Alternator shopping: boss mount vs case mount

[nichzimmerman]

I've had a hard time determining the style of alternator mount on my O-540, and I havent found a single forum online that thoroughly and pictorally describes the difference. It would be nice to have a few pictures of each style so future shoppers would know whether to select boss mount or case mount. So which is this one? That two-bolt mounting pad looks lower than some case mount pads I see online. Thanks.

Nic

[David Swartzendruber]

Nic, your engine will use a boss mount bracket for the alternator. The two-bolt pad is the "boss" that the term "boss mount" refers to. A "case mount" bracket attaches to the case using the case bolts that hold the case halves together. An engine that uses a case-mount bracket appears to have no provisions for mounting an alternator bracket. I included some pictures of a case mount bracket and a boss mount bracket stolen from the B&C website.

[nichzimmerman]

Thanks! Thats very helpful. I figured as much regarding the boss mount, but couldn't find pics of a case mount setup. And, I read that most narrow deck O-540s like mine were case mount.

[David Swartzendruber]

I've heard that most of the time, boss mount correlates to wide deck and case mount correlates to narrow deck but I don't know how extensive the exceptions are. The picture of your engine clearly shows a boss.

[rodsmith]

My engine which started as an O-540B4B5 narrow deck has the boss mount.