Sump plug washer - does it matter which way round?

I would have thought domed side facing the sump so that it compresses as you tighten the bolt.

I doubt it. Either way it's going to get squashed flat. Mike.

Not if you do the bolt up nice and tight!

The flat side should be in apposition to the sump.

On a discussion on o-rings/washers, where a reduced clearance gap makes for reduced movements of the seal, "Changes in Pressure/Vacuum" "Cycling pressure can cause the seal to move back and forth within the gland. This can be especially damaging to seals with poor dynamic properties or in applications with low compression, which will allow for more motion."

This is in agreement with the advice for the correct positioning of the o-ring water seal (battery cover) on the back of my wrist watch. "Note the triangular shape of the gasket material. The flat surface (the base of the triangle) is the bottom, which should be against the watch body when the gasket is installed. The pointed surface (the peak of the triangle) is the top, which contacts the battery cover." ~ User Manual.

Hmmmm....

Started off as a washer question and ended up quoting about cycling and watch batteries.

Lost me!!! :)

Look into the engineering principle involved.....I give up!

Tip: Never do a sump bolt up "nice and tight".

Which are totally irrelevant to the question asked. Think about it. It's simply a washer fitting between two flat surfaces. Mike.

With the domed face of the washer presenting a smaller area when tightening is started, that side has the potential to be squashed first because the pressure on it is greater (same force, smaller area). So, do you want it to be forced into the profile of the sump or the profile of the nut? Does it make any difference?

Andrew

Guessing here but instinctively I'd say that the right way to do it (not saying that it does make a difference) is to have the rounded edge facing the sump. Surely the recess for the washer on the nut is square cut, not rounded. Also when the nut is being tightened they'll be more friction on the flat side (bigger contact area) and presumably you want the washer turning with the nut. Lastly I'd imagine than the rounded edge being crushed provides a better seal than a flat edge.

Peter.

There must be a sump plug washer designer somewhere reading this rolling around laughing thinking that making it rounded for no reason would eventually make someone ask :-) Unless there really is someone out there who actually knows...

Like, why do ball/roller bearings that take more than a negligible load always seem to have an odd number of rollers? It just seems too trivial to email an actual bearing manufacturer.

Andrew

I can't remember on which side the recess, the gland, for the washer is. I instinctively took it that it's on the sump. The flat should lie smugly in the recess, and movement of the washer should be kept to a minimum (to alleviate long-term stress/damage) in the course of performing its prime function. The movement in this case comes from thermal expansion and contraction (of the metal, not the elastomer) which is allowed for by the unfilled gap. This clearance is the subject of the discussion. The consideration is further complicated by the washer not being subjected to a high compression. This is the message in the article.

If the recess is on the nut, then the flat of the washer should also be there inside it.

Now all of this may sound like making a mountain out of a molehill, but we are in the spirit of striving for uncompromising engineering excellence -- Aren't we? :) -- as I believe the OP's inquiry undoubtedly is.

Could be something to do with harmonics in vibration? That's to avoid running out of control of vibration when a critical harmonic is standing and being augmented ad infinitum.

The message from "Andrew Morton" contains these words:

I've asked my nephew (who works in one of Ford's engineering design departments) why the Mondeo has a left hand thread on the towing eye. Dunno if he'll ever get round to answering me - he's up to his eyeballs in leaking showers at the moment.

The message from "Lin Chung" contains these words:

It's also probably slightly easier to make them - with an even number of balls/rollers the core will have to fit between pairs in opposition which would highlight any tight spots. With an odd number this would be averaged out to some extent.

AFAIK most cars don't have a recess, or profile in either the insert or the plug, therefore it doesn't matter which way the washer is fitted. Even if one or the other did have a recess, I can't see it affecting which way the washer is fitted. IME sump plug washers are either stamped out of solid copper, or formed from sheet in the same way as those for spark plugs, and the washer is simply squashed between the flat undersurface of the plug, and the flat surface of a threaded insert fitted to the sump. Mike.

& I'm prepared to bet a copper washers domed as it's made accidentally rather than deliberately.

In message , Duncan Wood writes

I would concur with that. I would imagine it gets like that when its stamped out.

All parts stamped from sheet have part radiusus, radii?, on one side, and sharp corners on the other, but very often copper sealing washers are formed from thin sheet copper which is then rolled, to give in effect an open double thickness, like a spark plug washer. Presumably cheaper to manufacture as less material is used. Mike.