Shnorr washers are made to fit under cap screws. They are the same diamiteras the head but have a sedated face and can be dished to help friction and are Usualy hardened like a spring.

David

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I was avoiding the temptation to have my usual rant about how much better the Imperial cap-heads can be … but let me just reference this Unbrako page … which offers free downloads of some very informative documents:

**LINK** https://unbrako.com/catalogs-downloads

MichaelG.

If you cannot find them stack some on a mandrel/bolt and turn the required amount off. I know its a pain but quicker than sarching for and awaiting delivery.

regards

Well you did mention standard, this site lists 6 different standards from around the world such as ISO, DIN, JIS, ANSI, etc for metric socket screw counterbores and all are 15mm for M8 so I can see why most washers fit. I think there are a couple of closer and looser standards about possibly in Machinerys handbook. Also worth looking at the ANSI imperial which gives a larger dia than your McMaster Carr ones at 1/16 over head dia.

What make are yours and are you using metric ones or a 5/16 one which is rather close to the Ansi chart?

Edited By JasonB on 19/10/2022 18:11:53

Edited By JasonB on 19/10/2022 18:14:14

Have you put M8 x 13 washer into Google, plenty come up there eg Amazon, Accu, e-bay so they are available to suit your tighter holes.

Edited By JasonB on 19/10/2022 18:41:01

Too many on the net to be oversize metric and they tend to be called shim washers and come in a range of METRIC thicknesses eg 0.1. 0.2. 0.3. 0.5, 0.75, 1.0. That chap Ali who sends express delivery has a lot to offer 8×10, 11, 12, 13, 14, 16 which don't all tie in with imperial sizes

Just wondering if Bill's 13.4 counterbore is a 5/16 imperial one being sold/described as metric.

Edited By JasonB on 19/10/2022 18:57:40

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That’s probably a good call, Jason

17/32” nominal according to UNBRAKO

MichaelG.

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Pages 82 and 83 of the ‘Engineering Guide’ give a nice summary 

Edited By Michael Gilligan on 19/10/2022 20:01:56

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It’s probably because they have a long tradition of doing things meticulously, and 15mm would be abhorrent.

MichaelG.

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Here’s a rabbit-hole quoting a wide range of standards: 

https://engineersbible.com/counterbore-socket-din/

… haven’t followed-up the links yet.

Edited By Michael Gilligan on 19/10/2022 20:40:53

Not seen the last of the three but YG-1 list fine and medium screw hole as well as tapping size, all with the same 15mm for M8 heads

I suppose it makes sense as if the head hole was tight there would be little point in having the screw hole larger as the head would not be able to wobble about if there was no room around it. If you were using aircraft type counterbores then one 15mm would be needed and just swap pilots as needed to suite the clearance hole.

Don't know what standard those ones in the chart are made to but at least your washers are made and available at 12 or 13mm dia for M8.

Delighted to see Hopper's comment, because it fits my amateur thoughts on the subject.

I started by asking what washers are for? A quick look-see revealed three main reasons for using washers:

1. A washer is needed if the material being fastened is weaker than the bolt. Then washers provide support to the top surface as the bolt is tightened, which otherwise tends to break up around the entry point. The effect is most obvious on soft materials like wood, but it happens to metals like Aluminium, Copper, and Mild-steel as well. I imagine Cast-iron being weak in tension makes it likely to need washers too. The 'weak material' needing protection can be a layer of paint rather than the underlying metal.
2. They're designed to absorb vibration, split springs and so forth, which is important in anything that moves because alternating stresses are remarkably good at undoing threads. Not good when a wheel falls off!
3. To seal the connection against liquids, either to prevent leaks or corrosion.

As far as I understand washers from my armchair, none of the reasons for using them require the washer to be a particular size. So why do Imperial counter-bores come in different sizes? I suspect the reason is historic. Imperial tooling dates to a time before materials were made to a consistent specification, therefore the size of the washer needed depended more critically on what was being made. An unreliable material needs bigger washers than reliable materials, but drilling large counterbores is more expensive than drilling small ones. In this world it's a good thing that designers and production engineers had a choice.

Metric fasteners are a latter development, becoming popular after metals were made to a consistent specification. In this world, I think it's cheaper to drill one adequate size of counterbore hole sufficient for most materials. The metric system is deliberately simplified compared with predecessors, usually for the better.

However, although modern engineering strongly prefers using standard sizes it's always possible for the designer to decide otherwise, for example by specifying a larger or smaller counterbore. But they'd need a strong reason because it involves buying non-standard tooling in a brutally competitive world where extra cost is dangerous.

Dave