I had similar issues with B&C that they were going to fix for me but it's been over a year now with no fix.
I had an IO-540 narrow deck and a new IO-390, and had plane power 70A and 60A respectively.
I then bought brand new B&C BC460-H alternators and tried to upgrade.
First of all, for builders that haven't taken the time to actually look CLOSELY and measure these alternators, you're doing yourself a big disservice in alternator belt wear, and I can see how someone may say "Mine fit fine". This isn't something you should just eyeball, but you need to really measure.
On my planes, of them fit fine, the other did not, and was off by a lot.
I had a lot of back and forth with B&C about it, and ordered their other bracket, and that shifted it too far the other way. I gave them the proper measurements, and months later, when I thought they had actually listened to me and understood the situation, with them saying they would send a fixed bracket, I found out they indeed had not been listening. They DID have other complaints on the IO-390 with aligment, so it wasn't just me.
Their fix was to send out new alternators, believe it or not. They didn't do the simple thing of modifying the bracket and making one with the right hole spacings, they modified the alternator body, specifically the mounting swingarm lug dimensions, to machine it differently to make it fit.
I was very skeptical, but when I mounted it on the first plane, it fit perfectly.
But guess what? If you fix it on that plane, that means the other plane will now NOT be perfect. Sure enough, that was the case. Now the other airplane was misaligned.
The only thing I could do was, pick one of the 2 alternator styles, and oblong the holes in one of the brackets, so that it would work. I did that in early 2020, waiting to hear back from them on a fixed bracket. Then COVID hit, and I figured they may take a while.
Then, last summer I bought a brand new wide deck IO-540 and swapped out my old engine. Guess what. Yeah, my brand new IO-540 was now poorly aligned just like my brand new IO-390. And, neither bracket they sell made it fit properly.
If you have not checked your B&C 460 series alternator, I'd suggest you take the time to check it out. You may think it's aligned but if you have any new lycoming, it probably isn't.
As for B&C, they never contacted me again about it, and at this point I have both planes with doctored brackets. I don't like the fix because it was almost impossible to hand file the holes perfectly and now I have slop in the alignment and have the worry that things may not stay aligned if there is any movement of the bracket. I would love to have them just make correct brackets, for either alternator (they produced BC460-H an BC460-1 variants with different dimensions)
At this point, it's been about a year since they dealt with it and I'm still waiting today for a fix. My PlanePower alternators were aligned just great. It may well be that some day I go back to PlanePower. It's hard to pick a vendor when you don't have confidence that they are listening when you tell them things don't line up. My theory is that most people just slap the alternator in and don't look at it...and there are tons of these out there that aren't lined up properly.
If you measure it, on mine you find that the center of the drive belt hits about .093" or so off from the center of the pulley if I remember correctly. And that puts the V of the belt not at all aligned with the V of the pulley
I have another RV friend who put one of them on one of his planes with an IO-540 in it as well, and his suffered the same misalignment. He didn't believe me at first. I told him my alignment was off, and he claimed his fit perfectly. He thought I was crazy. Then when he finally had the cowl off one day, he measured it and found indeed his was off by about the same amount as mine.