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PostPosted: Fri Dec 17, 2021 7:03 pm 
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I'm tasked with reviving a Star ST901 for someone who would really like to use this since it was given to him by his late grandfather, and trying to identify and locate piston seal and breech seal.

The breech seal *might* have been leather originally, and is not present anymore, it's unusual (to me) because the breech block is angled since it mates with a transfer port that begins inside the centre of the compression tube then angles upward to meet the bore. The breech seat area is .510 o.d. and .312 id and height is ~.187 upper and ~.130 lower. Ideally a urethane or something more robust than leather would work but a leather one that could be trimmed would do for short term. Viewing a EVP of a Diana model 24 shows similar breech style and similar piston with integral sear but I have no idea those dimensions nor do I have access to one to measure.

The original piston seal was leather and held by a 5mmx0.80 bevel head screw and cupped and beveled washer. Original screw ~.193 dia at threads, sear is .393 od.

I've contacted a well known Canadian supplier who had a very appealing idea to convert the piston seal via a kit that would use Gamo/Crosman seals but I'm unsure the new screw, possibly being larger diameter, would leave enough material for sear strength after drilling and tapping, assuming the material is of a hardness that won't break a tap. I don't yet know an accurate thread size of that new screw so maybe nothing to worry about. The supplier also carries a Industry Brand kit that likely provides at least some parts for many old Chinese rifles but that isn't confirmed/denied yet that it will work here.

I could re-use the mainspring since maximum performance isn't a priority (owner has PAL) and above mentioned supplier indicates he has a quality spring solution, so deciding on converting a piston seal and aquiring a breech seal remains - I'm unfamiliar with these older Chinese guns and sourcing of parts for them, this one looks like a good candidate for a nice and proper restoration but mainly wanting basic functioning at this time.

Spring info in case useful to someone else: .740 od x .495 id , .118 wire , 34.25 coils from ground end to ground end , 8.437 L .

So I'm appealing to anyone who has already solved these challenges or has suggestions and hoping that this hard working, highly intelligent, extraordinarily talented, and uncommonly attractive crowd can provide some solution(s).
:)

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PostPosted: Sat Dec 18, 2021 6:31 am 
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Location: Eastern Townships
I have one of these fine rifles, and had to do exactly what you need to do. I think the work is well worth the efforts and time, as this is the best quality Chinese rifle (for the era) I ever saw, period. It truly is a competitor to the Diana rifles of the same era (I'm thinking of the model 27 here).

The breech seal: Yes the breech is slanted like on a Diana rifle, and this can be a problem if looking for a synthetic replacement. I first tried making a wedge-shaped shim to back an Oring, but that didn't worked too well. I went back to leather, as it conforms easily to the slanted face of the breech, and all is good now. I used a piece of leather from an old leather weightlifting belt.

The piston seal: The original leather one was shot, being cut in a few places, so it needed to be replaced. I found out the compression tube is a ''standard'' 25mm size, so a Crosman piston seal works just right. I had to make a dovetail adapter and modify an Allen head screw, but that was pretty easy work.The screw is a common M5x0.8 size, 15 mm in length I think. The dovetail adapter indeed need to be domed, but just enough, so it doesn't hit the front comp. tube wall. I made sure the tube was well de-burred and smooth, and the new seal went in nicely. I re-used the original spring, as it was in good condition.

When I got it my rifle shot in the 300s fps, now with the new seals it's shooting at a very consistent 454 to 463 fps with JSB RS Exact 7.33 grains domed pellets. The barrels on these rifles are very well made, with excellent rifling, I'm easily getting 3/8'' groups shooting upright, unsupported at 10 meters.

Hope that helps, and feel free to ask for more details if needed :drinkers: .

Some pics:


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PostPosted: Sat Dec 18, 2021 9:51 am 
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Thank you airmec for the reply, I agree it seems to be a nice rifle and looks worth restoring, but just getting it going is all that's needed right now. Your pictures are helpful for ideas, especially the piston seal retainer. For me though I don't have a lathe. I welcome a quote on making a retainer/screw to mail if you're still making parts but I am unable to get very far into this one financially. If it something you feel like doing then let me know. Initially I was hoping to just find a kit, or parts from another Chinese or more common rifle that can work in stead. My easiest route would be to purchase a conversion kit from someone but I hesitate because I don't know details of the fastener ie. if it's greater diameter than the M5 screw then how much material will be left of the integral sear up near the piston head if I drilled and tapped? Or, could the screw in the new kit be turned down and threaded to M5x0.8 etc? Modding the screw diameter properly is likely more than I can do with an electric drill and semi-decent chuck and have the result I am hoping for.

Breeh seal; if I can find some .250 thick hard leather I could try making a punch from pipe but I don't want to spend excessive time and effort on this when I have som much to keep me busy already at home. Airmec if you have leftover materials and want to make me one or two and mail them I am receptive to that idea too, let me know.

Going by the spring specs I guess-timated it would shoot around 450fps and that would be just fine for now.

