F-Clamp Quick Fix

Not really much exciting to report but something nonetheless.

One of my f-clamps kept falling apart whenever I released its tension. I’d back off the handle, and the jaw arm would then drop and slide down the whole length of the bar.

F-clamp that keeps falling apart. There is a linear relationship with my frustration and the number of times its fallen apart.

Normally, the jaw just slides down and stops at the end of the bar which is an annoyance. Some of my better quality f-clamps have a pin that has been riveted to the end of the bar so the jaw cannot fall off. However, this one is a cheaper construction and the steel at the end of the bar has been peened with a punch to deform the bar. Well this has worn out and let’s the jaw slide through.

That little punch mark no longer stops the jam from sliding off.

There’s only so many times you can tolerate this and the best solution is to fix the problem.

My solution was simple, drill a hole, tap an M4 thread and insert an M4 screw.

If only I could remember the drill size for an M4 tap, is it 3mm, 3.2mm or 3.5mm? Good opportunity to consult the Engineers Black Book 3rd Edition I got for Christmas (one of those things I’ve wanted for ages but couldn’t really justify buying because I only needed it’s information occasionally).

M4 fine pitch thread is 3.5mm drill.

Ok, time to fix this problem for good. Centre punched the hole, added a bit of sewing machine oil for lubrication and 3.5mm hole made in the bar within seconds. Nice long spirals of swarf too 😁.

Super easy steel to drill through.

Then tapped the hole, making sure the tap was square on each side. Again a little lubrication on the tap and making sure to rotate anticlockwise every full rotation or so to break the chips and not the tap.

Make sure your tap is square and remember to back off a bit with each rotation.

Hole trapped, now for a quick test fit. Perfect.

Threads engage well.

All done, slid the jaw back on and inserted the screw. No chance of the jaw sliding off now.


All up, it took about 5 mins to fix a problem that has caused a reasonable amount of frustration – I reckon that makes it a good investment of time.

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