Why do 1/2" pedal taps exist?

[xtinctionRebeller]

These are 1/2" taps for pedals:

When a crank arm is stripped by a 1/2" pedal, it wouldn't make sense to retap it with a 1/2" tap tool again because the threaded material is missing. If I understand correctly, the socket must be tapped with a bigger tool (e.g. 9/16") to ensure there's enough metal for new threads. Then one of these inserts must be screwed in with glue to pad out the extra space:

So I can see why 9/16" taps are sold in shops. There are also 1/2" taps and they're usually about 25% cheaper. My pedals seem to measure at 1/2", but all the pedals I'm seeing in shops are 9/16". So what if one of those 9/16" pedals strips? I would expect to find a tap size of 5/8". But I'm not seeing those.. so I must be misunderstanding something.

 

[markz]

5/8" tap - https://www.grainger.com/search?searchBar=true&searchQuery=5%2F8+tap

1/2" is for old style pedals, 1pc cranks usually.
https://sheldonbrown.com/pedals.html

Thread Sizes

Most pedals have 9/16" x 20 tpi threads.

Pedals for one-piece cranks are 1/2" x 20 tpi.

Older French bicycles used a 14 mm x 1.25 mm thread, but these are quite rare. French-threaded pedals are commonly labeled "D" and G" (French for "droite" and "gauche" (right and left). A French pedal will start to thread into a 9/16 x 20 crank (and vice versa), but will soon bind. Do not force it, or it will damage the crank. Aluminum French cranks are easily rethreaded to 9/16" x 20 TPI.

In the early 1980s, Shimano attempted to popularize a very large thread size, called "Dyna Drive." This system was designed to place the pedal bearing inside of the crank, so that the foot could be slightly below the pivot point of the pedal bearing. This was believed to offer biomechanical advantages, but the system was not a commercial success, and has been abandoned.

These threadings have 100 years of industrial inertia behind them, but none of them is ideal. A better system would have a conical surface between the pedal spindle and crank, to avoid fretting (wear resulting from small motions of the pedal spindle inside the crank). See Jobst Brandt's comments here and here.

The right pedal has normal threads and the left pedal, reverse threads.

 

[Balmorhea]

One-piece cranks use 1/2”-20 pedal thread, which is a standard UNF thread used in industry. That’s why such taps are relatively cheap and available.

Most other cranks use 9/16”-20 pedal thread, which is a special bicycle-specific pitch. Standard UNF 9/16” thread is 18tpi.

There’s no practical reason that one-piece cranks can’t use 9/16” threads, but they are made to a set of dimensional standards which is older than that of any currently produced three-piece cranks.

 

[xtinctionRebeller]

If 9/16" is the most common pedal, then why don't most bicycle shops have the 5/8" taps that would be needed to repair stripped out 9/16" sockets? Park Tool only makes taps sized 1/2" and 9/16" (not 5/8"). And why do so many bike shops have 1/2" taps, which would only be used to re-tap (non-existent?) 7/16" pedals? I suspect I'm misunderstanding how the taps work. Is it possible that people are gluing inserts in and then tapping within the insert? Or are people introducing solder into the stripped out socket then tapping that?

 

[speedmd]

1/2" pedals? Seriously, update the previous thread with what your loading is (your weight including back pack, shoes (10 pound hiking boots) and winter clothing and any other significant weight) on the pedals that your not able to keep snug. They (the vast majority) are good for no more than about 100 pound rider IMO. From the sounds of your post on the bearings locking up, they are well past their service life before you trashed the crank.

Bike shops know already that when you strip them out, they are not worth trying to fix in most all situations. Very difficult to get inserts to hold pedals straight never mind tap drill them straight and co linier with the bb axle without significant fixturing. They did you a favor. Get strong adult stuff that you can not bend easily just from standing on them and pushing a little. It's time!

