Right out of the box the plunger pin got stuck under the petal in the case. Oil and use didn't remove it. This means I had to unhook the air hose to stop it in order to change discs. In the end it turned out to be a fitting on the underside. Once removed, it opened access to the plunger spring. I took a thin piece of metal wire, pushed it through the shaft and pushed the plunger up to loosen it. A drop of oil on the piston and now it works. It doesn't have as much "power" as the orbital I had 25 years before he died. I am a professional finisher and I use my grinder every day. Let's see how long this lasts. You get what you pay for, it was definitely cheap.