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Monday, April 13, 2009

Bicycle Thread Gauges

Since bicycles have been around since the 19th century there are many different thread standards that have been used. Standards also vary from country to country.

Bicycle rear axles alone use 8 different kinds of threads, making bolt and nut threads an important part of a bicycle mechanic's knowledge. Identifying and measuring threads thus becomes the first step to dealing with problematic nuts and bolts (especially rusted one).

Threads are either measured in Imperial or Metric. ie. 3/8" x 26 tpi (threads per inch) or 10 mm x 1 mm.

The first number refers to the diameter of a bolt (or the interior diameter of a nut).

The second number refers to the pitch or size of the threads, the spacing between them.


With metric pitch its measured in the distance between threads, whereas Imperial pitch is measured by threads per inch (tpi). The tpi method is also known as British Standard Cycle (BSC) or Whitworth Gauge (G). (Confused yet?)

So if you see markings that say 24 tpi or 24 G, you will know its referring to 24 threads per inch.

All this becomes easier to understand when you try matching up the bolt using a thread pitch gauge (a tool for measuring threads). If the size seems a bit off, round to the nearest full number, either the closest 1/16th of an inch for Imperial gauges or the closest half number in the case of Metric gauges (Metric bicycle threads come in 0.5 mm increments).

So if its 4.9 mm, its 5 mm. If its 23/64ths", use 3/8".

A tiny bit of space is left between bolt and nut so it winds on easier.


Use a caliper to measure a bolt's diameter and measure from the side (perpendicular) of the bolt or sideways inside a nut's internal diameter. If you measure the bolt and you know the nut fits (but may be tough to turn because of rusted pits) its not necessary to measure the nut too (the size will always be a little off so the nut turns smoothly).


Equally important is whether the thread turns clockwise or anti-clockwise. You can determine this by holding the bolt up vertical and look at it from the side. If the threads point to the top right, its a right-hand thread. If it points to the top left, its a left-hand thread. (The result will be the same regardless of whether you hold the bolt vertically upright or downwards.)

Left-hand threads are commonly used on the left pedal of the bicycle, so that it remains tight and doesn't accidentally become loose due to use.

To determine thread direction on a nut, place a thread gauge inside the bolt, note the number of gauge teeth showing and then turn it clockwise. If more gauge teeth appear, its a left-hand thread. If less teeth, a right-hand thread. (The results will be the opposite if you go counter-clockwise.)


Proper lubrication makes a big difference. It becomes easier to tighten bolts/nuts and reduces corrosion.

With lubricants you have two basic choices: Oil or grease.

Grease = Larger/coarser threads

Oil = Smaller threads.

Don't use cheap oils like WD40 or 3in1. They repel grease and other oils, attracts dirt and can even damage your bicycle. You should get to know your oils so you know which ones will damage your bicycle.

For nuts and bolts an oil like Loctite is preferable. Loctite hardens/expands after use, but is not glue-like or sticky. Don't bother using Loctite if the nut has a Nylon insert (which prevents bolt slippage).

Oils are measured in viscosity (thickness). The higher the number the thicker the viscosity. Loctite 277 or 290 for example is for heavy duty applications, whereas Loctite 222 or 242 are used in more regular use. 242 is the most common one you should use.


Repairing a bolt or nut is a last ditch move when it becomes rusty or pitted. Replacing them is usually best, but if the bicycle is a classic Raleigh for example (which has Rs on all the bolt heads), its better to keep the original bolts/etc.

You will need a thread cutting tool known as a Tap for internal nut threads, or a Die for external bolt threads.

After determining the size, gauge and direction of your bolt or nut, take your Die or Tap and turn it onto/into your bolt/nut. Use a little bit of Cutting Oil to help the process be more smooth. It will start cutting metal filings out and you will encounter resistance turning the tool. When that happens, turn the tool back half of a rotation to let the loose metal filing out and then resume. Repeat this process until the whole bolt or nut has been cut to the ends of the threading.

A Tap and Die set is very expensive however and this will not be a good choice for many people. Consulting a professional bicycle mechanic may be your best choice.

There are also cheaper tools such as thread chasers and thread files, but these are for minor repairs only. Thread chasers don't actually cut threads, it just fixes existing threads that have been shredded a little. A thread file will only fix a little dent or hole.


We've all encountered this problem.

Removing a stubborn bolt or nut isn't easy and requires the proper tools. This task will be almost impossible without the necessary equipment.

#1. Use a very slippery, penetrating oil to loosen the bolt in the nut.

#2. Find the tool that best fits and can grip the bolt. If the head of the bolt is deformed, you may have to use vice grips instead of a wrench or screwdriver.

#3. If the vice grips fails, use a small saw (like a coping saw or Dremel) or a hammer & chisel to make a slot in the end of the bolt large enough for a screwdriver to fit snugly.

#4. Alternative you could file the bolt down so it has flat edges and try a wrench or vice grip again.

#5. The last ditch move (often the only thing you can do if the bolt head has been sheered off): Determine the direction of the bolt's threads, drill out the inside of the bolt, put a screw inside the hole and tighten it in the direction of the bolts threads. Once it has been tightened, further movement will be moving the whole bolt in the proper loosening direction.

#5b. Drill out the whole bolt using an electric drill, repair the threads of the nut using a Tap and Die set and replace the drilled bolt with a new one.

#5c. Drill out the bolt USING a Tap and Die set, but this will wear down the sharpness of the tool. Replace the bolt.

#5d. If possible cut off the damn f*cking bolt using a hacksaw or cutting torch, then repair or replace the damaged parts. This will likely be your choice if the bolt has become bent.

1 comment:

  1. How about the thread on a bolt that secures the sissy bar of a banana seat to the rear of a bicycle frame of a 1970's american made muscle bike. 1970 Western Flyer Buzz Bike to be exact. I went to the local hardware store and none of those threads matched this bolt thread. I would appreciate it if anyone could help on this subject.



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