I have bolt that is listed as being 1/4 x 3/4” – Its stamped on its head as “S30400 THE”

I need to clean out the threads of what it bolts into ……. do I just need a 1/4” TAP a la >>>>>

https://www.amazon.co.uk/Volkel-4-inch-UNC-HSS-Tap/dp/B00EO4JX38

If so …. can I “hold” the end with a spanner or do I need a tap handle?

all depends

is it bolting into something thats through and out or is it a blind hole.

if its a blind hole you need a bottom tap – the difference being that the tap cuts full depth threads all the way down instead of tapering.

your showing a taper tap for a UNC thread , you also need to know if your bolt is a 1.4 UNC (20 TPI) or 1/4 UNF (28TPI)

itll most likely be 1/4 unc as thats the most common but its not unknown to be the other….

i do find a quality tap handles good for getting the thread started paralel but no if theres an existing thread then you can use your spanner

Measure the threads per inch of the bolt should tell you if it’s UNF or UNC based on the info provided by trail_rat.

What vintage is the ‘thing’? Did it originate in the UK or the USA? You’d be better off getting / borrowing a thread gauge, as there are other possible thread forms.

1/4 UNC is a camera tripod thread so if the bolt goes into your camera base nicely then that’s what it is. A handy check before buying the tap. As for holding the tap a spanner will be fine. A cordless drill does the job well too if you go slowly.

Cheers guys.

Its a through thread rather than blind.

Just tried the camera test and its would appear (as it went in and out okay) that its 1/4 UNC.

Love this forum 8)

Thank you again.

Mark

A cordless drill is also a nice way to snap a tap.

Yeah bollocks to using a drill.

Tap handles are cheap and useful.

A cordless drill is also a nice way to snap a tap.

I suppose so, if you are ham fisted enough. 8) I don’t recall ever snapping one and I’ve cut and cleaned thousands of threads with a cordless drill. Very easy to maintain a constant speed and torque while set at a perfect and steady angle.