Changeable Gullet.... Not :( (rounded hex bolt)

Do you mean they are hex screws? These are round on the edge but with an allen key fitting in the middle, hubby does mine with an allen key and you can get them from all DIY shops. x
 
Another option is to try and get a torx key set (couple of quid from a DIY store) and try and hammer in the next size up. That should give enough grip - but a local garage should have it out in a trice if you take it along.....worth checking the silly ******* haven't cross threaded it, easy to clean up with a tap if they have.
 
As it is a hex bolt, I assume its a Thorowgood or Kent and Masters, it should be quite simple to get a mole grips on the side of the head of the bolt and get it out that way, or take it to your local saddler, they will be able to remove it for you, and any registered Thorowgood or K&M stockist should have spare bolts, I keep them instock, is it the panel bolts or the ones that hold the gullet plate in place.
 
Thanks everyone, it was a bit of a struggle, we ended up prising it apart a bit and then using a monkey wrench to turn the bolt. It's the panel one annoyingly as the others you can always get a wrench round. Now I just need to get a new bolt :)
 
Top