The Timing Belt
An ounce of prevention
In the last 18 months Iíve come across several 928s where
mechanical items have come loose and caused expensive damage Ė damage
which didnít have to happen. The first item goes back to motors now at
least 15 years old Ė the single cam 928 engines (4.5 and 4.7 litre), and a
problem that has started to appear in the last couple of years with the twin
cam engines (5.0 and 5.4 litre).
Single Cam = 1 cam per bank = 2 valve-heads, 16
Twin cam = 2 cams per bank = 4 valve heads, 32 valves
It concerns the torque of the single cam gear retaining
bolt. The cam gears on all the above engines are made of aluminium and what
happens is the torque of the on and only retaining bolt reduces with time
and mileage, and allows the cam gear (aluminium) to chatter (move) on the
camshaft machined end boss and keyway.
Note that the camshaft in this area is hollow to allow the
bolt to go through. When it loosens, it will only chatter for a short while,
causing only a slight ticking sound from the end of the cam, near the gear.
Then, shortly after the camshaft breaks just behind the gear.
In the early years many theorised that the reason for this
is that the aluminium cam gear on ďallĒ 928 engines butt up against a
machined flank on the camshaft boss, which is very petite. Over time this
area crushes slightly causing the torque holding it secure to reduce, and
the process then accelerates over time.
Result: One broken cam 'at least'.
On the 4.5 and 4.7 engines (single cam), usually the valves
will not hit the pistons. But on the 5.0 and 5.4 engines (twin cam), they
will! And depending upon the engine speed when it lets go, can do a lot of
damage. Bent valves, damaged valve guides, possibly destroyed hydraulic
lifters and, if really unlucky, damaged or destroyed heads and pistons. Plus
the broken cam or cams. All because no-one bothered to periodically check
and tighten the cam gear retaining bolts.
Iíve been checking these every time I have a new customer
come in for repairs, and then after every 40,000km.
Single cam 0.8 hours
Twin cam 1.3 hours.
The second item concerns all 928 engines, and the
tensioner roller that tensions the timing belt.
One case: Iíve gone to replace the
cam belt when we (myself and owner of vehicle) a bit before it was due.
The new owner had only recently purchased the vehicle only to find that
the previous person who replaced the cam belt years before made a couple
of what seem to be common mistakes.
a) No Circlip. For some reason the circlip on the
small shaft that is part of the old tensioner roller assembly that acts as
an end-stop. This circlip attaches to the tensioner arm, and is often not
removed from the old roller assembly and fitted to the new one (if assy is
replaced). This causes the roller to go in too far and touch the arm itself
when the roller bearing wears and gets freeplay. This in turn causes it to
run out of alignment. I can no longer get these circlips from Porsche spares
but can get exactly the same ones from Wurth Products.
b) Tired Tension Roller. Always fit a new tensioner
roller when belt is changed. Because the tensioner roller will sometimes not
last 2 complete ďlivesĒ of cambelt (i.e. 2 cam belts, fitted over
The point is the cost: if the roller
gives up and itís a single cam engine, not a huge drama. But itís a
twin cam engine, we have bent valves.
The third item is the hydraulic tensioner boot. There
are 3 different types used over the years and all are available from spares,
thank God. Always replace the boot when the cambelt is changed because the
boot goes hard, cracks, and lets dust into the tensioner. The small supply
of oil can also get out.
The fourth item is the step bolt and pivot bushes.
There are two types of stop bolts, depending on your year model:
These step bolts hold the tension roller arm onto the
engine, it is possible to break them if reused (the threads stretch). There
is only one, and it is quite petite. Again, always replace and once again,
if it breaks or falls off your twin cam engine, you have bent valves.
Note: Iíve been informed that
Porsche has now withdrawn the early type that does up to the 1980 model,
inclusive. If we all write or e-mail PCA and request it to be made
available, they may do so.. because it we donítÖ well, whatís next?
Unit 2/2 Paton Pl, Balgowlah NSW, 2093
(02) 9948 2651