My top end rebuild is well underway and I went to inspect the progress yesterday. On cylinder #3 there's clear evidence of leakage as per the attached photos. Note the carbon traces from 11:00 to 15:00 on the head outside the sealing ring groove. There's also a small amount of material missing on both the head (slightly clockwise of the stud hole in the upper right corner of the head photo) and the cylinder (slightly counter clockwise of the stud hole). There wasn't a broken stud or loose nut. Is this just from break-down of the sealing ring over time and then the combustion gases eventually finding a way out? Or some kind of fretting? Initial belief is that the head can be skimmed but that the cylinder needs to be replaced but more measurement is required.

Exhaust valve guides were worn as seems to be the norm. I would guess 0.8 to 1.0mm of lateral play of the valve head when just off the seat with commensurate wear signs and micropitting on the sealing areas. Leakdown figures were: 14, 10, 26, 8, 12 and 12 percent. These leakdown figures are seen by some as acceptable (excluding cylinder #3) but given the exhaust valve guide wear this seems to reinforce the statements made by several experienced folks on this board that leakdown figures need to be in the 6-8% range and no higher.

 

Porsche has updated the head studs over the years.. Please purchase ARP studs for your new motor. If this was my engine build I would weld the sides of the heads and install EVO stepped sealing rings.. What you have shown is common.

 

The only ARP studs I can find are part number 204-4206 but they're not listed for the 993, just the 911/930?

 

Thanks for sharing Felix - do you have any idea how your engine got like this....or is this typical for the mileage?

 

Kenny, as I posted... "Is this just from break-down of the sealing ring over time and then the combustion gases eventually finding a way out? Or some kind of fretting?"

 

Thanks felix.

I was wondering if this particular symptom could be related to your experience with the car and how it's used e.g. lots of tracks days, frequent short journeys, laying the car up - the opposite of these etc...

 

The plot thickens... the chap that showed me everything on Saturday wasn't aware that a piece of metal was found clamped between the barrel and the head! It would appear to have been there since build time. Hence the oily/carbon traces from the leak of the unburned combustion mixture but no signs of errosion from the ignited mixture as it was protected by the piston. They've saved it; I'll post a photo later.