Question for the enginners, 0.0016mm
 

The correct drill size for a 6mm tap is 5.2mm. As a decimal this is 0.2047.
My nearest drill bit is 13/64 which is 0.2031 a difference of 0.0016.

Will the hole be just that bit too small or does it more depend on the quality of the tap, I.E it does not snap due to being too tight?

Some drill bits are advertised as 5.2mm 13/64 but my Zeus book shows these are different sizes.

macafee2

If you are using imperial, then you want a 7/32, or 5mm for a 75% thread with an M6x1 Ian.

5.2 will give you a 50% thread, so slightly too big ;)

Brian :D

Thank "luck" I asked what I thought was a stupid question. Thank you Brian. I was sooooo close to ordering a 5.2 drill bit.
I think I will practice on a bit of metal.
How "hard" is the trailing arm where the hydraulic pipes are held?

macafee2

Yes 5mm is the tapping drill size for standard 6mm thread. Tip, if you're tapping into something hard, drill the 5mm hole then just offer your 13/64 in a couple of mm to help start the taper tap.

Just as a side note, I wouldn't ever be worried about 1.6 microns on anything to do with cars ;)
I think you meant 1.6 thousandths of an inch in which case grab your pair of feeler gauges and look at the thickness of the thinnest one. Isn't going to make a dot of difference in a hole of any size when running a tap down. You'll loose more than that drilling free hand.
Maybe you were thinking of m5 which has a 4.2mm tapping drill?

7/32 would be a bit big looking at the old Zeus book, just over 5.5mm. If you had to use an imperial drill you'll get away with a 13/64, which is about 5.15mm.
I have used 3/16 drills in the past, they are 4.75mm and it is very difficult to get an m6 tap started but will get you buy on soft materials.

There’s a word I haven’t heard of in 40 years, can’t even think what happened to my Zeus book

My Zeus is 1978 and I dragged it out to look at today.

I have had a practice with some flat bar and cut the thread ok using a 5mm drill and then putting a chamfer on the edge. I also used some cutting agent... amazing what you can find at car boot sales.

Tomorrow I will have a go at the trailing arm.

I appreciate all the advice given

macafee2

As a rule of thumb I've always times the thread size by 0.8 to get the drill diameter. :}

why 0.8? Never heard that rule of thumb.

macafee2

I've got three or four copies in various states of repair! My one for best stays in the tool box, the working copy is on its last legs but I recon it's at least 30 year old. I've also got a PDF version on my phone just in case! Very handy things to have but there are a lot of very good alternatives now. Some are free if you know where to look!