hung up. It scraped a small piece of the seal off. Had I sanded down the edges of the compression tube better that wouldnt have happened.

Other than those hangups, everything else wasn't too difficult. I thank you guys a ton for helping me out by answering my questions. If it wasn't for this forum, I wouldn't even attempt it. I'm so glad I took the time to do the tune myself. I learned so much about the workings of my gun and all the internals. I'm confident now that if I need to pull it back apart I can do it much quicker.

Thanks again for everyone's help with this project. I hope all the questions I asked will help others in the future

[gamo2hammerli]

Good job Trey....the first time is always the trickiest.  After that...it gets easier.  So far I've taken apart my Gamo Big Cat, Viper, Expotec, Crosman Sierra Pro (Twice), Storm and RWS Diana P5 Magnum (Pistol).  Learning more each time.  Strange how the foot of your cocking linkage got hung up going into the receiver's slot and piston....  All of mine's went in easy.

[ezman604]

Good job Trey, glad you completed your first home tune. I'll offer a couple ofsuggestions for your next attempt. I grind, sand and buff the spring ends. I also add a stainless steel washer at both ends of the spring is possible. I buff these out and add moly to them. It gives the spring a smoother surface to twist on when compressing and releasing. It helps eliminate torque.

Also, get you a piece of allthread. About an 18" piece will work. Should be small enough diameter that will fit inside the spring. Get two large washers and a couple of nuts to fit the allthread. Also get a wingnutsame size. Put two nuts on one end of the allthread to lock them in place. Side one large washer onto the rod. Slide your new spring onto the allthread, add large washer and wingnut. Crank the wingnut down to compress the spring completely and let it sit for 2-3 minutes. Back off wingnut, remove your now preloaded spring, lube and install. Your replacement spring was sticking out of the chamber that far because the spring had not been preloaded. That puts a dangerous strain on your compressor when installing aspring that has not been preloaded.

DEFINATELY use moly on the spring ends and a light application of tar along the spring length. Too much tar will dampen it too much and reduce you power. I'm on my 5th home tune and learning more each time. It's fun and I enjoy seeing the results. Good job and let us know if you get chrony numbers on it.

Happy Shooting....and tuning!!!!

Dave

[xschug3]

Dave, those are great tips to know. I will be doing more tunes in the future so those pointers will help out greatly. You're definitely right about a spring thats not preloaded putting more stress on a spring compressor. I was really beginning to worry as I continued compressing my spring. I started hearing some creaking and funny noises coming from the wood of my compressor. I immediately began releasing the load on the spring. I was doing all of this inside and the last thing I wanted was a spring going through my wall....or even worse, my face. I had to redesign my compressor and add a few pieces of wood to "beef" it up. Once I did that, I was completely confident in my compressor and its design.

I must say, the whole process was bittersweet. It was frusterating and tedious. My hands were very sore the next day and I got 2 deep cuts somehow during the tune. It really tested my patience. I had to stop and take some breaks to ask for advice. But, it really is one of the most satisfying projects I've completed. I learned so much about my gun. I'm much more confident in the my guns abilities also. I'm very pleased with the outcome and my Big Cat clearly shoots harder (I don't have a chrony), is quieter, and doesn't recoil as bad. Also, got rid of that nasty Gamo "twang".  I couldn't be happier with it