 

[Balmorhea]

1/2" pedals? Seriously, update the previous thread with what your loading is (your weight including back pack, shoes (10 pound hiking boots) and winter clothing and any other significant weight) on the pedals that your not able to keep snug. They (the vast majority) are good for no more than about 100 pound rider IMO.

1/2” pedals are standard issue on beach cruisers, which in turn are favored by super heavyweight riders. I’ve seen plenty of bent one-piece cranks and pedals, but not proportionally more than three-piece cranks and 9/16” pedals. The 1/2” part is only for fastening, after all. If it’s properly tight, the part that fails when overloaded will most likely be the cantilevered portion that has the pedal body on it (which has the same dimensions as an equivalent 9/16” pedal). All my pedals that have bent or broken, did so at the inboard bearing race or just outboard of it.

9/16” pedal threads fail or strip much more commonly, because so many of them are tapped into aluminum rather than steel. Aluminum threads are weaker, and the clamping surface on an aluminum crank allows more relative movement.

 

[xtinctionRebeller]

@speedmd - the situation is much different than you realize. The cranks I kept destroying were on my previous bike and I don't know what size those pedal bolts were. I recently bought my current bike used, and it came with a stripped out crank arm from the seller. I am actually in the process of buying a mid-drive kit, which will come with new cranks. I have no choice in that.. I'll use whatever cranks come in the kit, even if they are 1/2". To complicate things getting the kit delivered is taking forever. So I might re-tap the drive side crank purely as a temporary measure until the other project is ready. And if the pedals fall apart again I don't care because the permanent parts are coming eventually. So if I re-tap the crank, break that, and then also break whatever arrives with the mid-drive kit, then I'll have an opportunity to buy what's needed for 9/16" pedal bolts.

I really just need my questions answered, for my understanding, because I'm concerned that I'll be buying the wrong tap tools for the temporary fix.

 

[Voltron]

Usually the only time you use the taps in a shop is when someone had the pedal unscrew, and then rip out the last thread or two. The tap can fix that. But almost no shop will actually want to drill a bigger hole and helicoil a totally messed up pedal hole. It's incredibly hard to get the alignment just right to where you don't feel the pedal oscillating underfoot with each rotation, and much cheaper to get another crank. Many modern cranks are hollow except for the spot where the pedal hole is, so again, not helicoil friendly.

 

[Voltron]

And unless it's a speciality bmx crank, the only thing that uses 1/2" is one piece crank, so if it didn't look like this, it's almost guaranteed to be 9/16"

 

[Balmorhea]

And unless it's a speciality bmx crank, the only thing that uses 1/2" is one piece crank, so if it didn't look like this, it's almost guaranteed to be 9/16"

opc-chainwheel.jpg

I’ve never seen a crank with 1/2” pedal threads that wasn’t one-piece.

 

[Voltron]

Here's one...

 

[Voltron]

They're not common though.

 

[spinningmagnets]

1/2-inch taps continue to exist because people keep buying them. I don't know why they would buy them, but maybe it's not for bicycle pedals?

I once read about a British retiree who made it his specialty to rebuild Norton motorcycle transmissions in his garage. He didn't do cars, or engines. He didn't do the transmissions for Triumphs, BSA's, Enfields, or any other motorcycle. He would custom-cast the new plain bearings and then machine them to a perfect fit. He needed tools that were not common.

The French threading standard is now considered obsolete. It uses a 35mm x 1mm pitch. Both non-drive and drive side cups use a right hand threading. The shell inside diameter is approximately 34mm

The Italian Standard uses a threading of 36mm x 24 threads per inch. Both sides of the bottom bracket are right hand threaded. Parts in the Italian standard are usually marked as such

 

[xtinctionRebeller]

These are often sold right next to each other:

• 1/2" pedal tap pair (left and right)
• 9/16" pedal tap pair (left and right)

They often have "pedal" in the name. Left and right threading is also characteristic of pedals. They're often made by Park Tools, a company that seems devoted to bike tools. They're sometimes sold with inserts, which implies the need for padding (which is expected when doing a pedal repair). So I don't buy the idea that bike shops are selling these for an obscure market outside of that context. They're selling the 9/16" ones in the US; so although the French are sometimes rethreading their obsolete cranks to work with modern pedals (9/16"), that's unlikely to trigger US shops to stock them.

