MECHANISM, PERCUSSION

551 551 66D MECHANISM, PERCUSSION is essentially the main housing for the percussion piston in your rock drill. It’s where the piston cycles back and forth, building up and releasing all that energy to hit the bit. Think of it as the core chamber that directs the hydraulic flow and keeps everything aligned for that hammering action. Without it, or if it’s compromised, you don’t get proper impact.

When this cylinder starts to wear, usually what happens is you’ll notice a drop in your penetration rate. The impact just isn’t there like it used to be. Sometimes, you might even hear a different, duller sound from the drill head, or feel inconsistent impact energy at the bit. If the internal surfaces get scored or out of spec, the piston won’t seal right, leading to pressure bypass and a significant loss of power. You might also see increased oil temperature in the percussion circuit because of all that internal leakage and friction.

Replacing one of these isn't a quick job, so you want to make sure you’ve diagnosed it properly. When you do swap it out, always, and I mean always, check the condition of the percussion piston itself and the accumulator diaphragms. A worn piston can quickly damage a new cylinder, and bad accumulators mean the new cylinder will be working harder than it should, shortening its life. Also, make sure all the internal seals and O-rings are new and properly seated – a tiny nick can cause big problems down the line.