Too many o-ring sizes?

Question

On trying to use a Karcher 520 pressure washer which I hadn't used for a couple of years, the output pressure was obviously low, and water was coming out of the plastic case. Looked like a seal or two had gone.

I downloaded the service diagram and found that Karcher had used at least six different sizes of o-rings (and that didn't include the large gasket seal on the motor). Why so many different sizes? There is 12 x

2mm, 11 x 2mm, 9 x 1.5mm, 6 x 2mm, 4 x 2mm, 3.69 kpl (that's what the parts list says)

Is is usual to have that many different sizes in a pressure washer? Some of them aren't in the o-ring sets available from Screwfix or Hilka.

But it could just be Karcher - the first problem was getting the case off. No posidrive or even hex-headed bolts, of course. Karcher have to use Torx. And not just one size, either. There are three different sized screws, 20, 25, and 30. Why? Couldn't they find a single size which worked?

newshound · Accepted Answer

I don't think it is unexpected, there are so many different locations in  a pressure washer.

DerbyBorn · Answer

Sub assemblies from different suppliers maybe? Perhaps value engineering  defining the most economical size without defaulting to rationalising to  make assembly and stocking easier?

newshound · Answer

O rings are available in a huge range of standard sizes. There are several different functions in a pressure washer: normally at least three seals in the pump, then there is the trigger valve and the lance connection. It would be crazy to engineer those components to take a single size.

Jeff Layman · Answer

Those sizes are for the pump part only. The lance uses yet more sizes! There are also parts which include metal and o-rings as a unit where the sizes are not stated, but appear to be different from the other o-ring sizes. For example, see parts 11, 12, and 13 in diagram F2, and compare that of the o-ring in part 13 with that of the o-ring numbered as 7 in part part 6 here:

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I'm not questioning two or three very different sizes might be needed, but 12 x 2mm, 11 x 2mm, 9 x 1.5mm? Surely they could have used just 11 x

2, and designed the metal parts to fit. And, with reference to my comment above, why sell some o-rings spares separately and some only as part of a unit assembly?

But it seems other manufacturers also use umpteen different sizes of o-ring in their pressure washers. For example, see page 3 here:

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If, when I get the pump apart, I find several different sized o-rings have gone, the expense of buying a replacement kit might not make repair worthwhile, and the money might be better spent on a decent replacement (Nilfisk?).

Dave Plowman (News) · Answer

Production engineering is about keeping costs to a minimum. If you can save 0.001p on a part by using a smaller one, it adds up when making thousands of them.

Just how easy it is to repair if ever needed is not really a concern. Even more so for DIY repair.

Chris Green · Answer

Not necessarily true for this sort of product surely. I doubt if the quantities are into hundreds of thousands and in the thousands sorts of quantities it might well be that 20000 of a single size of O-ring is cheaper than 10000 of size A and 10000 of size B.

I doubt if all that muc thought goes into 'design' on these things, they just throw together a collection of standard functions and that's it.

Too many o-ring sizes?

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