Crosman Flattop Piston Design/Material

[BigBird]

Hi,

Please move/ delete/edit this post if need be.  It's my first.

My 550W lathe motor was smoked by a tantilizingly cheap 100 pack of supposedly 8 amp fuses (probably 12 amp) a while ago.  I was testing my new (to me), random, unmarked 120 DC motor on my lathe last night and decided to try to make a flattop (FT) piston for my 1322.  I have some 5/8" 1018 CRS that I bought locally but was fooling around with some very large SHCS I had laying around. I didn't look at the YFS 12.9 grade "headstamp" until after I was done (hey, motor works! albeit very low RPMs).

Anyhow now I'd like some information about FT piston design and material.  If this could work I'll use it or mod it, otherwise test complete.  Here are my questions:

1. Will this "screw" work if I can get the axis hole properly placed at the bottom?  Any advice about a non-adjustable piston (how to get perfect calibration?

2. Do I have to make the bottom .620 for proper alignment? (If so, I'll turn down, thread and also make some fixture for a wiper).  FYI - I have 3/4" and 5/8" CRS, 1" Aluminum, 1.25" 303 and know of a source for 5/8 303 fairly cheap

3. Why don't people use mild tool steel for this?  Weight? RPMs required for finish?  Corrosion?  You are magnetizing it with every pump?

4. If you are able can you post some designs/pictures of your FT pistons?  I've searched the forum and did not see any/many DIY for these.

Caveats: I often do things that cost more or don't make sense just to learn or maybe have some "guy" self accomplishment moment.  I know Mellonair has them very reasonable (I even know his wait time is now lower ).

[rsterne]

It will be difficult to get the perfect length without some provision to adjust that.... The usual setup is a piston and a yoke, with a threaded rod between.... The middle item on the left in this photo is an example of an FTP.... It was actually made from the front of a spare valve....



You also have to machine the end of the valve flat to match it, as in the item to the right of that FTP....

Bob

[BigBird]

Thanks Bob!

Yeah if I was starting with solid AL rod or brass I could start on the muzzle end and shave off the face of the piston till it was the right length.  That's part of my caveat (doesn't make sense).  After I flat top the valve I'll get creative.  My brain needs some problem solving sometimes.

I know it sounds crazy but WHAT IF I cut out the crotch a little at a time till it bottoms out (at the top).  I could then know the proper area for the hole?  I'd have to make sure everything is centered with some plastic rings or something.  I can always fall back on chopping it off and threading.  It was a $0.25 screw at a thrift shop (opportunity cost ignored).

Thanks and Happy Easter.

[Machinist]

I made a FTP years ago and I'm pretty sure I used aluminum.  Aluminum rubbing in the steel tube wont scratch the tube.  I put a plastic wear band on it anyway, that defiantly won't wear the tube. 

I think I made it two pieces that thread together.  Then add a shim between them until the piston lightly hit the valve.

Sorry I don't have pics, I did this a while ago.

I don't have the gun here, I lent it to my friend.

Steve

[BigBird]

Thanks.  Ok that's a good design.  So both designs (nuts or shims) must work well.  I always thought a nut might move over time.  Looks like Bob uses a lock washer on the top and probably bottoms out at the bottom with the additional bottom nut providing lateral movement support.

I know this mod isn't the most important but I figured it would be a little more doable than making some other internals.  Last night I broke my groove cutter holder.  Now it's broken, I hate to part with it.  I have one on order but it will take a couple weeks.  Then I'll be grooving again.  I have a non-carbide, HSS cutter but that doesn't work well on this.  Maybe I can use the HSS for the FT valve, which I still need to do. 

When cutting and facing, should I

wrap

wrap

the valve half with tape to prevent damage in the lathe chuck?  Or what is recommended?

For the piston, I do need to check if there is enough diameter at the threaded area (minor) to even cut a crotch (forgot that key part).  If not I'll just turn it down and take this advice and thread it.  I got some more very large screws (cheap) today 5/8" diameter that would be easier to do a one piece cause I could put the oring groove anywhere on the shoulder.  Still a lot of work and stupid time-consuming work at that.  Don't worry you can tell me that.  I can take constructive criticism.

[Machinist]

Since you asked....

I'd use a material that machines quickly and easily.  Why work harder than you have to? 

As far as smashing threads in a chuck, how hard do you have to really clamp on the thing?  Light marks on the threads won't matter as long as the parts screw together right?  You could protect the threads with a threaded sleeve if you think you need to.

Steve

[rsterne]

Both the fork and piston are threaded 1/4"-20 and the lock nuts are just there to prevent either end from "self-adjusting".... The joiner is a short piece of "ready-rod"....

Bob

[BigBird]

Did a valve, not THE valve, a 760 valve for practice.  Hmm, 760 valve, I wonder, can I Tap That?  Are they the same size as a 1322?

Blue tape on both ends of the valve, I had to put it in the lathe to grip it to get it apart.