[schoir]

Originally Posted by The Radium King

with patience and some valium i intend to put my engineering degree to practical use and learn how to assemble/disassemble my own engine. in a previous post i acknowledged that the LN cost didn't include labour but the Raby work did. I consciously chose not to value my own time in this comparison. in the roughness of my initial estimates i left out 'shop parts' like rings, etc.

i have a 3.2 that is starting to burn oil. i also have access to an inexpensive 3.4 with d-chiunk. i am in the early stages of analysis of my options, and i think that both should be fixable with new liners. with the new liners comes opportuninity for bore increase. while i can assemble/disassemble, i would have to outsource any machine work. hence my efforts to confirm the necessity of head reworking, etc., to accompany any bore increase.

again, not to discredit Jake in any manner (i have made every effort to value his work and input in this thread and continue to do so) but i am trying to separate unessential modifications from essential modifications. your definition of this will differ from mine. some hit on 19, some don't. some change ims bearings, some don't.

before i do things i gather information - lots of information. it is part of the process that i quite enjoy. it is necessitated by my remote location and need to ensure value for money. the internet is often my first stop, so that i can be informed when i take the next step and start working with vendors, etc. most of the conversation in this thread should have happened on the phone with Jake or whomever, but i was actually looking for input from someone who had increased bore without ancilliary mods so as to form a baseline.

if any value has come from this conversation, it is the acknowledgement that not everyone on this 986 board wants to (or can) put big $ into their car. everyone has different values and needs and will phrase their input accordingly. whatever; when it comes to opinion you get what you pay for on the internet. vendors should carefully analyse how they address this disparate group, however, lest they alienate a growing percentage of their target market.

my $.02; trk done with this thread.

Here is a refreshing take from a mechanic in England (named "Bazhart" on PCGB), who seems to have a knack for clear explanations when responding to a question regarding common failures:

"Sorry, I thought the weak spots or faults were quite well known now.

The most common failure in a Boxster or 996 at relatively low mileage (say 20 to 50 K) is a failed intermediate shaft bearing that seems to aflict all models.

The next most common in higher mileage 996's is a cracked liner. This should be more grequent in Cayman S and 997 models as the cylinder wall thickness is the same and the load higher - but they are newer and time will tell.

Then we get a mixture of broken chains, crankshaft bearings/shells, cracked cylinder heads (mainly 996's and a few Boxster S's).

All the above repeat quite often and form a list of typical failures we come to expect (can often therefore diagnose over the phone) and have done something about improving or repairing.

More rarely we have had odd things that have not repeated YET.

Some engine with the later variable valve lift system have had the canshaft lobe wear right through the tappet top face.

Sometimes - on all models - we have had core plugs coming out and a siezure of a piston on one side of the piston only - on the thrust face - sometimes a std 996 - more often a supercharged version - sometimes one with very large piston clearances because the combustion gasses pass the piston and burn the oil on the piston face where the rings no longer seal the now oval bore sufficiently.

It is this last fault that we have only seen once on s Boxster 2.5 or 2.7, a couple of times on a Boxster S, a few more on a 996, but increasingly more often with a 987, 997 3.6 and 997 3.8 - which is strange because they are newer than the other models and therefore generally have covered less miles.

Usually - if a piston seizes on the thrust face - it is a problem of thrust load on the cylinder face from the piston and a reduction in the ability of the oil film to support the piston resulting in it rubbing too hard on the cylinder lining. This is usually due to the oil film being too hot and the resulting reduction in support from the oil thinning with the heat.

To investigate if this explanation fits the engines we have seen we checked which side seizes - finding always the 456 side of the engine (or bank 2).

This side has a more tortuous route for the coolant to travel before entering the block, and the additional oil cooler to reduce flow slightly. Also - as the coolant enters from the bottom of both sides of the block (into both banks) the coolest coolant hits the bottom of the cylinders first and heats up as it passes accross the cylinders. However on bank 2 the thrust face is on the top and on bank one it is on the bottom - so the thrust face will be running hotter on bank 2 than bank 1.

This means that bank 2 will have the thinnest oil supporting the thrust face of the psiton.

Unusually - all the engines listed above split the amount of coolant that can pass into the cylinder block and the cylinder head so only about 1/10 goes into the block and 9/10 into the head and it mixes together again on its way back to the thermostat and radiator.

This makes the cylinders running hotter than the head and hotter than they would be if the coolant passage was like say the 944, 968 etc where all the coolant goes into the block first - which is therefore the coolest. Furthermore with such small coolant passages the coolant speed in the block will be reduced and the temperature rise higher and therefore noticeably higher on the top or thrust face on cylinder bank 2.

The models with variable valve lift create much higher torque at low revs (giving much better performance without revving the engine), however that increase in performance comes about because the piston is pushing harder on the cylinder wall at lower revs and probably more often as drivers enjoy the increased performance throughout the range.

We think there is a link here because all the engines with higher thrust on the hottest side are failing in this mode sooner than before - it makes sense to us anyway.

Always when weak spots occur in engines - they only ever afflict a small number - so the design can reasonably be called marginal or a weak spot. Consequently (for example) although most were OK - when a Rover K series loses coolant most garages assume correctly the head gasket has gone etc. Designers could always improve the design and erradicate the problem - if they had time and the intention - in the same way the we have provided reliable solutions to all the problems we have found.

However it is perfectly clear that many of the problems were not re-designed and have been with the engines for a very long time and so it is either because the engineers at Porsche found it impossible to identify the problems and rectify them or they could have but didnt't. I will leave it up to you to ponder on the likelyhood that if our minute business was instantly able to analyse and resolve the problems and find solutions - whether the Porsche engineers were capable of doing the same or better - or not!

Regardless of all that - we can only re-balance the cylinder temperature problem upon rebuilding the engine and so - my strong advice - to make sure you have a warranty you can trust - especially with variable valve lift engines.

We are testing a cooler termostat that will lower the overall temperature as a simple low cost preventative measure to protect engines and will report on this soon.

Baz "

Regards, Maurice.