I think Voltron gave the most plausible answer: that someone strips just ~2-3 turns of threads and the shop uses a tap to clear that out. As for why bike parts dealers aren't selling 5/8" pedal taps, Voltron suggested that repair shops aren't keen to risk having a pedal that's not 100% perpendicular.. or that it weakens the wall of the socket too much. I guess I'll roll with that answer. The important thing is that if I decide to attempt a repair, it's probably correct that I go up one size and use an insert. But since my crankset is 3 pieces, I'm not trusting my ability to eyeball the difference between 1/2 and 9/16ths when holding the pedal up to a ruler. So I'll probably scrap the crank repair idea anyway and just wait for my kit to arrive.

I was also originally thinking I should just have the tap tools on hand to deal with routine future breakage. But perhaps Speedmd's conjecture that migrating to bigger pedal bolts will stop the cycle of breakage is sensible. I've already got the torquing tools coming to make sure pedals are tight going forward (with plumbers tape as well). Guess I'll also start carrying a pedal wrench wherever I go so I can tighten on the fly -- waiting till I make it home before tighening the pedal is historically part of my problem.

 

[spinningmagnets]

someone screws up ~2-3 turns of threads and the shop uses a tap to clear that out

Aha!...I've done that, I think it's called "thread chasing" / "chasing the threads"? makes sense now...

 

[amberwolf]

fwiw, i've abused bike parts of many kinds, including both sizes of pedals, both on bikes and for other purposes, and so far haven't broken the threaded area, or damaged it, on either size, in any of the forms of abuse.

but i *have* broken the actual pedal spindle itself, just at the inboard bearing on the pedal spindle, at the stress riser of diameter change closest to that, each time with the 1/2" one-piece-crank type; though these were being abused in non-bike projects placing "unnatural" loading on them.

so i don't think it matters much, if at all, which of those size threadings the pedal/crank interface use, regarding strength.

The French threading standard is now considered obsolete. It uses a 35mm x 1mm pitch. Both non-drive and drive side cups use a right hand threading. The shell inside diameter is approximately 34mm

The Italian Standard uses a threading of 36mm x 24 threads per inch. Both sides of the bottom bracket are right hand threaded. Parts in the Italian standard are usually marked as such

btw, the above does not refer to the cranks or pedals, as this thread is discussing, but the bb shell itself, part of the bike frame.

 

[Voltron]

someone screws up ~2-3 turns of threads and the shop uses a tap to clear that out

I've done that, I think it's called "thread chasing" / "chasing the threads"? makes sense...

And usually done from the inside end of the pedal hole and chasing it outward as it makes the initial alignment of the tap easier

 

[efMX Trials Electric Freeride]

if you have three piece cranks with aluminum crank arms then the pedals are 9/16.. those taps are used mostly for thread chase not making new threads.. badly damaged crank threads require drill and helicoil or insert .. often cheaper to replace crank arm due to labor precision, cost..

 

[speedmd]

Good Hardened steel spindles are twice or more the strength of low grade spindles used in many of the box store bikes.

Some good 1/2 ones out there for certain. They all have weight limits, the cages, each bearing and the flex (in %) in the spindle. Steel on steel threads certainly more secure than alloy arms can offer. But also important are larger support shoulders at base of the pedal threads (as on most quality spindles) that can dissipate higher loads without allowing thread movement. Threads are not designed to be tight and it's the screw force - friction under the shoulder that keeps it in place and greatly reduces bending stresses at base of thread. Good Grade 8 (10.9) steel in a 9/16 setup would have multiples the endurance limit of the average 1/2" setup.

I know of only a few that allow over a 175 pound rider limit. Would be nice to know what we are dealing with exactly.