For whatever reason, and this is observational, case though bolts tend to leak or break on 993 engines. We began seeing this around 2010.
For whatever reason, since 2015 or so, and not before that, we have seen a fair number of 993 engines with case through bolt leakage. In our experience this is the #2 upper bolt from the front and invariably on the 1-3 side. In order to replace case through bolt o-rings, the cylinders need to be removed in addition to heads/cam towers etc. We do this as part of our top end. A top end generally does not include rings. We now always include the case through bolt operation in addition to any top ends, along with front and rear crank seals, intermediate shaft seal, oil pressure console seal and oil breather gasket.
A customer who purchased a 95 993 engine with a top end upgrade (cams, upgraded valve gear) and redline set to 7500 had a pair of case through bolt break. I have not seen this since, and the replacement bolts, as far as i know, have held. It should be noted that GT3 engines, which are architecturally very close to the 996 twin turbo, which, itself shares the bottom end with a 993, have a modification of an additional four o-rings on the center lower case though bolts.
There are 14 case through bolts/studs
- All seven of the upper case through bolts site in a pressurized oil channel, since the bore is used to distribute oil to the bearings and case squirters from the main oil gallery.
- The first lower bolt from the front, inside the cam box is a stud so it’s supported.
- The second lower bolt feeds oil to the intermediate shaft so it’s pressurized.
- The seventh is either a stud (all 911 through 964) or a shorter bolt that threads into the case.
- This leaves four of the lower through bolts in a bore supported radially only by their ends*.
Thirteen case bolts/studs are sealed with o-rings (23 total since the last two are studs (964) single threaded into the case (993) and one is inside the cam box)
*Interesting note: In the GT3, and the GT3 ONLY additional o-rings are inserted around the bolts which are not used as oil galleries (4) where the case splits when the engine is assembled. These are to mitigate vibrations due to natural frequencies at high RPM.