Author Topic: A question about Tapco vs stock gas tube and piston sizes  (Read 4938 times)

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Sgt_Pork

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A question about Tapco vs stock gas tube and piston sizes
« on: January 18, 2016, 02:26:08 AM »
I've read here and there that the Tapco gas tube and piston are both sized slightly larger in diameter then their stock counterparts.  I'm looking to confirm if this is so, because I have an SKS that seems to have a gas tube that is slightly too large for it's piston.  This apparent mismatch in size seems to be causing a failure to fully cycle. 

I tried the test where you hold your thumb over the end of the gas tube with the piston inside it, and then turn it vertical to see if the blocked air pressure causes the piston to slowly glide down (which indicates a good fit), or drop quickly with the normal speed of gravity because the air has room to quickly get past the piston head (which is bad - too much room), and it is the latter situation (too much room) that holds true for this gun.

All of which makes me think that this mismatch of gas tube to piston size might possibly be because the tube is one of the slightly larger Tapco replacement tubes, while the piston is still the slightly smaller stock one.

So my question I guess is this, are the Tapco gas tubes and pistons indeed slightly larger in diameter, and if so then by how much?  Are there specs out there somewhere on the Tapco parts vs the stock ones with respect to gas tube inner diameters and piston head diameters that I can reference to check and see what I've currently got?

If it is a larger Tapco gas tube mismatched with a smaller stock piston that is causing my failure to cycle, then it's possible that all I will need to do is put a matching Tapco piston in to get the gun cycling properly once again, but I'd like to be able to check the sizing before I start spending good folding money for a part I may not need.
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Rocketvapor

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Re: A question about Tapco vs stock gas tube and piston sizes
« Reply #1 on: January 18, 2016, 02:28:42 PM »
There's a good bit of info out on the different gas tubes and piston with the exception of the Tapco ones, or the NC Star ones.
The bore on all the examples of Chinese and Russian gas tubes I have appear to be right at 13mm (I measure 0.512" which is about 0.0002" larger than 13mm) with the pistons being a little less.
A piston diameter of 0.5112" is the largest I have and gives a really strong suction, while the smallest @ 0.5101" gives a weaker suction but still cycles fine.  Pulled another and 0.5115" is the largest out of 6, keep getting about 0.0001 or 0.0002" difference each time I measure :banghead:

Sorry, but no info on the Tapco dimensions.

My guess is that a small strip, about 1/4" wide, of plain old printer paper should bind up a piston inserted head first into a gastube.  More clearance than that and you might have a problem. 
Someone want to try that and report back?

 
« Last Edit: January 18, 2016, 03:09:29 PM by Rocketvapor »
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Sgt_Pork

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Re: A question about Tapco vs stock gas tube and piston sizes
« Reply #2 on: January 19, 2016, 02:40:20 PM »
Well I tried the paper trick on the gastube/piston in question, and I was able to just slip the piston in without binding.  At the mouth of the tube I get just slightly larger then 13 mm with my analog calipers (I guess need to join the 21st century and get a digital one for a more precise measurement), but deeper in I think it widens out some more.  Would corrosion inside the tube from poor cleaning do this?  If so then it might not be a Tapco tube size mismatch with stock piston issue, just a worn out stock tube that wasn't properly maintained...
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Rocketvapor

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Re: A question about Tapco vs stock gas tube and piston sizes
« Reply #3 on: January 19, 2016, 06:17:42 PM »
The very end may be peened down in diameter a little.
Rust inside the bore of a poorly maintained gas tube could also account for some increase in diameter.
Rusts, cleaned, rusts, cleaned, etc.

I still don't have any idea about the Tapco dimensions. 

And good analog instruments are still valid tools.
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Sgt_Pork

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Re: A question about Tapco vs stock gas tube and piston sizes
« Reply #4 on: January 19, 2016, 08:51:23 PM »
Well thanks for the info Rocket.  Still looking for that info on the Tapco tube (might need to email them), but your posts have been educational for me, and are much appreciated.
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