Life’s Tough Decisions: Tap a Hole With Fine or Coarse Threads

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There are are two spots on “Cal” that need some rehab in terms of damaged fastener holes.

The first is one of the mounting bolts for the rear brake pedal.  The prior owner broke off a bolt in the hole, and it took three separate sessions over three weekends for me to to drill it out.  The hole is now clear and ready for me to re-tap it.

The second spot is one of the primary drive cover bolts.  The hole is clear, but the threads are definitely damaged.  I am unable to apply the required 15 ft-lbs of torque without the bolt basically spinning and jumping threads.  Without a proper seal on the cover, the enclosed primary will slowly leak oil over time.

With these two mini-projects in mind, I began thinking more and more about “how” the holes should be re-tapped.  Tool-wise, I am going to use a set of HSS bits and taps, but I am referring more to the type of threads to be used – fine or coarse threads.

Something to keep in mind in both the situations described above (and a common problem working on motorcycles in general), is that space is limited.  In particular, I am going to have to drill out each hole slightly bigger in order to then cut new threads.  The amount of “wall” space left outside each hole is very limited.

Below is a little table I put together discussing the plusses and minuses of choosing fine or coarse threads.  Like many issues in the motorcycle and mechanics world, this one is hotly debated although some of it comes down to basic common sense and/or physics/science.

Fine or Coarse Threads

Stronger in tension and shear due to their slightly larger cross sectionLess likely to strip in the future
Less likely to loosen under vibrationMore resistant to accidental cross-threading
Easier to tap into difficult (hard) materials due to shallower cutting – less likely to damage tapInstall faster as there are fewer turns required
More strength where depth is limitedIf subjected to extreme heat, they are less likely to seize
Easier to tap into thin walled sections where remaining material is limitedEasier to tap into brittle materials
Require less tightening force to achieve equivalent torque
Good for situations where the fastener is used for fine adjustments – adjustment screws

After reading the table, you have probably come to the conclusion that fine threads are better than coarse.  I will concede that they have some very notable benefits.

That said, my personal opinions on fine or coarse threads are as follows:

-Manufacturers should use coarse-threads where they are being fabricated in a factory with the highest-quality machinery and “space” limitations can be controlled – to counter-act loosening during vibration, liquid thread-lock materials can be used.  The coarse threads themselves are simply stronger and more durable.

-For the home-mechanic, who is drilling out and re-threading an already damaged hole to a larger size, I think fine-threads make more sense.  The factors of there generally being less room to work with and fine-threads being easier to tap out-weigh some of the other considerations.  Nothing is worse than trying to tap a hole and breaking the tap.  Trust me those things are even more impossible to drill out!

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