Re: EKO181K - 1972 Porsche 911T 2.5
Posted: Sun Nov 25, 2018 11:09 am
...and yesterday I went along to see where we were.....not how I expected the engine to look when it was taken out for a 'service' a few weeks ago....
....whilst I've said previously that this engine has been rebuilt a couple of times that's not strictly the case......the first issue with the pistons touching the heads involved Nick Fulljames having special steel gaskets made to solve the issue, work he carried out free of charge even though the engine had done several thousand hard miles......the second issue with the chain case failure meant the engine was stripped to a short motor and bearings replaced.....the engine cases have never previously been apart since the original build........however this time around we're starting from scratch and making sure everything is perfect and within spec....case line reemed and conrods measured, balancing checks, everything polished, time certs checked/replaced etc etc....
....one thing that was noted is that the mating surface of the barrel decks was not completely flat.....hard to see here but with the cases bored out for 89mm barrels the 2R/3R cases are stretched to their limits, just how they would have been for a 2.5ST engine built back in the day....
....and Gary believes these may well have been the cause of the problem.....
....as Gary explained the steel bolts have a far different expansion rate than the magnesium case and alloy heads and a with a case that can expand by as much as 3mm over a heat cycle this has put extreme pressure on the bolts which in the first instance caused the copper barrel gaskets to be squeezed outwards causing a loss of compression and piston heads to touch and you can even see a slight depression on the later steel gaskets.....it's little wonder the barrel decks surfaces have changed....
....the steel bolts are fine in a regular engine but we'll now upgrade to Dilavar bolts, as did Porsche in the late seventies, which will match the expansion rate of the cases and heads whilst new gaskets will be manufactured and barrel desks machined to suit.....this is all very fine margins of a few thou or so but work that will make this fabulous engine stronger than ever
....whilst I've said previously that this engine has been rebuilt a couple of times that's not strictly the case......the first issue with the pistons touching the heads involved Nick Fulljames having special steel gaskets made to solve the issue, work he carried out free of charge even though the engine had done several thousand hard miles......the second issue with the chain case failure meant the engine was stripped to a short motor and bearings replaced.....the engine cases have never previously been apart since the original build........however this time around we're starting from scratch and making sure everything is perfect and within spec....case line reemed and conrods measured, balancing checks, everything polished, time certs checked/replaced etc etc....
....one thing that was noted is that the mating surface of the barrel decks was not completely flat.....hard to see here but with the cases bored out for 89mm barrels the 2R/3R cases are stretched to their limits, just how they would have been for a 2.5ST engine built back in the day....
....and Gary believes these may well have been the cause of the problem.....
....as Gary explained the steel bolts have a far different expansion rate than the magnesium case and alloy heads and a with a case that can expand by as much as 3mm over a heat cycle this has put extreme pressure on the bolts which in the first instance caused the copper barrel gaskets to be squeezed outwards causing a loss of compression and piston heads to touch and you can even see a slight depression on the later steel gaskets.....it's little wonder the barrel decks surfaces have changed....
....the steel bolts are fine in a regular engine but we'll now upgrade to Dilavar bolts, as did Porsche in the late seventies, which will match the expansion rate of the cases and heads whilst new gaskets will be manufactured and barrel desks machined to suit.....this is all very fine margins of a few thou or so but work that will make this fabulous engine stronger than ever