Virgin 1377 Build

[MichaelM]

Ok this is going to be an ongoing thread over the next few weeks or so as I build this 1377 from start to finish.. everything will be fabbed up/machined in house.. I had a 1377 as a kid in the 80's (great old sliding breech rear cocker...) and for the last couple years since I been back into airgunning I have wanted to get another one.. I have done a massive amount of research over the last year or so and have formed my own opinions based on research and have finaly set it all down on paper and got a virgin 1377 for 40 bucks a few weeks ago off amazon AND NOW THE BUILD BEGINS!!

lets first start off with a pic

Virgin RIGHT out of the box 1377


I plan on keeping this in a pistol format so..
Build plan as I have it so far
Reduce headspace in the pump assy by..
  Flat top valve and adjustable flat top piston (piston will be 6061 alu to solve melting/ heat issues)
  Secret mod that I have not seen/heard of being done so we will see if it works/has any effect.....(will share once its done )

Reduced valve volume
  Not going for ultimate in power but eff in the lower pump range as I am keeping this a pistol format anything with more volume would just be blown out the end of the barrel increasing noise so I am going for effeciency...

Pinned Valve
  Even though I am not going for big power with a stuffed valve the pressure will still increase rapidly... after researching alot of the pinned vs non pinned threads there DOES seems to be a trend with the ALU valves that by pinning for stiffness(not so much for safety) there seems to be a decent increase in FPS... that and I figure it cannot hurt.....

Extended bolt probe
  I plan on experimenting with extended probe with the stock bolt/breech untill I get my custom breech built and then it will have its own extended adjustable probe...

Highraise Breech
  The breech I plan on machining is going to have a slight raise about 1/8 inch or so so a 3/4" shroud can be installed if wanted... it may or may not be a multishot but if it is  it will be made for the 5 shot stick mags that crosman makes for most its basic pump rifles.... there will not be enough room to do a repeater with a m-rod mag with such a short rise in breach height....
  ALSO it WILL be ambidextrous with a rear cocking bolt handle that can simple be rotated for left or right handed use..... I am a left hander so really the stock righty breech layout is good for me in a pistol BUT I also plan on this breech design to be a basic platform for other models with slight mods to the breech..
  "Stubby" length the basic breech will be "short" at about 4.5 inches in length... a longer breech can easily be made from the basic design by simply adding a inch or so to the front barrel clamping area for those that want a longer breech/dovetail..
  Barrel clamping design... the barrel will be clamped into the breech... setscrews can mar/dimple the barrel and even malform the bore... and a clamping setup is MUCH sturdier anyways then a few setscrews....
  The breech will be made from Fortal HR aluminum.. this stuff is amazing and stronger then alot of steels... its a expensive very cool 7075 T651 series of aluminum that is basicaly twice as strong and almost twice as hard as 6061... It machines like a dream and makes all other aluminums feel like butter in comparison.... you can actualy take a plate of the stuff and hit it with a mallet and instead of "donk donk" like most alu it rigns like a bell.... anyways its VERY cool stuff

Front Plug/Pump Pivot
Once the breech is made I will figure out something that matches the general shape of the build lol....


And last but not least is the general replacement of screws and pivot pins and such for better/and or stainless counterparts... AND maybe try my hand and forming some wood into a pleasing shape lol......



  Ok so I shot her a couple dozen times to help work out the gunk and get everything seated decently for a chrony test... I took an average of 3 shot across the chrony at each pump level from 1-10 then did a 12 14 and 15 pump as well for the giggles.. all shots where with a break in between to start out with a "cold" system to be as fair across the  strings as possible. Its really cool to see the pattern laid out and how the EFF/pumps drops drastically as you go

Pumps  FPS
1          198
2          299
3          359
4          402
5          435
6          467
7          489
8          510
9          524
10        543
12        558
14        578
15        590

Interesting parts is right at the 9-10 pump mark you can REALY notice the heat buildup... a second or two after that last pump the heat would hit the pumptube and you could feel the tube get considerably warm.... at 15 pumps it would get rather warm and this was all done with smooth even pumps...

Looks like I have a healthy example to tear down and build

[MichaelM]

Ok teardown and inspection plus flattopped piston

Ok so I got onto tearing her down and inspecting averything so a couple things to note

I think I definatly NEED a extended probe.... look how much of the transfer port is covered by that pellet not to mention the damage that HAS to be happening to that skirt as high pressure air slams it from the side.......



this is one of the newer valves with the nose trimmed to match the piston cup better


and as you can see it actualy seems to fit very well to the piston as you can see from the imprint of the nose onto the piston cup.... so for the couple people I have read other places saying the flat nose is a manufacturing mistake... I say bullcrap... I think crosman made the same easy design change to better fit the cup/piston that I have seen quiet a few home modders do themselves....



