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What
follows is a fairly detailed description of a "top end "rebuild
of a 911 air-cooled engine. The basic process applies to virtually
all 911 air-cooled engines but of course there are specific differences
to specific models. This piece is designed for you, the customer,
to understand exactly what goes on and where you are spending your
money.
Once it has been decided that the rebuild needs to be done, usually
after careful consideration, the engine and gearbox are removed
from the vehicle and the two units split. With the clutch and flywheel
removed the engine as a whole unit is bolted into a purpose made
engine stand. This rotates 360 degrees in order to aid disassembly
and assembly. The inlet manifold and injection system are removed
along with the black painted tinwear. This leaves the air-cooling
system behind along with the cooling fan and alternator. This is
then removed as a complete unit. After rotating the unit 180 degrees
we then tackle the exhaust.911 exhausts are notoriously difficult
to remove and great care and skill is needed to undo the exhaust
manifold nuts without the studs breaking. If this happens it adds
to the cost of the job along with the extra hassle of drilling out
the offending items. Oxy-acetelyne welding torch is used to heat
the nuts in order to remove them without the studs breaking. We
usually have a 100% success rate! Sometimes though especially on
older engines the studs need to be replaced anyway as they rot so
badly you cant fit a new nut to them.
Once the hard bit of the exhaust is overcome you are basically left
with what is known by some as the "long block". This is
the basic engine with all its ancillaries stripped off leaving just
the main body of the engine, comprising crankcase, barrels, cylinder
heads, camshaft housings and camshaft drive.
The top and bottom cam covers are removed as well as the front timing
covers. This exposes the camshaft valve gear as well as the camshaft
drive. Once the drive is removed we then undo the cylinder head
retainers and remove the head and camshaft housing as one assembly.
These assemblies are then stripped separately on the bench, where
the heads (6 individual) are also stripped and carefully checked
and measured against Porsche specs. All our heads are bead blasted
and have new valve guides fitted along with a special valve seat
cut. The valves are then lapped in and the 6 heads are reassembled
with new valve stem seals as well as having the vale spring heights
set up with shims, again to very tight factory specs. With all the
rockers and camshafts carefully checked the whole unit is assembled
as one, ready to refit onto the barrels. This is repeated for the
same assembly on the other side.
The barrels and pistons is another area that we study very carefully.
The 6 individual barrels or cylinders are removed as well as their
related piston, taking care to keep them matched together. The pistons
are bead blasted and crack tested as well as measured, again to
Porsche specs. The barrels are also checked for ovality and general
wear. When refitted they have new piston rings and new base gaskets
ready to take the rebuilt head/camshaft housing assembly. With regard
to 964 engines we also replace the crankcase through bolt seals.
The through bolts bolt the two halves of the crankcase together,
but they also go through the main oil gallery. Porsche fitted a
tiny rubber seal at each end of the bolt to seal up this gallery,
but they didn't get it quite right with the 964.Basically the type
of rubber used got very hard and brittle and lost is elasticity
very quickly and thus failed to keep the oil in its place where
it should be and not on your garage floor! The same applies to the
seal known as the base gasket. This is a rubber "o"ring
fitted to the bottom of the barrel to seal the barrel into the crankcase.
For the first time Porsche used rubber rather than the traditional
copper version. One can only surmise that cost played an important
part in this decision.
So with new base gaskets, through bolts seals and piston rings fitted
the bottom end is ready to take the rebuilt heads and camhousings.
Once these are bolted down and the cam drive re-assembled all that
remains is the all important camshaft timing. This is done with
a DTI gauge, which measures the amount of inlet valve opening at
a specified point. It measures down to 100th of a millimetre, so
it's pretty accurate. Porsche specify, say for a 911SC 3.0 liter
engine, a lift of0.9 to 1.1mm with the ideal setting of 1.00mm.We
strive to get the camshaft timing absolutely spot on as well as
getting it matched for both banks as the end result is a more powerful
and smoother running engine.
All that remains is to refit all the ancillaries and exhaust system.
This is split into its component sections and rebuilt using new
nuts, bolts and gaskets. With painted or powder coated tin wear
and all the relevant bits cleaned up the engine starts to take shape.
If the engine is has an electronic injection system , all of our
engine rebuilds benefit from an injector overhaul on our ASNU injector
machine. This cleans the injectors ultrasonically and along with
new filter baskets and spray nozzle caps ensures they are in perfect
order ready to run in the new engine. As part of our rebuild package
we carry out a major service as part of the rebuild for no extra
cost. As most of the service work is involved with the engine it
basically entails the chassis related work. It also means you get
a service stamp in the service book. If you can co-inside your rebuild
when the service is due its like having a free service, e.g., top
end rebuild menu price for 1990 964 is £3500.Major service
menu price for same model is £450 so if engine job is done
at the service due time the real cost for the engine is £3050.All
prices are plus vat.
Once the engine and gearbox are refitted into the car, the vehicle
is road tested for at least 25 miles (usually more). If all is ok
then its over to you to run it in for 1500 miles. Once run in we
recommend that the vehicle be returned to us for a first engine
service. This entails an oil and filter change, check and adjust
valve clearences, check tightness of rocker shafts and cylinder
heads and a final check on the tune.
We hope this has been of help to anyone contemplating having this
type of work done, and would be glad to answer any further questions
by phone or email.
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