[nickfracture]

Don't grind your cases. Just put the belt tensioner on there that's meant to be on there. You'll need a bigger (correct) size belt to go with it. The tensioner pulls the belt up and away from that boss as belt comes around from the front pulley.

Here's a pic of one with the belt and tensioner

I had a similar probelm to you a while back, couldn't figure out how to get the right belt tension. I needed a tensioner. I ended up making a DIY one from some bearings on a bolt, to work with a tin primary: Primary Belt + rotary top + round swingarm, too loose

 

[nickfracture]

Nick, I was thinking of doing that but I had read in a few places that that style tensioner wasn't recommended for closed primaries? Not sure if they meant an oil filled primary (in which case, you wouldn't want a belt anyway!), or if they were referring to a dry but still tin covered primary (maybe because of heat?). In any case, until I can do more research on tensioners, I'll vent the cover to keep the heat down, and just keep a cautious eye on belt condition 😀

If you had an oil filled primary, you'd be running a chain. In that case yeah, that would be the wrong kind of tensioner. Chains have the ones that sits below the chain, pushing them up.

The one I mentioned would be all good in your bike, open or closed. This is what they look like. It says "open belt" but they're for closed primaries too

 

[nickfracture]

Thanks for the additional info. And your post about building your own tensioner also rocks! Great job 👍

No problem man. Hope it helps! Let us know how you get on 🍻