Confused Mom/ Tap and Die Question...

J

June

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Can anyone help a single mom figure out how to complete the task of re-tapping a piece of metal? My teenage son has an old moped whose pedal doesn't stay attached to the pedal arm. The opening on the pedal arm where the pedal screws in is stripped. It's an old moped, so finding a replacement part is almost impossible. I don't know how to figure out what size tapping bit to buy or where I would buy it.
I really don't understand "UNC", "NC", "UNF", "STI-UNF", "STI-UNC, "STI-METRIC" etc (and I can't find a source that explains these terms or even a "how-to" guide). All I know is the pedal size is 9/16ths; but how do I figure out how close the threads need to be (is this thread pitch?) Do I just buy a 9/16ths tapping bit? Thank you in advance for "any" help anyone may be able to offer.
 
If the internal thread has stripped, a simple retapping won't help. The material needed to form the thread is lost. A remedy may be to use a threaded insert which basically the hole is drilled out and tapped for a larger thread size. The insert is threaded for the thread on the pedal.

If there isn't enough material to support an insert, the next option would be to have the damaged hole plugged by welding and the drilled and tapped for the thread on the pedal.

Chances are that the external or male thread is also damaged.

As to tap size, a start would be to determine whether you have a metric or inch thread. If the moped is newer or a foreign brand, it would most likely be metric. An older US brand would likely be inch. The next option is coarse or fine thread. A measure of the threads per inch will determine that. A simple thread gage will determine that.

You will most likely not be able to find a suitable tap at a hardware store or DIY. It would need to be ordered from a machine shop supply or or industrial supply like Granger or McMaster Carr.

As to on line information, Wikipedia is a good place to start. It will have more information than you need. https://en.wikipedia.org/wiki/Screw_thread
If you provide more info on the moped on this site (model, year of manufacture, etc.), more than likely someone will have some knowledge of the moped.
 
Hi June,

Welcome to the site!

I agree, there is lots to know there.
I am NOT trying to make you more confused, but a couple things to (hopefully) help.

Be aware that the "STI" that you quoted above is an acronym for "Screw Thread Insert"
These create an internal thread that is "oversized" of the typical standard thread (UNF, UNC) so that an insert (helicoil, time-sert, etc.) can be installed in the over-size hole to allow the original fastener size to be used again.

See here:
https://www.travers.com/sti-taps/c/297765/

it states:
S.T.I. Taps

S.T.I. Taps are cutting tools which are used in maintenance or repair applications to prepare a hole with an oversized internal thread which will accomodate a helical coil wire screw thread insert. Screw Thread Insert Taps have a larger diameter but the same pitch as a standard tap in order to accomodate the wire insert.
Click to expand...

It may be kind of expensive to buy an insert package (drill bit, special STI tap, insert installation tool, etc.) for one-time use.

Q1) Any idea what the metals are? Are they magnetic?
(steel is magnetic and much harder than aluminum, so it is more difficult to tap)

Stick with us.
I promise this group will try to help....heck if I was anywhere close I'd offer to fix it for you!

-brino
 
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Another consideration is that sometimes one of the peddle threads will be left-handed so that the peddle does not unscrew while using it.
 
Sorry for the repeated answer. Firstram beat me to it.
 
If you don't mind a little experimenting, you could drill and tap the peddle with nearly any size you like then put the peddle in the crank, and thread the new bolt with washer into the peddle and tighten it up. There is not too much force on the peddle in the direction of the bolt so it should stay put. Probably should also use a lock washer with the bolt.
 
Yes, Left side pedal on a moped or bicycle will be left hand threaded. If the threaded hole is completely striped out you will either need a new crank arm or, if the part isn't available, the hole needs to have a threaded insert installed. A machine shop can do do the insert for you.

The thread is 9/16-20 R & L depending on the side of the crank.
 
June,

I hope you are not feeling over-whelmed !

The first step is identifying the original thread.
Since you have both the male and female sides hopefully you can figure it out.

As @RJSakowski mentions above if you provide the moped brand/size/model we may have a better chance of helping.

One simple and cheap tool for finding the fastener diameter is this:
1615644763067.png
I would think you could get one for under $10 at any hardware store, home-depot, etc.

For finding the thread pitch a gauge like this is often used:

1615644838847.png

However, you may not find that an your local hardware store.

Then (as also mentioned above) you still need to know left-hand or right-hand thread.

-brino

Here's an example of a helicoil kit:
1615644622376.png

from here:
https://www.mcmaster.com/helicoils/thread-size~9-16-12/

Unfortunately, that might be prohibitively expensive for a single use.
 
How about some good closeup pictures of this area?
The messed up threads on both pieces affected.
 
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