Engine Bolt Thread Cleaning

If a nut or bolt is hard to remove, it will be just as hard to tighten. So it needs to be cleaned before re-assembly.

thread chaser or tap to clean dirty bolt threads

Dirty threads will give false readings while final-torquing bolts.

And there's a worst case scenario that you really want to avoid: Damaged threads or crud in a blind hole can prevent a bolt from being completely installed. This may ultimately lead to continued tightening and cracking the hole, resulting in total part failure.

clean and tap bolt threads

When bolt threads are properly clean, you'll be able to thread a bolt in completely using just your fingers. If they don't, you've got cleaning to do, but what is the best way?


Cleaning vs Tapping

Thread chasers are designed to safely clean debris from bolt thread holes. They are not designed for cutting new threads--they are best suited for cleaning rusty or damaged threads.

Threading taps are designed to cut threads where none have existed before. They can remove material from existing threads when used too aggressively, and make old threads looser.

A lot of techs, including myself, use taps to clean bolt threads.

Here's how to stay out of trouble when using a tap to clean threads; use fingers-only force. If it requires more force than two fingers to thread in, you may be be cutting new threads!

cleaning vs tapping bolt threads

The Difference Between Tap and Thread Chaser

Thread chasers are similar to taps, but are slightly smaller in diameter and don't have an edge on them that actually cuts.

clean and tap bolt threads

Thread chaser in foreground, threading tap in background

Thread Chaser or Tap?

Most thread holes an auto restorer comes across won't see more than 40 or 50 ft/lbs of torque; for example; intake manifold or accessory bolt holes in cylinder heads. Being super-critical here isn't necessary. A hand-held tap will get the job done.

When To Use A Thread Chaser

So what are thread chasers good for ? They are safe to use in more critical spots. Examples would include engine main bearing caps, or cylinder head bolt holes, which call for higher torque values. A thread chaser will not remove any excess metal.

Gen One Small Block Chevy Build

This would be of particular concern on a vintage engine, like this 1966 Chevy small-block.


Cleaning Crud Out Of Bolt Holes

Dirty bolt threads can hide dirt, rust, grease, thread sealant, or a combination of these.

thread chaser or tap to clean dirty bolt threads

My usual method for cleaning bolt threads: Run a tap into the hole as far as I can using fingers only until it stops

Unscrew it out and clean the crud off the tap

Spray in some brake parts cleaner

And a few blasts of compressed air.

I'll repeat this process until the tap comes out clean.

When I think I've removed all the junk out of the threads, I'll do it again.

Sometimes it will take 4 or 5 passes for the threads to be completely clean. You'll be surprised how much crud can hide inside a bolt thread!

When done, you can now get precise torque values when assembling to ensure an even clamping force.


Clean Bolt Threads

Remember to clean the bolt threads, too.

wire brush in vise

Before I had a bench grinder, I cleaned bolts on a wire brush mounted in a vice.

bench grinder for garage workshop

A bench grinder with a wire wheel does the same thing, it just cleans it quicker.

how to clean dirty bolt threads

Norton Commando cylinder bolts cleaned and ready to be installed


Tap Storage

One by one, as needed, I've accumulated quite a few taps and thread chasers.

clean and tap bolt threads

To keep them organized and find them easily, I bought a couple small parts organizers.

small parts organizer for garage


Tap and Die Set

If it's in the budget, consider purchasing a tap and die set.

tap and die set for automotive engine build

Not only will get you a large assortment of sizes, you'll get handles and accessories, and a case to keep them in.


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