I put a new coupler in my SC last weekend and noted serious grooves inside the housing where the action takes place. It spins freely, but what could have caused this? I'm probably losing some boost, but I'm not worried about that. Is it a sign that something let go on previous owner, or a sign of impending failure?

Sorry, no pic, but can describe it as hundreds of grooves 1/16 in wide, give or take, the full length of the housing. Looked like it was made that way if I didn't know better. Blades look fine with some buildup on them but the tips were clean. Gears were fine. Oil was nasty, never been changed probably, and the SC did not appear to have been cracked open before.

Chunks of carbon from your EGR valve scored the inner housing of the SC. We all have it.

I can grab pics off my spare charger tonight for you to compare if you'd like.

I've got two casings..one is nice with almost no grooves. The other has serious groovage.

No that'* fine. I just couldn't find anything about it in the forum so thought I'd ask. Thanks