BOS Deville air spring rattle

My venerable old BOS Deville was rattling, and it felt harsh (like it ramped up far too quickly). I pushed down on the fork and it went “clunk”, and then again as I let it return. It wasn’t the sort of crown/stanchion interface creak I’ve heard on other forks – more of a clunk coming from inside the air spring side.

I searched the web. There’s no service manual floating around. Very few people have posted videos or service notes for this exact model of fork. I found various promising forum posts, but none of them answered my question. Others had what appeared to be the same issue, but (perhaps predictably) no solution to it.

I pored over pictures of the dismantled fork, trying to figure it out. I ended up trying to make head or tail of a video in Russian showing a complete disassembly of the fork. That proved to be the key to the mystery.

When I took the top-cap off the air spring, there was a beautifully machined metal part which looks a bit like a trumpet. It sitting on it’s own, at the bottom of the air chamber. It’s supposed to be attached to the top cap. I’d never noticed it when servicing the fork before – perhaps because it was still attached where it should be. And to be honest, I was far more interested in sorting out the other side of the fork – where nearly all of the oil goes.

So I fished it out (probably the hardest part), re-greased it, and re-connected it to the top cap. Put it back together, pumped it up, and went for a ride.

The rattling stopped and they’re back to feeling nice again. Almost too plush, but I can work on that.