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I'm trying to restore an old park bench, and I am struggling to unscrew the bolts. The heads have this wide (~12-14 mm) "flat cross" pattern that I have never seen before (see photo), and I don't have any screwdriver that nearly wide enough to get any kind of grip on them.

My googling attempts to find the right tool have been futile. It might be because I don't know exactly what to search for, but common screwdriver bit sets don't seem to have anything approaching that kind of size.

What would be the right tool to unscrew these and where could I find it?

bolt head

And, for what it's worth, square nuts at the back:

squarebut

Update:

Drilling successful! One down, 23 to go...

drilled bolt

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    Imagine there are nuts on the bottom of bolts, you will need a BF(big f...king) screwdriver or cut/grind a piece of flat stock steel. If possible a grinder will cut the ends/nuts off and just wack the bolt heads out. First look seem like they were slots, second look they seem to be raised, if raised maybe a pair of small locking/vise grips might hold. – crip659 Jun 7 at 11:10
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    Are you replacing the wood? If so then remove the wood and grip with mole-grips or equivalent. Often found it is easier and quicker to shear the bolts and replace compared to trying to undo them... – Solar Mike Jun 7 at 11:32
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    With the condition of the nuts, would not bother with trying to undo them. Use a grinder or cutting torch to cut them off. Would probably just damaged the wood trying to undo them by turning the nuts off. – crip659 Jun 7 at 11:43
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    If the nut and bolt is fairly frozen in place, you could drill out the bolt through the nut and possibly preserve everything except for the nut and bolt. If it is not frozen, you could hold the nut with a wrench and drill it out. – WoodAirGrille Jun 7 at 13:42
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    A regular flathead (slotted) screwdriver provides no grip? Seems like a 3/8" or 1/2" head could work. You might want to spray some WD-40 first and let it penetrate for an hour or so. – MonkeyZeus Jun 7 at 14:03
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Looking at the condition of the nut and the minimal profile of the head, I'm afraid the better question may be "how do I get these out with minimal damage to the wood?"

I'd take a drill to the end with the nut. Hold the nut with pliers if the nut-and-screw assembly is prone to spinning. Choose a drill slightly larger than the screw itself. You can either drill all the way through the nut until the remnants of the screw and nut are completely separated, or drill most of the way through and then hammer a punch or nail set into the drilled hole until the screw/nut shears away.

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    There are also "nut splitters" which might work to split the nuts off the end. – Fresh Codemonger Jun 7 at 19:16
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    Since the nut is proud of the green whatever below it, you could use a Dremmel or angle grinder to cut downwards towards the green object, rather than trying to cut parallel. At a certain point you will have two half nuts which might be levered apart. You might do less damage his way. – nigel222 Jun 9 at 14:44
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I would use a small grinder ( Dremel) with a cut-off wheel and cut the nuts off.

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    Ahah. Somehow, my brain could only see the nut as being completely inside the table, flush with the metal. I now understand that the nut is protruding, and can be cut off. – Eric Duminil Jun 7 at 22:31
  • Less precise (but effective nonetheless) would be a reciprocating saw with a metal cutting blade. Less still would be an angle grinder (a good 1mm kerf should suffice) – Machavity Jun 8 at 14:05
  • I used exactly this method to restore an old bench and swing that are similar to the one in the photo. – Theodore Jun 9 at 18:24
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Normal flathead screwdriver in theory

I believe these are simply used to make it easier to apply a flathead screwdriver with at most 90 degrees of turn adjustment needed. So in theory you can just put a flathead on it, a spanner on the square nut, and you're done.

In reality, better with a Dremel on the nut

Considering the level of rusting though, it's highly unlikely you're going to get anything to part company. Most likely the screwdriver slots are going to give way, or for a second possibility the head could shear off. I'm with @blacksmith37 here - the best solution is to simply cut the nut off, and then use something of the right diameter as a drift to knock the bolt out of the front.

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I would use a grinder attachment on an electric drill to carefully grind off the nut. Next I would use a “EZ out” to back the bolt out of the hole. The EZ out is available at most hardware stores for a few bucks.

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Depending on the aesthetic requirements, an angle grinder from the back may be the fastest option.

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I'd start by soaking the threads with penetrating oil of your preferred type. Then the next day fit a suitable 12 pointed socket over the nut and see if it spins off.

Option 2 is to get your assistant to hold the socket still while you use an impact screwdriver and hammer on the front.

Third, try adding some heat to the nut, ideally a small gas torch or perhaps a high-powered hot air gun. Avoid burning the wood though. Then repeat with the leverage and impact screwdriver.

Last resort is to drill either end. It could work out easier to drill the head because its already got a crosshair there, and you theoretically have less metal to drill through.

When you choose new bolts, consider stainless steel or at least galvinised steel, and pre-grease the threads, ideally with an antiseize compound.

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    "Avoid burning the wood" - no, really? – Michael Harvey Jun 9 at 12:11
  • @MichaelHarvey well - would you prefer answers to be incomplete and miss important details ? – Criggie Jun 9 at 12:47

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