Too many o-ring sizes?

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?
--

Jeff

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On 11/4/2016 7:24 PM, Jeff Layman wrote:

I don't think it is unexpected, there are so many different locations in a pressure washer.
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Sub assemblies from different suppliers maybe? Perhaps value engineering defining the most economical size without defaulting to rationalising to make assembly and stocking easier?
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On 11/4/2016 8:42 PM, DerbyBorn wrote:

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.
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On 04/11/16 21:10, newshound wrote:

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: www.ereplacementparts.com/images/karcher/K_520M_DI-Plus_(1.069-700.0).pdf
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: https://www.sparesgiant.com/media/pdf/champion/Champion%20CPW1300%20Red.pdf
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?).
--

Jeff

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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.
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*Why is "abbreviated" such a long word?

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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That utterly mangles the real story, as always.
If it was that simple, they'd all have large numbers of only slightly different O rings and fasteners in them, and they don’t, particularly with the much more common stuff like cars.

True.

The same considerations apply to repair by the pros except as far as the use of special tools to do the repair is concerned.
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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.
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Chris Green
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wrote:

True.

Have fun listing any appliance that is more likely to be designed. Vacuum cleaners clearly are. The only relatively more common thing is cars now.

I don’t believe that, particularly with the kraut products.
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IMO it was 'designed' by a fuckwit.
I have a Gerni myself, but havent needed to fix it.
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On 05/11/16 03:07, Rod Speed wrote:

Seems that Gerni are Nilfisk, and those seem to be recommended here as being reliable.
--

Jeff

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