Homemade High Pressure Pump.

[Mitchza89]

Nah its not that, the groove is on the top nut, not the bottom. I filled the bottom nut a bit too small. The piston off the shockpump wont stratch anything at all, it's polished aluminium.

[Hawkeye]

It makes far more sense to groove the bottom. The seal is formed on the down-stroke so it will work more reliably if the o-ring seals on a continuous surface. Otherwise the piston will seal when you pull the handle up. The air will still leak past at the top of the stroke but why fight a vacuum?