I've taken pictures to help explain but can't easily upload them – I can email them to someone to upload if there are any volunteers, otherwise similar pictures can be found on our website
www.hartech.org
The normal potential problems with the cylinder design are scoring/scuffing & cracking.
Scoring isn't as common in the 996 3.4 engine as the 3.6, 3.8 & 3.4 987 models but it can still happen, particularly with the Ferroprint pistons.
There are numerous contributory factors with scoring but the underlying issue is that the cylinder bore running face (Lokasil) isn't quite up to the job in many cases. This is why we choose to go with Nickel Ceramic plating (like in the Metzger engine) as a running surface which is much more durable – it is electro plated onto an aluminium alloy wet liner which shares the same expansion and contraction rates as the rest of the block so is nice and stable in both the deck height and integrity of fit rather than a different material with half the expansion and contraction rate.
Cylinder cracking is a totally different issue and comes about because the original design is essentially a tube of metal just surrounded by water without any support at the top. The thrust loading over the years pushes against one portion of the cylinder and it gradually goes oval and eventually cracks. Our new cylinders are a 'top hat" design so naturally stronger and are locked against the casting in a precision machined diameter - so cannot move so will not migrate oval and crack. The original design is an 'open deck" whereas the wedging of our liner against the other part of the block makes it more of a 'closed deck". (Incidentally, this isn't a classic problem with the smaller engines as the cylinder walls are thicker & stronger).
The 'closed deck" element of our liner can be re-produced by infilling the space with a support ring of metal.
The pictures you showed me yesterday David are original Lokasil cylinders but with support rings fitted – hence them being described as 'closed deck". They are NOT liners nor Nikasil plated. They don't look like our work either (well certainly not anything we've done for the last 15 years).
Going off tangent a little, there are 2 classic areas in these engines which will eventually lead to a failure through natural wear & tear/degradation – cylinders gradually migrating oval eventually cracking & crank shaft bearings wearing out. There's a pattern where both seem to start happening at similar mileages (90k-100k upwards). There are obviously lots of cases where engines last longer but there are also others which don't quite make it (we have 2 in at the moment – 1 at 75k and 1 at 80k).
Cylinder cracking isn't too much of a drama – a re-build is needed but the damage is isolated but crank shaft bearing failure is another story and a disaster. The crank & rod will usually be wrecked and potentially the carrier. The debris then usually spreads and will kill the oil pumps and, more often than not gets into the cylinders damaging pistons and potentially the bore surface material. In the very worst cases the rod breaks and the whole engine can be smashed and written off. Cranks, carriers etc are getting much harder to source & second hand values are creeping up.
Consequently, there's a very strong case for replacing the crank bearings as the mileage gets higher before they've failed, particularly if the owners plan is to keep the car for some time.
Hope that helps !
Grant
Porsche 991: 2011-2019
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