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Anyone _successfully_ rebuilt a Bosch mechanical metering unit
In message <VA.firstname.lastname@example.org> Chris Wilson writes:
> Has anyone actually had a Bosh 5 cylinder mechanical injection metering unit
> apart and _back together again_ ?
Judging by the swathe of dust I just left around east Northamptonshire - yes.
> I have had dead ones apart,but hear tales of woe about it being impossible
> to reseal the separator "shim" between the top and the main body.At least 2
> people have stated that in manufacture the shim is micro deposited with a
> sealant,that is impossible to recreate...Sounds a bit suspect to me.
No, it's true. If you get "the kit" from Bosch, the replacement diaphragm is
clearly coated with _something_.
> When one fails,and starts to do "odd" things,is anyone au fait with them
> sufficiently to say _what_ wears,and if there is any possibility of
> rectifying them?
(Using knowledge only recently acquired from Tim at BR Motorsport ...)
Yes. The major single cause of failure is failure of the upper O-ring.
Tim and I dismantled my metering head a couple of weeks ago, on the search for
an elusive problem. I had tested, cleaned, renewed and rechecked just about
every other component - Tim checked my work, and then pointed the finger at the
It's easy to take apart. A few Torx screws. Wash the heads out with WD40
before you start, then wipe it down. On the MB head, the central (control
pressure) port is proud of the centre of the head and makes a useful gripping
spot for the vice.
a) Avoid all sources of dust, filth, rubbish, etc., with passion. Go to
b) Dismantle with care. Drop _anything_ onto the workshop floor, and you
might as well throw everything in the bin.
c) After dismantling, dry all components _very_ thoroughly.
d) With the head clamped upside down, the upper O-ring is on the central
surround to the plunger, which remains in the upper half of the head
when you lift off the bottom. This will probably be your problem.
e) Be careful with the tiny O-rings around each internal port. Make sure you
have an acre or two of lint-free bench space to lay everything on.
f) With the O-ring replaced, re-assembly is the reverse of dismantling.
g) The factory sealant can be imitated with hydraulic sealant - this is sold
under brand names like "Loktite". Look for the type that's intended for
high pressure hydraulic seals. Put 1/4 ounce in a small container, and
paint it onto the flush surfaces of the upper and lower metering head
halves with a fine brush. Don't get any into the ports. I hope you
dried it thoroughly.
h) When reassembling - there is a _MINUTE_ (and I mean _SMALL_) hole in the
diaphragm that needs to be aligned with holes drilled in the upper and
Tim (who showed me how to do it, and has done it dozens of times) took around
half an hour for the disassembly, and around an hour for the assembly. I have
never seen anyone work so slowly and carefully.
There's only one critical adjustment, and that's the play in the spindle.
Before you take it apart, use a _good_ digital depth gauge (or the end of a
digital caliper) to measure the free movement in the spindle, and use the same
gauge to reset this value when you're done. I think this part, at least,
should be discussed in Bentley - it's in the Audi microfiche. The official
adjustment tool is a Hazet 2655-2. I've only ever seen a picture.
Committee Member, UK Audi [ur-]quattro Owners Club