Kevin Ross

Hi Everyone.

As you may remember, my engine is in at Porsche undergoing a rebuild. They have come back to me and made some recommendations while the engine is in the state it is in (see pics)... and I want your opinion if you will...




Just some background:
Check Engine light came on,
Porsche did a compression test, and 2 Cyls showed low compression
They have taken the engine apart to the current state (see pics with 2 cyls open), and found that the valve guides were the culprit.

So, while the engine is as it is, they have recommended, amoung other things, the following:
do the through bolts
replace piston rings
maybe go the whole hog and replace the bearings

Of course, the further I go, the steeper that $$ slope becomes. I have decided to do the through bolts for sure, but it is the other two I am hesitating on. I understand that just the rings (parts only!) will cost US$1650. With labor that's around $2000!!

I have a friend who is a Porsche Mechanic who has said that in his opinion the rings and bearings are fine (you can still see the factory machine marks on the inside of the piston chamber).

Any input?

911/Q45

I would either leave it be or go 3.8 while it's apart. How many miles on the motor?

Kevin

Kevin there are 2 avenues that most people address.

1) Most dealership's will only recommend fixing what needs to be fixed and leaving everything else..

2)Being pro-active and you have already covered the labor bill, go and replace a few nickle and dime parts and give yourself some piece of mind and some resale ability in the future. I'd replace every ring in that engine, I'd also pull off every rod and put new rod bearing in. I'd go one step further and send the rods out and resize them back to specs and put ARP rod bolts in.. I'd install camshafts and do a complete valve job on the cylinder heads.. All of these mentioned mods will insure a longer life, and if you sell your car, will bring something to the table.

Kevin

I'd also backdate your camshaft sprockets to early 993 or 964, so that you can get precise camshaft timing between each bank.

Geoffrey

Depending on the mileage and reason for the inspection, I might not go any further into the engine. If the engine is showing signs of high leakdown, it will usually be 1 of 2 things, either ring sealing (not necessarily the rings) or valve seats. I have seen the 993 exhaust valves become pitted due to poor quality valves used. Additionally, the guides are soft and there is some question as to the specs used for clearance. The valve sealing is most likely the cause of your leakdown numbers, and any excessive burning of oil will be worn valve guides. Further, it is usually the pistons that wear, not the rings. Due to the wide bore and short rod, the piston rocks enough to rub the side of the cylinder. This wears the piston and to a lesser degree the cylinder. Simply replacing the rings as Kevin suggests does nothing to help sealing if the cylinder has been scored from the pistons. The rings don't wear out, they might break due to detonation, but it takes lots of mileage to wear the rings.

If I thought the cylinder looked scored thatn I'd probably pull 1 cylinder and measure the bore and look at the piston. However, know this is at the dealership, you are stuck with the "replace" mentality.

I would however, replace every valve guide, and every valve with aftermarket parts of higher quality. I'd also replace the cam sprockets as Kevin mentions to ensure the timing is proper and remains proper. If I was going to modify the engine later, then I would take the opportunity to replace the rod bolts with ARPs and have them check/resized as necessary.

viperbob

$1650 for rings? Maybe from Porsche. OE rings are about $200 here in the states for a 993TT.

Peter S 993tt

I'd do the head studs(since you are pulling them) and the valve guides (and valves since you will be pulling them anyway also). Forget the bottom end - you shouldn't have a problem. In the future, I suggest leaving the under tray off - that is most likely the cause of the worn guides - the excess heat.

I'm just curious what's this about the cam timing sprockets? Is this a known issue?

weston98

I've also heard the rods on this engine are lightweight and weak. You may want to consider Pauter or Carillo replacements and ARP rod bolts. Maybe valve springs in addition to the other good suggestions made in this thread. I'm no expert, just what I heard.

Peter S 993tt

Funny - I've heard that the bottom end is rather stout.

Kevin

The rods in the 993TT are the weakest design to date. However, you will have rod bolt failure prior to rod breakage or bending..

With regards to my statement about replacing the piston rings, one has to realize that as the pistons move in the cylinders the rings have got to have motion. They have to be free to expand. When a valve guide has failed, or oil has been introducted into the combustion chamber, carbon builds up in the ring lands. Once the rings freeze (from carbon buildup) there ability to seal the combustion gases deminishes. You start to have blowby into the crankcase. Combustion gases pass the rings and pressurize the engine case. And this recks havok on your turbocharger sump system to work, because the added pressure is keeping the oil from flowing through the turbochargers.

So, when you have 2 or 3 cylinders failed. One should pull the pistons out and clean the ring lands. If you have gotten this far, why not replace all the rings.. It's like, replacing spark plug, you can sand blast them, but most replace them with new ones.

Jean

Please do the rod bolts as Kevin mentioned, head studs are a good idea as well. Inexpensive and very useful if you decide to go beyond stock hp. The rest of course depends on your budget, but I would do as much as possible, especially everything in the heads, and spec out the P&C.

Kevin Ross

ok, I'm just absorbing everything said thus far... thanks so much for the input!!

I have to say I have no idea what head studs are, but I can say that ALL the valve guides are being replaced not jus exhaust ones... and the throughbolts are being done... plus some other stuff.

It was explained to me that the bottom end is perfect and not to touch it, and that the rings appear fine (not that they themselves have been inspected, but given the wearing on the actual chamber (?) apparently it looks like it just rolled out of the factory!

But keep it coming... just give my non-tech mind time to catch up - you guys are guru's and I need to absorb it all!!

chris walrod

One thing to consider with regard to reringing. One must measure ring land on the piston looking for excessive and out of (or near) wear limits dictated by the factory.

If these lands or grooves are wider than ideal, installing new rings have been known to accelerate wear local to ring grooves. This is from the increased stiction of new rings. In other words, new rings grab the cylinder bores more than that of seated rings.

Too much axial ring clearance will lead to ring breakage. Maybe this is too over the top, but a thought nonetheless.

Kevin

There are a few Porsche engine builders will send pistons to Total Seal and have them cut the ring lands when the land is out of spec as Chris has mentioned. Oversized rings can them be installed. You can then hand file the ring gaps to your specs based on you cylinder wear.

However, it is critical to measure your bores to see if they are within spec.

I seen recently that Road Race in LA has a few surplus piston/liner for sale..

Kevin

Kevin are you having rod bolts replaced with ARP ones? You really need to consider doing it..