The Top of the PE fire chamber (which is also the secondary burn chamber) isn't welded/bolted/wedged in place. It sits on top of a stainless steel ledge that extends across the back and both sides, with a single pin holding it in place at center rear. This allows the hottest part of the stove to expand and contract, (and even change shape slightly over time as metal stove components will), without putting undue stress on itself or the firebox. This was a pretty revolutionary idea 20-some years ago when PE's engineers came up with the design. Back then, most stoves, including the then-new "clean air" models, had welded-in or bolted-in baffles that caused considerable pressure on the firebox when the 1100-degree secondaries lit up, shortening the life expectancy of the internal components. We've retired many stoves over the years that were in perfect shape but for the burned out baffle, which, being a welded in part of the firebox assembly, was impossible to repair or replace. In the years since, we've seen several other manufacturers adopt some sort of stress-relieving engineering in the secondary burn area, but PE's "floating firebox" blazed the trail.