Bicycles Asked by Nathan Knutson on January 1, 2021
There are some 5-bolt 110mm BCD double cranks out there (FSA Tempo Adventure) that put the large ring on the inside of the spider tabs. I want to figure out a way to use whatever 110mm rings I want with them so I’m not tied to a specific, hard to get, iffy quality replacement FSA ring. What’s the simplest way to cut a serviceable chainring bolt counterbore into a ring with an existing hole and counterbore on the other side?
What I did in the end was this:
Correct answer by Nathan Knutson on January 1, 2021
First, note that there are three different bolt holes on a 110 mm circle. There's a 5 hole pattern which is symmetric, and there are different Shimano and SRAM 4 bolt hole patterns (...because Standards!)
Fortunately you want a 5 hole, and there's only one common 5 hole format. (But as OJS points out, there's a 5 hole Campagnolo standard that is not symmetrical. It's likely the price of a campy part will make that obvious at purchase time!)
I would look for a countersink bit to fit your drill press, that is the correct side slope, and wide enough to make the hole at the widest end. Ideally it will have a hex shank, not a round shank because they tend to turn in the chuck and lack concentricity.
Now, onto the real chainring.
I suggest you get a couple of cheap used chainrings from your local ebay, or community bike fixup group, and have a go at drilling them for practice.
Other options:
A center finder might help, but they tend to be for milling machines with moveable tables and are overkill here.
You may choose to make a jig, perhaps two pins that just fit through the two adjacent bolt holes. The first lining-up would be no faster, but the remaining 4 should be quicker.
Some other thoughts:
Keep your existing original chainring as a template. Never ever throw it out.
Another handy resource for chainring shenanigans is the Wolf Tooth BCD PDF. Just print it at 100% no scaling, and check that the 1cm and 1inch test lines are exactly that long. A printout can be stuck to almost anything with spray glue in a can for ease of working.
If you can't find a good chainring candidate with the right 110 BCD hole shape, you may get away with drilling 5 holes around a 4 hole chainring, However you need enough metal to do the job, which might be hard to find.
Finally, if all this sounds too much, there's absolutely no shame in taking the job to a machine shop and explaining what you need. They would probably use a fancy endmill with the same side slope, and run it in a milling machine, perhaps even on a rotary table. It would take longer to explain to them, than it would for them to do the set up. I bet the drilling would be done in a couple minutes tops and won't cost the earth.
What NOT to do - this:
Answered by Criggie on January 1, 2021
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