PPS STS - Shock Transfer System for M4 Rifles

I've been using this STS for some time in my M4. The shell cracked under the 120 spring. Found that the retaining bolts that thighten the little pistons stripped and let free the hole assembly. Tried replacing the screws but that didn't work. Finally replaced both the little pistons and springs with two 4mm bolts cut to size. The upper and lower are a very snug fit but hold the cracked shell.