Hole in tank too small for 5/16" bolt

I'm looking for a smaller diameter brass bolt for the bowl-to-tank connection. One hole accepts the new 5/16" bolt just fine, but the other hole is too small. The old hardware is smaller diameter but its really corroded and I would like to replace it.

I can't seem to find a bolt with a large enough flat head on it to get a good seal with the rubber washer. I've tried using a stainless washer on top of the rubber washer but water get in between the bolt head and the steel washer. I could use another rubber washer in that joint, or maybe JB weld the steel interface.... Well you see where this is going.

Any advice where I can get the right size bolt?

 

They come in 1/4" too, if that helps.

 

American Standard uses flimsy little bolts if you can find some.

 

Use a stainless washer over the leaky new one

 

You can probably enlarge the hole slightly with a Dremel tool and a silicon carbide or diamond grinding bit. Use a bit that is too small to fill the hole so you can't wedge it in.

 

Any chance the hole has calcium or mineral deposits around it making it smaller? I'd try a small file or something to clean it.

 

No its a Central Am. Std. Toilet that came with those cheesy 1/4" zinc plated steel bolts....
Hecho en Guatamala we have outsourced toilet manufacturing to a contry where most of the population doesn't have one!

 

Any decent hardware store will carry 1/4" brass round-head screws and nuts to match, SS washers, etc.