So airmec I don't have a lathe which means I'm still only able to buy something already made. This is the kit I mentioned (see below), and don't know the screw details, but typically a user drills a .312 hole for the screw to pass through and the sear is only .393. I suppose pre-recording the sear position then drilling out the sear from the seal side and welding the sear to the nut then that assembly to the piston *might* work if I do my part it's just a lot more than I want and am able to do right now.

https://scopesandammo.com/storefront/po ... hp?pID=851

It is difficult to say for certain, but if the head of the screw in this kit is nominally 5mm in height like an original Crosman piston dovetail then it appears the threaded portion diameter is about the same size as that 5mm head, then maybe it's an M5 screw?? Hmmm... I wish I knew, because if it's 6mm I probably can't rethread without a heli-coil type repair.

And... I cant tell for sure but the front of the Star ST901 compression tube might be concave slightly, if it is then I will lose some compression by using a 'flat' piston head/screw, if so then that migh remedied by a build-up of JB Weld then re-shaped with valve grinding compound around a silicone sprayed (as a release agent) hex key section to hold the shape and the left seal off, then finish the shape and reinstall the seal?? Ugh, I do hope it's a M5 screw... so tempting lol.

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PostPosted: Sat Dec 18, 2021 4:01 pm 
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There is another screw from the rifle that looks similar style to the one in that kit above ie. head is beveled and Philips drive, so I measured it's diameter at the threaded portion - it's a M6x1.0 screw, bigger than the M5 threaded hole in the piston and the M5 screw ( .193 od ) and... the "drill size" for a M6 is... .193, so maaayyybe it is possible to drill and tap for an M6 if the screw in the kit is in fact M6. This partially depends on whether the steel in the piston is hardened - I don't want to break a tap in there.

Just an idea... only recording my thoughts here in case it helps someone else.

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PostPosted: Sat Dec 18, 2021 4:51 pm 
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Location: Ontario, Canada
Spring: The spring measurements that you posted match the Crosman B-19-00-4B spring ($12.03).

Piston Seal: Possibly you could use the 1" Crosman B18-04-1B piston seal ($2.28).

Piston Seal Adapter: Possibly you could get someone to machine you a simple tapered piston adapter to use the $2 Crosman piston seal. You can take your original piston screw to a machine shop to get the thread size checked. The adapter could look like this with a thread to match what you need (see link).
https://target-tune.in/international/pr ... fullSite=1

Breech Seal: Possibly a #109 o-ring will work. Size is .505 OD, .299 ID, .103 CS but they stretch. You could use some silicone or hot glue under the seal to get the height you need. Forum member Ricksplace used hot glue to get his Ruger breech o-ring leveled out when the barrel was shortened.


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PostPosted: Sun Dec 19, 2021 12:43 pm 
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Phantom Payne, I sent you a P.M.


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PostPosted: Fri Dec 24, 2021 7:58 am 
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TCooper wrote:
Spring: The spring measurements that you posted match the Crosman B-19-00-4B spring ($12.03).

Piston Seal: Possibly you could use the 1" Crosman B18-04-1B piston seal ($2.28).

Piston Seal Adapter: Possibly you could get someone to machine you a simple tapered piston adapter to use the $2 Crosman piston seal. You can take your original piston screw to a machine shop to get the thread size checked. The adapter could look like this with a thread to match what you need (see link).
https://target-tune.in/international/pr ... fullSite=1

Breech Seal: Possibly a #109 o-ring will work. Size is .505 OD, .299 ID, .103 CS but they stretch. You could use some silicone or hot glue under the seal to get the height you need. Forum member Ricksplace used hot glue to get his Ruger breech o-ring leveled out when the barrel was shortened.



Thank you TCooper for that information. I hope to re-use the spring if it's able to achieve anything above 430fps since I need to avoid any cost I can on this one, it's not mine.

I hope to use a Crosman piston seal due to availablity and cost, the S&A conversion kit looks great but I was unable to order it since I never heard back about the screw thread specs. I found another kit that seems perfect at https://www.airgunsource.ca/us/vortek-a ... ent-s.html and it has the correct screw. Fortunately for me airmec was able to supply one as seen in his post above, it's on it way now.

The seal is going to be leather for the short term at least due to airmec being able to supply a quantity sufficient to create one, I hope I get it right the first time lol. But... since I really, really would like a synthetic solution of some kind I will scout around for an o-ring like you spec'd and experiment. The challenge is going to be getting any built-up materials at the correct matching angle to the face of the breech block, then getting the height to match whatever is used to provide "correct" amount proud to the face. Will probably just close up the barrel and use feeler gauges to establish the gap then attempt to end up .025 greater than the gap.

Busy times at home right now but as soon as the parts arrive I will get at it and hope to post a couple pics and results. This does seem like a nice rifle for what it is and I wish it was mine, lol, because I would like to spend more resources on it and re-blue it, but that's not the objective this time.

Thanks again TCooper.

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