Pro Tip – Chase Those Crusty Threads

The restoration process is about more than just taking a vehicle apart and putting it back together. It’s about total reconditioning, and quality restoration shops often spend more time doing work that’s never seen. Occasionally the professionals share their secrets with us, and here is one such example.

It’s no secret that eliminating rust is one of the most common obstacles any restorer will face, professional or otherwise. Naturally, a lot of time is typically spent on body panels, chassis components, and visible trim and hardware. But what about the rust that lives between these parts in the threads where hardware meets bare metal?

We recently saw a technician at one of our local British sports car restoration shops chasing threads with a tap and die as he reassembled a beautifully rebuilt engine on a 1950s MG. Despite the fresh new paint on the outside, he couldn’t trust that the treads were as clean and true. So, before reassembly, he ran a tap through each threaded hole and a die over each stud to be sure.

Following his lead, we took some time to do the same procedure on Project Sunny, our Range Rover restoration. In this case, the threads were tapped into the floor structure to hold the rear seat assembly in place. Of the eight bolts that were originally fitted, two of them broke off during removal and the other six were badly corroded. The tapped holes run all the way through the floor, leaving the bottoms exposed to the elements. Here’s what we did.

Remove the old hardware

It seems obvious, but make sure the old bolt is completely free and hasn’t partially broken off in place. Give the two seized bolts, this was a real possibility.

Confirm the correct tap size

In the case of our Range Rover, the tapped holes used metric bolts in a M6 x 1.00 size. Our tap and die set contained exactly the tap we needed.

Lubricate the tap

You might think running a tap through an already tapped hole wouldn’t require lubricant, but we still recommend using a cutting oil. This will not only make for a smoother, more accurate chase, but will also lengthen the life of the tap. Any tool oil or cutting oil will do.

Carefully start tapping

If the existing threads are severely corroded or worn, you run the risk of easily cross-threading them if you’re not careful. Always re-tap by hand, taking care to ensure you’re correctly aligned with the old threads. Turn the tap half a turn at a time, backing it out slightly in between to clear any debris. If there is drag on the tap but you’re sure you’re in the original threads, apply a little additional oil.

Clean out the hole

When you’re done, confirm the threads are clean by installing a new a new fastener before flushing out the re-tapped hole with compressed air or a cleaning fluid. If you’re doing this task at the early stages of the restoration work, be sure to leave some oil in the hole to prevent rust from forming. If you’re doing this during reassembly, use a solvent to remove the oil and any debris to ensure proper torque is achieved.