Cylinder Head Studs

Broken head studs in 911 motors is an issue that could be viewed as a chink in Porsche's vaunted design armor. From the 2.7 all the way up to the 964 3.6-liter motors, Porsche has implemented (some might say experimented with) different head stud materials in an attempt to rectify the breakage problem. A variety of materials have been used, some with more success than others. For the SC and Carrera motors, steel was the material used for the upper intake row of head studs, and a special "dilivar" material in the lower exhaust row. The steel units generally work well and create few problems, but the dilivar units are infamous to Porsche owners and mechanics alike. The dilivar alloy was designed to have a similar thermal expansion rate to that of the aluminum engine case, cylinders, and heads. In theory, this would help balance the stresses involved during expansion and contraction of the aluminum parts. In reality, larger displacement motors with their additional stress and corrosion issues have seen a significant number of failures, regardless of mileage accrued, thanks to faulty head studs.

While Porsche finally seemed to lick the problem with the fully threaded steel units installed top and bottom on the 3.6-liter 993, I wasn't going to roll the dice with my 3.5. Chris Brown from ARP recommended his company's high-grade head stud kit that comes beautifully packaged complete with the appropriate (24) studs, washers, 12-point nuts and special moly assembly lube. ARP is well respected in motorsports circles for its engine fasteners and hardware. The head studs are manufactured from premium grade 8740 alloy steel and heat-treated to 200,000 psi. After heat-treating, the studs are centerless ground to make them as concentric as possible. The ARP head studs are also thread rolled after heat-treating, providing for ten times better fatigue strength than studs threaded prior to heat-treating. ARP head stud products are not the cheapest on the market, but their quality and attention to detail does provide for a premium part that should be considered a necessary replacement for any 911 motor using dilivar head studs.

Popular Piston/Cylinder Sets for 3.0L SC & 3.2L Carrera Motors

3.0L SC Displacement Options

BoreDisplacementPiston TypeCRStrokeWrist PinCase BoreMachine Case?Notes
95mm3.0LCIS/Motronic8.5:170.4mm22mm103mm 1978-1979 U.S. spec SC
95mm3.0LCIS/Motronic9.3:170.4mm22mm103mm 1980-1983 U.S. spec SC
95mm3.0LCIS/Motronic9.8:170.4mm22mm103mm 1981-1983 Euro spec SC
98mm3.2LCIS/Motronic9.8:170.4mm22mm103mmNo3.2L "Short Stroke", Twin Ignition optional
98mm3.2LCarb/MFI/EFI10.3:170.4mm22mm103mmNosame as above, Twin Ignition required
100mm3.3LCarb/MFI/EFI9.5:170.4mm22mm105mmYesTwin Ignition required, rarely built
98mm3.4LMotronic/CIS9.8:174.4mm23mm 103mmNo3.2L Carrera crank & rods. Twin Ignition optional
98mm3.4LCarb/MFI/EFI10.3:174.4mm23mm103mmNosame as above, Twin Ignition required
100mm3.5LMotronic/CIS9.8:174.4mm23mm105mmYessame as above, Twin Ignition required100mm3.5LCarb/MFI/EFI10.3:174.4mm23mm105mmYessame as above
100mm3.6LMotronic/CIS9.8:176.4mm23mm105mmYes964 3.6L crank w/ 3.2L rods.Piston mods & Twin Ignition required
102mm3.7LMotronic/CIS9.8:174.4mm23mm107mmYes3.2L crank, aftermarket rods
& Twin Ignition required

3.2L Carrera Displacement Options

BoreDisplacementPiston TypeCRStrokeWrist PinCase BoreMachine Case?Notes
95mm3.2LMotronic/CIS9.5:174.4mm23mm 103mm 1984-1989 U.S. spec Carrera
95mm3.2LMotronic/CIS10.3:174.4mm23mm103mm 1984-1989 Euro spec Carrera
98mm3.4LMotronic/CIS9.8:174.4mm23mm 103mmNoTwin Ignition optional
98mm3.4LCarb/MFI/EFI10.3:174.4mm23mm103mmNoTwin Ignition required
100mm3.5LMotronic/CIS9.8:174.4mm23mm105mmYesTwin Ignition required
100mm3.5LCarb/MFI/EFI10.3:174.4mm23mm105mmYessame as above
100mm3.6LMotronic/CIS9.8:176.4mm23mm105mmYes964 3.6L crank, Piston mods
& Twin Ignition required
102mm3.7LMotronic/CIS9.8:174.4mm23mm107mm Yes3.2L crank, aftermarket rods
& Twin Ignition required

Mahle original cast and Mahle Motorsport forged piston & cylinder sets distributed exclusively by SSF Auto Parts and Andial. JE Pistons also offers custom forged pistons in a variety of sizes and compression ratios for the engine builder that may have a special requirement.

Rod Bolts

The connecting rod bolts are another source of concern for 3.2-liter Carrera motors. Prior to the introduction of the 3.3-liter Turbo motor in 1978, Porsche rod bolts were 10mm in diameter and provided the extra safety margin Porsche usually engineers into its motors. Unfortunately, when Porsche borrowed the 74.4mm crank and rods from the 3.3-liter Turbo to make a 3.2-liter engine for the Carrera in 1984, the smaller 9mm diameter rod bolts first seen on the earlier 3.3-liter Turbo were used as well. The stock 9mm rod bolts are considered a weak point and the motor should not be revved past 6800 rpm with any regularity. Many professional engine builders recommend replacing the rod bolts with higher quality units whether they're doing a stock rebuild or a race prepping a motor.

Although I don't anticipate my 3.5, with the original injection system and relatively mild camshafts, spinning past 6800 rpm, this was another area where the factory offering could be improved upon. The most important fasteners in any engine are the rod bolts, as they mate the rod halves and must support the tension loads caused by each cycle of the crankshaft. Wanting to ensure a virtually bulletproof bottom end, I once again turned to the ARP catalog and selected the Pro Wave ARP2000 rod bolt kit. According to the company, these ultra heavy-duty rod bolts are made from an exclusive hybrid alloy developed to deliver superior strength (about 220,000 psi) and better fatigue properties. The patented Wave-Loc design contacts the rod and cap for optimum alignment and provides a snug fit for all OEM connecting rods despite wide ranges of factory rod bolt hole tolerances. ARP rod bolts are requisite for any 3.2L-liter rebuild that will see more than 6800 rpm, whether it's intentional or not.

AASCO Performance, Inc.
709 E. Adele St.
CA  92805
Automotive Racing Products, Inc. (ARP)
ANDIAL Road & Racing
3207 S. Shannon St.
Santa Ana
CA  92704
Ollie's Automotive Machining, Inc.
By Ralph B Hollack
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