Now to the only machining pic... lol I pulled the valve apart and prepped/cut it to its flat top... the alu used is VERY soft but seems to machine well with a good sharp bit so no problems there..... and here is the new flat top valve!



since I am stuffing the valve for more eff from lower pumps I am not going to open up the transfer port any..(maybe even experiment with smaller???) but I will angle the transfer port to give a smoother pathway for the air.... I may loose a FPS or two overall bt angeling it from extra volume but I think I will make up for that with more consistant shot to shot.....


thats it for today... I have all the material ordered to make everything and just waiting on its arrival to start on the piston... (HOPEFULLY by the beginning of next week if not this weekend!)

And PLEASE if anyone has anything to add or something I may have overlooked PLEASE pipe in and let me know what you think!!!!!

[fredfunk]

Very cool thread... I will be watching with anticipation! You don't waste any time either
I got a free 1377 in an airgun depot contest a couple years ago and I made some custom wood grips and an aluminum breech/extended bolt probe. It was my first machining project, and there is always room for improvement so I'm looking forward to stealing some of your ideas for sure.

[AmBraCol]

I look forward to following this thread.  There's an old 1377 kicking around that gets tinkered with now and then.  There are several things I'd like to do it it so am always looking to see how others' projects turn out to see how my own ideas might be on track - or not.

[rsterne]

I notice that your F-T-Piston has a fair radius on the nose.... You might want to face it off flat again and cut the O-ring groove only about 0.020" from the end to reduce headspace as much as possible....

Bob

[MichaelM]

Bob I prolly should have taken a side pic think its a trick of the light the face is absolutly flat but the o-ring groove is .030 from the face.... think I should go a touch thinner?  take another .01 off? I was actualy a bit scared at .030 thinking it was thin lol

[Gertrude]

I still have my old sliding breech rear cocker, but it needs a rebuild.
I recently picked up a near new 1377 on local CL for $30, yay!
I'll be watching this thread too ! Thanks !

[rsterne]

No load on the front of the groove.... it only has to keep the O-ring from falling off.... Don't drop it though..... *LOL*

Bob

[MichaelM]

No load on the front of the groove.... it only has to keep the O-ring from falling off.... Don't drop it though..... *LOL*

Bob

LOL bob yea! and I agree with the no dropping it going that thin ... that being said seems there is ALOT of deadspace in a few other areas I have not seen anyone address and I am looking to see what can be done about it

[MichaelM]

Ok I promised updates and here it is one!! lol and for those that requested more machining pics....  well this update should keep ya happy.....

OK today is the flattop piston and build,

First I chuck up some 5/8 6061 rod then I face the end square and center drill and bore it to 5/16 so I can tap it for the 3/8x16 rod I will use... I essentialy do this two times... one for the head and one for the tail of the pump setup..





I do this once and part it off for the tail then do it again for the head but also start the oring groove then part it off



I didnt get a pic of the next step but that was essentially putting the head back into the lathe and machining the oring groove to final depth and width and then facing the "nose" of the peice to final thickness(about .030)

next is over to the mill I throw the tail end into a square 5c collet holder this allows me to do all my machining for the pump arm groove then just flip the holder and be exactly 90 degrees to the slot when I drill the hole for the pin that holds it all together.... these 5c holders are just the things for quick repeatable setups...

here we are finding center of Y axis with an edge finder and my DRO(also a couple great pics of my awsome adjustable vise stop I made )

Spinning it up and looking for that edge


There it is..


and now for the other side



so now we hit the 1/2 function choose y and move the table till we are zeroed out



now I start cutting with a 1/8 bit(unfortunatly I had to go slow and easy because all I had that small is a 1/8 ballnose cutter.. A square end mill would have been more effecient and then used the ball nose for the last bit to finish up the cut.. Or if the 1/8 square  end mill was long enough I could have turned the part sideways and just cut it in..... either way this is what I had so its what I used )





Next simple lay the peice down and flip it 90 degrees with the holder find the edge on the X axis (we didnt need to find Y again as with the collets being indexable we are still good to go) and drill the pin hole





Once we find that edge we set X to -0.10 (tip of the edge finder is 0.20) now we move X in from the end about 3/16 of an inch(.1875) and drill us our hole for the pin..





I also put the head of the pump into the mill and milled 2 flats to help with adjusting and tightening the system up......(no pics of that lol )


next do alittle deburring and fit everything together...

the rear does NOT have a locknut like I see most of them have... instead it was thoroughly degreased and the threaded rod was locktited in place and tightened up...... then the head and locknut is threaded on... once the locktite sets up everything was put back together and the piston adjusted correctly so it JUST snaps over and holds itself closed well with no air in the gun.... Not to long that its stressing the tube holes and not to short that the pump arm just falls open.....







and here it is with a little 303 stainless cross pin I made to make it much easier to take it apart




I have not done any real chrony testing yet that will come a bit later today-tomorrow.... but I have been shooting it a bit and a few pellets thrown over the chrony looks like I have picked up between 30-40 FPS on the top end though I still need to check and make sure my transfer port isnt leaking and such

[Bentong]

Now you had a pellet inside...need to change your title to "ex Virgin 1377 Build" Nice build on the FT piston. Does the back of the piston scratch the tube? I must have been used to seeing delrin guide FT pistons. :0)

[MichaelM]

