On an older bike I have with a Sugino crank, I was wanting to replace the inner ring of a 52/40 with a 38-tooth inner ring I picked up a few months ago. The old bolt heads are 8.5 mm diameter and I just noticed that the new ring takes fasteners with 12 mm heads.

Are there any fasteners/adapters available that'll make this new ring work? I tried my LBS to no avail.

Thanks!

That's a tough one. I would first go to my local hardware store and search through the washer bins, crank and ring in hand- you may get lucky. Barring that, I would probably just live with the 40 tooth ring.

I'm not sure what you are describing. Most chainring bolts require a 5 mm Allen wrench. There are some very cheap cranks that have the chainrings riveted together and/or to the crank arms and these can't be replaced.

Here's a photo, not good but adequate. Old inner ring (8.5mm) atop spider, new one (12mm) atop all, current fasteners, apart and together atop old ring.

^ Other than machining some special sleeve inserts for each 12mm hole (to bring it down to 8.5) on a small jeweler's lathe, the only thing you could try is to make little rings out of the right diameter steel wire.

So 12 minus 8.5 equals 3.5 and divide that by two, you get 1.75mm. If you can find ordinary galv steel wire of 1.75mm diameter, you're in business. The way to make rings is to wrap the wire around the shank of... say a 9mm drill bit so that you end up with something that looks like a coil spring. Then place the coil on a vice and use a fine hacksaw to cut longitudinally from one side until you get the number of rings that you need. Make the spring long enough so that the vice can have something substantial to grab even after the number of rings you need fall to the floor. You'll have to twist the rings a bit so that it lines-up at the joint.

Might be worth a shot...

.

It sounds like your crank uses the old TA standard for chainring bolts.
https://www.specialites-ta.com/produits/trans_gb.htm
https://www.ebykr.com/?p=47