anewman

Some old bolts have rusted in and snapped off my battery tray. Was considering trying to use a tap, rather than a bolt and nut either side of the tray or cable ties.

I don't have a reversible drill and would probably try doing it on the cheap with a hand tool something like Information... Would these be likely to produce an acceptable thread that I could use to screw a standard bolt into?

Thanks in advance

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Tom_vRS

Absolutely they would do just the job.

Make sure you drill the bolt out to about 0.5mm less than the tap you want to use though, they aren't ideally suited to new holes, more retapping existing ones.

I'm sure you could get the rusted bolt out. I've had good sucess with "bolt outs" before, although I know they aren't ideal all the time.

Irwin 5 Piece Bolt Grip Nut Remover Set - Screwfix.com, Where the Trade Buys

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cheezemonkhai

Lots of plusgas after you have used a hacksaw to cut a srewdriver line in the broken bolt and then leave it for a couple of hours. Come back, add a bit more plusgas and with a suitable large driver you should have it out in no time.

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TeflonTom

not so, if it were only 0.50mm under tapped size you would almost certainly break the tap off in the hole, or if you did manage to put a thread in it would tear the thread out when you tighten the bolt up....

it depends on the pitch of the thread, an M8 tap needs a 6.8mm pilot hole for standard 1.25mm pitch and 7.1mm hole for 1.0mm pitch, although you could get away with a 6.5 or 7 respectively.

for M10 standard pitch(1.5mm) use 8.6mm hole, for fine pitch(1.25mm) use 8.9, or you could get away with 8.5 and 9.0


as a rough fule of thumb, to work out the size of the hole you need for any given metric thread, subtract the pitch from the diameter; ie M10x1.5, 10mm - 1.5mm = 8.5mm hole

there are differnent rules for unified threads and BSP and whitworth but you'll never see these on a modern car anyway except maybe on an oil cooler so you dont need to worry about them

those taps would be ok for tapping new holes, it's a plug tap you would difficulty tapping a new hole with... the most difficult thing with tapping new threads is getting them square

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anewman

Unfortunately there's no protuding bit in which to cut a screw driver receptacle. The bolts snapped off before I got my hands on the car, but it looks like someone came to needing to replace the battery and the heads of the bolts snapped clean off.

Thanks for the replies guys. will try the tap first and nut and bolt or cable ties will be last resort.

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andy2407

Will the OP need some form of cutting compound?

I'm no expert in these matters but I seem to recall seeing it used?

Maybe someone that knows more can post a link for him if its required.

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TeflonTom

wd40 will do

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pinkskud

you can use any old grease to be honest, just remember when using a hand tap, that you need to brake the sworff. to do this you need to turn the tap 1/4 back for every 1/5 forword.

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anewman

Ah I needn't have bought this then Toolstation

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rigsby

You should have said about the broken bolts alan, when you came over, i would have drilled and tapped them out for you

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Boris the Mad

just a quick note... you will never find the centre of the hole. drill out the bolt until you can drift out the remaining thread and then chase the thread with the relevent size tap.... come on guys lets not complicate things......if aerospace can do it !!

KenONeill

I knew there was something bothering me about this. Boris is dead right about how to fix this one! Oh and do not use a "bolt extractor"; they always break, and leave you needing to use spark erosion to clear the hole.

cheezemonkhai

What about just drilling the bolt out then? Failing that bolt croppers are cheap.

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pinkskud

drill it
tap it
helli coil it
locktight it

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rigsby

Before anything else , these are just my opinions and not ment to offend
1 Boris, i dont know what you do for a living, but if you cant find the centre of a broken bolt, to drill it out, you wouldnt last long in my garage. And as for drifting it out, you are more likely to damage the threads than being a bit off centre with the drill.
2 Ken, I dont know what make of bolt extractors you have used, but in the last 25 years ive only broken a couple , and one was when i whacked it to hard with the hammer getting it started in the hole
3 cheezemonkhai, how are bolt croppers guna be any good,when the bolt has snapped off?
4 pinkskud, we are talking about an 8mm thread in to a captive nut on a battery tray. The use of a helicoil is going a tad over the top.

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Boris the Mad

Rigsby not at all offended. My method has worked for me on many an occasion. As for working in your garage, your right I wouldn't last long. For my living I prefer making parts for the medical industry, aerospace and F1 cars. Oh and time served. However we all work in different ways to get a job out.

Lummox

Shame your so far away, dying to christen this...

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KenONeill

Rigsby, on (2), it's not just me. It's the general opinion of every other (meaning not you) mechanic I've known express an opinion on the subject of bolt extractors, so I'd imagine the make, and degree of tapping skill available, to be variable.

anewman

I've drilled out 2 holes to 6.5mm (both off centre, oops). Unfortunately my m8 tap won't turn in it (even afer tightening the chuck with mole grips) so perhaps I need to get a 7mm drill bit.

That's a great looking kit Lummox. Must have almost every size and pitch possible

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KenONeill

I can't see how you can get a tap drive tighter with a toggle grip, even a proper Mole one. The RH drive in Ross's photo is the tap drive, and any taps (or extractors) I've seen are like that with square tops. I can see them locking in undersize holes, but that's another story.