flat top piston mod - expectations
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Topic: flat top piston mod - expectations (Read 570 times))
HOSPassassin
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Posts: 335
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Real Name: Brendan
flat top piston mod - expectations
«
on:
July 14, 2022, 02:41:37 PM »
I just bought another 880 for the purpose of experimenting with mods so that I can gain some experience before I try anything with the guns I have that shoot really well already.
The new one shot 7-grain RWS Basics at 726 fps on 10 pumps and 766 on 14. Further pumping gained no further velocity. The accuracy with the Basics was acceptable at 10m, so I decided the gun was good enough to keep and to try flat-topping the piston. I bought some high-psi epoxy and sandpaper from the hardware store, tore open the gun, and gave it a go. I am not sure that the dimple in the piston is perfectly flat, but it is more filled in that it was when I started. After it hardened I sanded the face of the piston, cleaned it, put the gun back together, lubed it, and gave it a chrony test.
Grr. It seems that I have lost about 10 fps per pump from 1-10. Now it hits 713 at 10 pumps, but I gained 16 fps at the top, going from 766 to 782 on 15 pumps. Woohoo, I guess? Baracudas shoot at 678 on 15 pumps for 10.9 fpe, so I guess that's good? I'm not feeling it.
Unless I have done something wrong, in hindsight modding the piston seems to be more trouble than it is worth.
What is a reasonable expectation for this mod? Is it critically important for the piston face to be perfectly flat?
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USA, OH, Columbus
Nukall856
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Posts: 470
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Real Name: Nukall
Re: flat top piston mod - expectations
«
Reply #1 on:
September 04, 2022, 01:41:32 AM »
I used JBWeld to get the Limiter Slot filled. Sand paper to make the head flat. When i first put the piston in it was touching the valve so i removed more jb from the top. Also i took out the piston oring , cut a strip of teflontape as wide as the o-ring, placed it in the groove, tight 1round plus just a little overlap to complete the seal. Replace oring. You should feel an increase in pumping resistance. Also you should gain abit more fps fpe with the teflon oring shim.
NVak.
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pblawler
Squirrel and Rabbit
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Real Name: Patrick
Re: flat top piston mod - expectations
«
Reply #2 on:
September 13, 2022, 01:58:23 AM »
I bought a 880 to design a few parts for and tinker. I'm going to trade it to a friend when I'm done. I am curious about using Alumiweld on the piston head and barrel to valvebody interface. I ordered a spare pump head assembly to give it a go. They sell sticks of it at Harbor Freight. Looks like it should work good. I've seen it used to braze dissimilar metals too so fingers crossed.
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USA, AL, Falkville
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pblawler
Squirrel and Rabbit
Sharp Shooter
Posts: 632
Know where I hit'em by the sound of the whaap!
Real Name: Patrick
Re: flat top piston mod - expectations
«
Reply #3 on:
November 11, 2022, 07:35:17 PM »
This is probably a better solution than modding the factory piston and probably within a dmfew dollars of the cost.
https://www.gatewaytoairguns.org/GTA/index.php?topic=203896.0
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flat top piston mod - expectations