Thread size, bolt/tap sizes.

[castaway]

Question for the engineers please.

I need to cut a couple of new threads on my very old Colnbrook furling gear, this is to rehabilitate it for a further years service.

The old holes have lost there threads due to the usual corrosion between the s/steel bolts and aluminium torque tube. It should be no problem to drill and cut new threads once I can get hold of the correct size tap.

The bolts are the 'commonly available' s/s pan head variety available in bubble packs everywhere and I think M6 (they measure 5.85 mm across the threads.

I thought I could pop down to my local Draper Tool S|top and just match up the bolts with a suitable tap and hey presto.... but no.. They only appear to do a finer thread size.

So now it would appear that I have to send off or buy online so need to know exactly what to ask for rather than just take it down and match it up.

Can anyone help me with a correct size/ thread pitch from the info above ???

Thanks in anticipation

Nick

 

[Norman_E]

The usual M6 thread is 1mm pitch, requiring a tapping drill of 5mm. (in metric threads the nominal size minus the pitch gives you the correct tapping drill size)

Are you sure the old threads were not imperial sizes if it was UK built equipment? Quarter inch Whitworth thread is 20TPI, and therefore coarser than a slightly smaller 6mm thred.

If you have to drill and tap new holes, just go for 6mm and buy new Stainless bolts, but get yourself a decent quality HSS tap from a proper tool dealer, and I do not include Draper stuff in my definition of decent quality.

 

[lw395]

Standard M6 has a pitch of 1mm, so hold the screw next to a steel rule and check the mm lines over at least 10 threads.
If indoubt test the screw in a nut and test the nut on the tap!
Tracey tools in Dartmouth will sell you an M6x1mm tap. As will many other places, try to get HSS ones, don't buy a set of cheapo's, just get the ones you need even if they're secondhand.
If you can tap all the way through a 1st cut or taper tap is enough, if its a blind hole you'll need to finish off with a plug tap, in this case ask for a set of M6x1 taps.
Drill size to tap is 5mm.
M6 1mm pitch screws to fit are indeed everywhere, don't forget to use either copper grease, threadlock or duralac on them to avoid corrosion with the ally.
Hope that helps.

 

[old_salt]

HI. Get yourself one of theses little books Worth their weight in gold. web page
Your local Proper Engineers Wholesalers will have all you need, they will be far better than the DIY Draper Tool Shop.

 

[Topcat47]

You can't just re-tap the existing holes and fit the original bolts.

The tapping drill for 6mm bolts is 5mm. (if you're making new holes)

The tapping drill for 7mm bolts is 6mm (if you're widening the existing holes).

These are standard ISO metric coarse threads.

I don't know the Colnbrook system so I can't advise you on which is the preferable course of action, although I would tend to try fitting the over-sized bolts first, if it were mine.

6mm threads are:-4.77mm core dia , with a pitch of 1mm and a thread depth of 0.61.

7mm threads are:- 5.77mm core dia but otherwise identical

Hope this helps.

 

[vyv_cox]

Is the Zeus Precision booklet the longest enduring unchanged publication? I had my first one something like 40 years ago, several others periodically. I see from your link that they still look exactly the same! Remarkable.

As said, this little booklet will tell you everything you need to know about thread sizes. Measuring them can be a horse of a different colour, though.

 

[lw395]

I quibble whether there is an animal called standard M7!
M7 is rarely used, compared to M6 and M8. There are fine M7's used in tube fittings and god knows what, so pitch could differ!
If hole are already too slack for M6 and M8 is too big, helicoiling to M6 is not difficult.
Good luck.

 

[Topcat47]

The m8 tapping drill is 6.8mm dia. Make sure you have room for the larger head before you go this route, though.

BTW, I have 7mm metric bolts in my come in handy box. They aren't as common as 6 or 8's (which is why I'm hanging on to them) but they are used in a couple of places on my Jap diesel, my Chinese generator and a couple of other devices associated with my boating life........

I get my taps from Buck & Hickman......not cheap but very good. Machine Mart will do into Aluminium though.

 

[damo]

Sounds like the same job I have done. I drilled and tapped for the next size up and used hexheaded bolts cut short to the right length.

It is harder to do the foil sections though - especially in situ /forums/images/graemlins/grin.gif I dangled off an abseil rope to drill and tap for panheads, then filed them as flush as I could.

 

[leomagill]

Instead of just tapping a bigger size why not use a helicoil kit to repair the existing size thread, the kit will come with the correct size tap for the inserts and they are dead easy to fit.

 

[Topcat47]

I doubt that the tube would be thick enough to take a helicoil.

 

[andy01842]

A 6mm helicoil tap needs a 6mm hole or a striped M6 bolt hole. I would run a 6mm drill down the hole to clean it out. Tap out with the helicoil tap and you will have a thread that is stronger than the orignail. Some good quility kit has helicoils as standard.

 

[castaway]

Thanks for all the answers on this....What I am actually going to do is to simply rotate the torque tube round through 90deg and re drill and tap into the ali collar it is seated onto.

I'm afraid I will have to fettle the ali extrusion pieces also...unfortuately the joints on the extrusions always suffer when the forestay is slacked off or removed...my mast has been lowered 2 in the last 18months and its taken its toll on the joints.

Thanks Nick

 

[Clyde_Wanderer]

I would suggest you go into a chandlers or hardware shop, even B&Q size a 6mm bolt against your bolt and see how the threads compare, it sounds as if yours could be unf size, I think Whitworth is a bit old for your system.