Bicycle Mechanics - What size drill to use to bore hole for a 9/16" pedal tap

I know that taps recommend a specific drill sizes so that you cut the threads properly.

My googling is turning up lots of tap/drill size combos, but not for the cycling specific 9/16" pedal taps.

Anyone happen to know the recommended drill size?

Anyone happen to know the recommended drill size?

33/64" or .516" inches is correct and will yield a 73% thread which is the close enough to the 75% industrial guideline.

You can also use a 1/2" if that's what you have or if that's the limit of your drill chuck. However using a 1/2" tap will increase the tapping torque and load the tap up faster, so keep that in mind.

Here's a link to a nice site that list tap drills for most threads (http://www.newmantools.com/tapdrill.htm), and has a calculator for those not listed. Note you need to enter the decimal, not fractional diameter, and use 75% thread as this is as strong as 100% thread, but will reduce strain on the tap.

Pakk Tools makes a set of 9/16x20 pedal taps: http://www.parktool.com/product/pedal-tap-9-16-x-20-tpi-tap-6 They recommend the 33/64" tap drill FBinNY mentioned.

Remember you need both of these two taps as the nds pedal is left-hand threaded.

You don't drill at all if you are retapping 14mm threads to 9/16"!

I sort of want to know what the OP is doing that he needs to tap something

Taps a re tapered at the end, unless specifically a Bottoming tap.

measure the bottom of the V of the thread in the tap with your caliper.
then look for that drill size.

I sort of want to know what the OP is doing that he needs to tap something

Maybe planning on giving Mark Stonich some competition in the crank-shortening business?

Maybe planning on giving Mark Stonich some competition in the crank-shortening business?

That had crossed my mind but that seems a tricky venture without a good drillpress or milling machine

Upgrading the pedals on a one-piece crank?

I've got a bare aluminum Sugino triple crankset with 175 mm arms that is sitting around doing nothing.

I'm going to mount a 24t 74 mm BCD chainring to the inner position, then grind and file away the excess spider arm material, put pedals at about 120 mm, and cut and grind off the rest of the arms.

After final filing and a bit of polishing, this crankset is going to replace the Ashtabula crankset on my daughter's bike. I got a bottom bracket adapter from Harris cyclery.

Why, you ask?

Projects like these keep me from drinking.

And I'm intrigued by the concept of a high performing kid's bike. They use them all the time; they might as well have decent bearings and components.

Cool! Don't take it personally that our attention span is short and we tend to move quickly from the "how" to the "why." ;)

just looking at all my cranks and am wondering whether their is enough width at 120mm from the spndle for a pedal eye. but i suppose anybody that has enough ambition and knowhow to attempt it has already addressed this issue...

edit: then again it's not as if Eric Heiden is going to be riding it. :)

just looking at all my cranks and am wondering whether their is enough width at 120mm from the spndle for a pedal eye. ...

edit: then again it's not as if Eric Heiden is going to be riding it. :)

I think the edit covers it. there's probably adequate strength for a child, though not for an adult. Anyway children heal faster.

You know some guys swoon over some of those older triple cranks.

It does sound like a interesting idea though.

You know some guys swoon over some of those older triple cranks.

It does sound like a interesting idea though.

When she outgrows the bike, I'll take off the crank and put it on eBay and someone will be the only one in the Rivendell Reader circulation with a 120 mm Sugino triple, and they can regale their friends with the advantages of short crankarms, ("I barely even have to bend my knees!") as is suddenly popular.

33/64" or .516" inches is correct and will yield a 73% thread which is the close enough to the 75% industrial guideline.

You can also use a 1/2" if that's what you have or if that's the limit of your drill chuck. However using a 1/2" tap will increase the tapping torque and load the tap up faster, so keep that in mind.

Here's a link to a nice site that list tap drills for most threads (http://www.newmantools.com/tapdrill.htm), and has a calculator for those not listed. Note you need to enter the decimal, not fractional diameter, and use 75% thread as this is as strong as 100% thread, but will reduce strain on the tap.

Thanks! and thanks to HillRider, too.