Dimensions of machine screw head

[elcaro]

Hi I need to undo a machine screw. Iris slotted but very tight. A screw driver as broad as the slot tends to just twist and would damage the slot. I need a screwdriver to square drive bit as thick as the slot - any ideas pls?
Terry

 

[Burnerman]

You may find that an impact driver bit (black, hardened) would fit the bill, but if you are good with a small cold chisel it's possible to get a start with that on the head of the screw......so long as the screw isn't recessed, of course.
John

 

[Burnerman]

Is the screw rusted in place, or could it have a locking compound on the threads?
John

 

[elcaro]

Hi John, no not rusted or anything. It’ll a level plug for the gearbox oil in an outboard motor. The manual quotes a torque when re-fitting - not sure how to torque a screwdriver!

 

[Burnerman]

Aha, then go for the impact driver bit, which will likely be 1/2” drive and go for your socket set!
It’s unlikely to be ferociously tight but press the bit well home with your hand as you undo it.
You could do the same when tightening up, but a torque wrench is hardly necessary.
Chunky things, impact bits!
John

 

[elcaro]

Is this half inch socket on one end and a screwdriver tip on t’other?

 

[elcaro]

Like this

1/2" Screwdriver Socket Bits - Slotted | Cromwell Tools

Cromwell Tools - Experts in Hand Tools, Power Tools, Cutting Tools and PPE

www.cromwell.co.uk

?

 

[Burnerman]

Yep, that’s the sort of thing.....I have the Laser set which has slotted and Phillips drive bits.
For sure, you’ll shift the screw with that.
John

 

[elcaro]

Yeah looks like the driver bit would survive, but would it damage the slot if it wasn’t the same thickness as the slot, 2 mm or thereabouts?

 

[Burnerman]

Million dollar question Terry.....but there’s no reason for the screw to be that tight anyway.
Before you splash out, take the biggest driver you have, press it into the slot with your chest, and turn the blade with an adjustable spanner on the flats of the driver.
There’s a better than average chance it’ll shift.
John

 

[elcaro]

Thanks John. The book says tighten the level plug to 7 Nm 5.2 ft-lb. Not sure how tight that is. I have a torque wrench (my Dad’s) (and I’m 75!) so I don’t know how accurate it is. Don’t want to strip the thread! Or for the plug to come out while running, though it might survive that.
Mind you I had to get a mole wrench to undo the plastic (!) engine oil cap so I don’t think the bloke who serviced it knows his own strength!
Another way might be to say well there’s no sign of any leakage, no oil on the shed floor where it’s been for 2 years so should ok leave well alone

 

[elcaro]

It may push trace amounts of oil along the prop shaft in operation but then it hasn’t run since last service.

 

[Burnerman]

Always worth persevering with I think, if only to ascertain there isn't any water in there.
Hope you get there!
John

 

[elcaro]

Thanks John, might give it another go then!

 

[Old Salt]

5.2 ftlb so just nipped up. Imagine a bag of spuds hanging off the end of your spanner/screwdriver