Hexagon Head v. Allen/Cap Head screws/bolts

By far the worst culprits for rounding the hexagon sockets are Imperial countersunk hex socket screws. Anything below about 2BA and you need an electron microscope to find the socket... The metric ones are better but I've lost count of the ones I've had to remove either by battering a Torx bit into the chewed-up socket, or using a centre-punch and hammer to try to unscrew it.

Yes, sorry for my typo earlier - I wrote 8000kPa and meant 800.

Spanners will slip off tight hex heads a lot more easily than Allen keys will slip in a socket head. So, a lot fewer busted knuckles with socket heads.

**LINK**

The link is to an interesting read on fastenings and torques. The recommended maximum torques for small fastenings are very low, I think that most problems encountered with rounding of heads on whatever form of fastening is due to over tightening when first fitting.

How many people actually have or use a low value torque wrench in the less than 10lbft range.

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[Hand shoots up] ... Me, Sir

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Note: I bought the first one in the early 1970s to work on Hillman Imp camshafts

Edited By Michael Gilligan on 27/04/2018 10:49:51

Torque wrench less than 10 lbft, +1

If I need to get a hex head tight without rounding it I use a socket or a ring spanner.

I'd think most of us do - if we've got one, and if it's not itself bu99ered from some previous misuse, and if we can get at the hex head with it, and if we can apply the torque without canting the socket or ring and chewing up the head that way... if ...if...

If you are tightening steel bolts into an alloy cylinder head / engine block etc, you'd be daft not to use a torque wrench. If you overdo it (eg 4-5Nm for an M6 etc), you get to strip it down, Helicoil the hole and start again. Undertighten them and you get oil leaks. Best to do it right....

Murray

On an aluminium engine that is regularly stripped down I would be inclined to helicoil small tapped holes to preempt them stripping during rebuild. Tapped holes in aluminium castings seem to let go despite never being over tightened.

Mike

Unless the thread is fit & forget, always helicoil aluminium. In for a world of grief otherwise.

Regards

Richard.

The cost of the gear to do Helicoils or other brand inserts is vey cheap compared to a machine shop cost so just crack on and install them. They are also good in Perspex to prevent wear on the threads and effectively you use the next size up of thread but keep the fastener neat and small.

Mike

If possible I use studs in ally for exactly this reason. When I worked in aerospace, all tapped holes in ally were helicoiled.

Thanks for all the thoughts re my original query. I suspect that for my purposes, since I'm not into anything drastic like road going vehicles or really heavy machinery, it boils down to whatever is most suitable, probably Allen screws if space is tight otherwise probably hex head.

In respect of helicoils and aluminium, since I tend to use aluminium having obtained a length of busbar type ali (yes, it's darned soft), I have taken to making T-shaped steel inserts which are then glued in position. The T bar is placed such that the strain is such as to force the head onto the aluminium, and the upright of the T is tapped. Thus all the weight and wear of fastening/unfastening is primarily taken by the steel.

Once again, thanks to all,

Peter G. Shaw

A hex head with rounded corners can be unscrewed with a Metrinch socket or ring spanner, or one of the many "flank drive!" imitations. Am not so keen on the open end Metrinch spanners, to my mind they still seem likely to slip.

The advantage of Metrinch is that one size will deal with Whit, A/F or Metric, although at first, the "sloppy" fit seems very odd. Wouldn't be without mine now, if anything tight comes to need unfastening.

Soft Allen keys are a PITA.  Still using the Unbrako  ones that I bought in '58 as an Apprentice

Howard.

Edited By Howard Lewis on 28/04/2018 11:18:21