Now you had a pellet inside...need to change your title to "ex Virgin 1377 Build" Nice build on the FT piston. Does the back of the piston scratch the tube? I must have been used to seeing delrin guide FT pistons. :0)

so far there has been no rubbing EXCEPT from the front of the gun to about midway down the slot itself which would be a bit expected and doesnt/wouldnt effect the sealing parts of the tube.......

after doing alot of research I decided to go with 6061 instead of delrin(delrin WAS my first thought) because delrin has some heat issues and alot of people have had/noticed heat damage to the front of the  piston(think even our Mr. Stern has mentioned heat problems with his experiments as well...) I also kicked around the idea of a hybrid piston where the front is aluminum and the rear being delrin but there have been reports of wear on the pivot pin hole on the delrin pistons as well....

  now that being all said I am still looking to maybe add another o-ring to the rear part of the piston or maybe if I dont like that widen the oring groove about 1/4 inch and make a "split ring" of delrin that would snap into the groove and help the rear of the piston stay smooth in the tube....

 when I first decided to go all aluminum this was part of my plan and I just need to figure out the best way to do it now that I have the actual piston fitted into the gun and seeing where it would help the most....

[MichaelM]

Update Chrony Results

ok I ran a bunch of pellets through to get the system broke in and then like the first test took 3 shots at each pump # and averaged the results and this is what I got

Pumps   FPS
1          197
2          295
3          361
4          412
5          449
6          481
7          508
8          531
9          554
10        576
12        595
14        617
15        631

as  you can see I picked up alittle over 40 FPS on the top end but what I find interesting is at 10 pumps I am a full 4 pumps faster over the stock piston/valve setup ... there is a massive amount of wasted air as the sound levels start getting very high in the upper pump range... would be neat to see just how much more effecient this mod would be with a longer barrel... personaly with the stock 10 inch barrel I feel that anything over about 550 fps your really starting to fight a losing battle as the barrel just isnt long enough to use all that extra air.....

 Will see if this seems to hold true when I extend the bolt probe.... once I do that(next couple days) and get another chrony run in it will give me a better picture of what I will be trying for when I stuff the valve... would be very nice to reach the goal with as little as 5 pumps..... we will see....

[rsterne]

You are correct, a longer barrel will increase the velocity significantly at higher pump numbers.... Any "pump and dump" gun wastes about half the air when you get close to the point of retaining air (but don't).... For good efficiency, you want the valve to close when the pellet is only 1/3rd of the way down the barrel or less, certainly before 1/2 way.... but that's just not what happens in a 13XX unless you design for it.... What is your goal?....

Bob

[Hanabata808]

Great build. I love all the pics you took. You have a really nice setup!

[MichaelM]

Hanabata,
Thanks I have put alot of work into building my little shop up over the last few years!!!

Bob,
I dont have a set speed goal in mind per se.. I am looking at this as a learning experiance with a specific build idea in mind..... my first post pretty much lays out the build idea and why I have decided to go the routes I am taking...

   BUT if I had to sum it up I am trying to get the most speed out of the fewest pumps while keeping the 1377 in a pistol format.... and keeping it on the smaller side is whats fun and challanging with the build

[MichaelM]

ok another update,

today I took a few and got the extended boltprobe made up.. I machined off the nose till I had a shoulder about .030  to hold the oring in place... then drilled it 1/16  added a bit of 1/16 drill rod, then I machined a touch off the back and now just waiting on it all to setup before I put it back together and test





seats a pellet nicely and clears the transfer port


and still leaves me enough room to load a pellet

[AmBraCol]

Are you thinking of eventually changing the breech out, or leaving it standard plastic?  By the way, thanks for sharing this build with us.  It gives me a few ideas for my own, someday.

[MichaelM]

Are you thinking of eventually changing the breech out, or leaving it standard plastic?  By the way, thanks for sharing this build with us.  It gives me a few ideas for my own, someday.

I will be machining a new breech for it

[MichaelM]

update,

Ok curious chrony readings with the extended bolt probe I am not going to list all the pumps#'s like before but a sample to give an idea of whats seems to be happening.... Basicaly at some pump levels performance is the same but at others I am getting an approx 15 fps gain but its kinda a slow gain to a slow drop back to a slow gain.... over about a 5 pump cycle.... for example

pumps    pre extended  post extended
5                449               460
10              576               575
15              631               647

 dont get me wrong I think the #s with a stock barrel and only a flattop piston/valve setup are good #'s but with the extended probe its kinda a puzzle..... I have some theories as to why this is happening but I need to experiment more......

Essentially I am thinking the extra space between the valve and the base of the pellet is letting the air expand more and with the shorter barrel your getting more of the detrimental effects... where a longer barrel and more pumps would benifit from the better flow the lower pumps/shorter barrel that extra space is causing the weird up down on the speeds... next experiment is to get my hands on some stock 1377 bolts and build some extended probes with with the same over all length but with progressivaly shorter noses ... from full sized stock nose with a short probe to the full on shoulder for the oring and longer probe like I built... see what happends with the different probs at different airflows and barrel lengths.....