Cheers for all the replies. Reason it has to be a grub screw is that there was originally a smaller bolt in the hole and this bolt secured a clamp which holds down a diesel injector – there's a hole in the clamp, which if I make it any bigger will probably break the clamp. This bolt had worked its way loose and stripped all the threads, so have tapped bigger thread in order to get bigger bolt in it, but obviously a new bolt wouldn't fit through the hole in the clamp, so if I put a grub screw in and then screw original sized bolt into middle of grub screw it might work. (Have actually enlarged hole in clamp to 9mm but looks dodgy to take it up to a 10!)

It probably is 12.9 as I'm sure that's what's written on the black bolts of same type. Is 8.8 softer? I could try and find a softer grub screw I suppose, that's definitely one option. Or chopping the head off a softer bolt, drilling and tapping that, then make it take a flat screwdriver. Will also try a multipurpose drill bit to see what they're like as well.

Being a proper amateur with not much knowledge of these things other than when necessity dictates (though I do quite enjoy it in a strange way!), I have no real access to heat – does this annealing process last? Now this may be a stupid question, but could I just chuck it in a fire and then fish it out once it's cherry red, as one of you described, and if so would this last for good, or does the drilling have to be done straight after it's had its initial cool down? Plus, if I was to do the same thing once the hole was drilled out would this make the tapping with an hss tap any more likely to be a success?

PS carbide tipped bit definitely unsharpenable – think I've burnt all the carbide off with my enthusiasm on trial bolt!

Thanks again, and sorry about all the questions!

Just an update to say that me and the van have made it back home in one piece at last today. Have been walking 7 miles each way to my unit for last month or so to try and sort this headache so in a fine mood here! Thank you very much to everybody for their very helpful advice – basically pinched bits from each one and it came good in the end – was slowed down by my cheap (but pretty useless!) bench drill holder from Machinemart – wouldn't have managed without it, just a £20 thing you put your power drill into and lower it down, but the vice that came with it broke, meaning that I had to G-clamp the flimsy thing shut all the time.

After further slight complications, ended up tapping the top of the hole to an M12, which is the only time it cut first time nice and easily, maybe it's easier to do big jumps in tap sizes in aluminium, when tried to go from M8 to M9 and then M9 to M10 was really hard to get a thread started. Made a grub screw from softer steel threaded rod.(Now an expert on my 8.8s and 12.9s, handy pdf document by Thomson rail, quite informative for the beginner I thought – just type in 'A Short Guide To Metric Nuts and Bolts Thomson Rail' and it comes up on Google). Drilled a 5mm hole with a cobalt bit into the softer steel, then tapped that with an HSS tap to M6, and hacksawed a slot for a screwdriver. This caused a problem as took me ages to find an 8mm slotted screwdriver bit, which I needed to use to properly tighten it down, the 7mm just kept falling down M6 hole, and couldn't use a normal 8mm screwdriver as bonnet was in the way! Didn't look particularly professional, but threadlocked it down and no sign of anything getting worse after doing 30 laps of my industrial estate, so took the gamble and drove home tonight.

If I'd known it would have ended up with an M12 would definitely have used Hellicoils, and if I had access to a lathe would then have made a stepped stud. Have read up on drilling speeds, and now a bit wiser, but definitely of the opinion that cobalt not much better than HSS and should get solid tungsten carbide drill bits in for all tricky jobs in the future. Plus why do they produce M7, M9 and M11 taps when it's virtually impossible to buy any corresponding bolts?! Not everyone has their own boltmaking equipment, although I am tempted to one day buy a lathe and start selling custom made bolts of strange sizes through eBay – the only M9 bolt of any length available in the whole world it seems is something from BMW, which has one of those male torx heads on it, and if someone doesn't have those sockets to hand, that's another £20 away! I think a fiver a custom made bolt might be a winner.

Anyway, waffling now, but wanted to say thank you for everyone's help, and will drop in from time to time, but unlikely I'll be able to assist with anybody's problem with my sparse knowledge, but you never know!

Cheers, Ian Dodds

I just used engine oil cos was skint, so no idea whether that helped that much! Being very much an amateur didn't know if they were actually going to be much better (the marketed cutting oils), than normal oil, but I take it you've seen this yourself. I've had to do quite a bit of chainsawing of logs last couple of months, and switched from chainsaw oil at £5 a litre to sunflower oil at £1 a litre with no noticeable change, so did wonder whether it was a lot of extra money for like a 2% difference in performance!

Cheers

But why sell the taps if you cant buy/sell a fastener is my query! All I made in end was an M12 grub screw with a threaded hole for an M6 bolt to go through in the middle of it in the end. It's my lack of knowledge and experience that makes it sound so complicated – but had to drill through 30mm bolt/grub screw, which proved too much for cobalt bit in 12.9 bolt, but the tungsten carbide bit went through it easily. Ended up knackering the carbide tipped bit in the end (by, emabarrasingly flicking the switch to anticlockwise inadvertently!), so had to use milder steel for bolt / grubscrew used in the end, which was 8.8.

Cheers, Ian

I thought it was meant to be 5 for 6 in everything, shows how much I know! What's the reasoning behind all this, just so the threads don't have to cut in so deeply? Will this not mean theres a bit of wobble with a gap between the shaft of the bolt (the narrowest bit in between threads) and the outer material in the hole, if that makes sense?!