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Using Counterbores

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Colin Heseltine04/06/2020 20:24:51
409 forum posts
110 photos

I am intending to counterbore two holes which will have M5 caphead bolts located in them. The holes are drilled M5 and the bolt fits nicely. I have a set of very good quality counterbores with the usual range of sizes and several individual counterbores.

The issue I have is that the locating spigot on all the counterbores are larger than I would have expected. i.e I would have expected the M5 counterbore to have a 5mm dia spigot but it is around 5.23mm. Is this 5.23mm diameter the correct sized clearance hole for M5.


JasonB04/06/2020 20:27:17
18113 forum posts
1995 photos
1 articles

Yes, they are intended for generous clearance holes, you can get some that use a smaller range but still more than nominal.

Think M5 standard is 5.3mm clearance.

Michael Gilligan04/06/2020 20:36:57
15712 forum posts
687 photos

If you want ‘toolmaking’ tolerances for capheads ...

avoid the ISO Metric system and use Imperial fasteners.

... I will leave you to find the comparative proportions angel



Edited By Michael Gilligan on 04/06/2020 20:37:47

Tony Pratt 104/06/2020 20:47:59
1125 forum posts
5 photos
Posted by JasonB on 04/06/2020 20:27:17:

Yes, they are intended for generous clearance holes, you can get some that use a smaller range but still more than nominal.

Think M5 standard is 5.3mm clearance.

Jason is correct & another point is you do want clearance around a screw as generally they are not used as a location component.


Martin Connelly04/06/2020 20:57:40
1369 forum posts
159 photos

I've looked through the British Standard for metric threads and there is no mention of clearance holes. The tables in my Zeus books say 5.1mm is the clearance drill for M5.

Martin C

Chris Evans 604/06/2020 21:08:04
1658 forum posts

Most (all?) of my counterbores are also bigger around the head than I would like. One day if I can be bothered I will spin them down on the cutter grinder. Got to get energy levels back after a recent stroke first though.

John Baron04/06/2020 21:14:09
297 forum posts
122 photos

Hi Guys,

One thing that that needs to be accounted for when counter boring for cap head screws is that the shoulder under the head is a fraction larger than the clearance diameter. So the counter bore pilot is bigger to allow for that.


Edited By John Baron on 04/06/2020 21:14:27

HOWARDT04/06/2020 21:29:03
556 forum posts
15 photos

Cap heads have a radius under the head, the clearance hole diameter should clear this. Therefore the clearance hole is the screw diameter +twice the radius under the head + tolerance on position. Of course in the model world you just ignore all that, make the hole as tight as you want on the screw diameter and tighten the screw.

Michael Gilligan04/06/2020 23:05:35
15712 forum posts
687 photos


It may be useful to look at this thread from 2017 : **LINK**



Edit: or this one, from 2015

Edited By Michael Gilligan on 04/06/2020 23:15:58

Colin Heseltine04/06/2020 23:28:20
409 forum posts
110 photos


Thank you for that link. Very useful information.


JasonB05/06/2020 07:08:19
18113 forum posts
1995 photos
1 articles

Thanks Michael, I knew there was a standard

I did see some counterbores with 5mm pilot and 8.7mm head for those wanting a really close fit but they did have larger head sizes too. There is also the option to get counterbores with removable pilots so you can make and fit one to whatever size you want.

Bo'sun05/06/2020 10:06:29
141 forum posts


If you desperately want a "size for size" fit you could still use the counterbore tool you have, providing the pilot length is shorter than the depth below the counterbore. If that makes any sense? Simply drill the pilot diameter depth to the length of the pilot + the depth of the counterbore. The rest of the hole diameter can then remain at 5mm.

Hope that helps.

Vic05/06/2020 11:06:37
2500 forum posts
14 photos

From a purely aesthetic point of view standard metric counterbores don’t produce a very neat result. I’ve therefore used a variety of other tools including some home made ones to produce something that looks a bit better.

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