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?
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
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:
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:
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
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.
*Why is "abbreviated" such a long word?
Dave Plowman firstname.lastname@example.org London SW
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.
The same considerations apply to repair by the pros except
as far as the use of special tools to do the repair is concerned.
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
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