Long story, but I decided to replace the OEM t-stat housing with a new SS housing. Cross referenced and found that Merc OEM # 8M0061381 (2001 Gen VI) crossed to Sierra 18-1989 (SS housing). Specifically, stated that it fits the 502. I tried to mount it today and found that the mounting holes are not ithe same as the OEM and caused the mounting bolts to bind up and not tighten. The ss housing is wider from hole to hole. Has anyone else changed f-stat housing from OEM to stainless using the Sierra model 18-1989? Is it possible that it was cast incorrectly at the factory?

Ive used those in the past with no issues, are you mounting it on a stock mpi intake ?

Yes.

where did you buy it ? who makes it

Stainless Steel Thermostat Housing - Hardin Marine

I bought that same Sierra set up and had to ream the mounting holes a bit to get it to fit right. I had same issue with the bolts that were binding and wouldn't line up right, also has to use allen headed stainless bolts to clear. Keep in mind this was not the OE set up for me and I believe the 502 MPI used 12 point headed bolts too.

From Marine Parts Source in Tennessee, via their Ebay store. I called them this morning. Super nice. Their guy, Kevin, double checked everything and said the Sierra housing (18-1989) was the correct one for the 502 and is now checking with Sierra to find out what the deal is. I measured it last night hole to hole and the ss housing is wider than the OEM which is causing the bolts to bind up when tightened. Said they will suspend all sales on them till they can find out what the problem is, from Sierra. In the mean time, I guess I will go to plan B, which is to have the old OEM housing, which is in ok shape, blasted and powder coated and then hope for a solution on the new housing.

Not 100% sure, but I think that is the Sierra 18-1989 for BBC. I'm betting CPP drop ships it direct from Sierra.

How long ago did you get yours? I'm wondering if there could be a bad batch. By the way, that looks pretty dang sweet!! Mine is off by almost .080.

maybe you got a china knock off, one fits all

I bought it out of the swap shop new in the box a couple a months ago, not sure how old it was. When I googled it there were a few folks stating they had to ream it out as well.