Slipstream - November 2019

maverickpca

The monthly newsletter of the Maverick Region of the Porsche Club of America

There are still a gazillion air-cooled 911s on the

road. A problem that has forever plagued the ’76-‘87

911s is the cylinder head studs.

A brief refresher: Since the start of production of

the 911 in 1964 and through 1967, Porsche used a

sand-cast engine case. The sand casting method is very

expensive since a foundry tech has to make a mold for

every engine. Also the cases are heavy because of the

amount of aluminum used. Around 1965, Volkswagen

started using the pressure-cast method of producing

engine cases. The molds are reusable with the pressure

cast method.

Manufacturers are always looking for ways of

reducing cost. So in 1968, VW engineers started using

a magnesium/aluminum alloy for the cases. They could

use less material since the magnesium would strengthen

the alloy. This probably worked great in the frigid

climes of Europe, but not so much here in the American

southwest. The cases were so rigid that the pressure

on the case exerted by the cylinder head studs (caused

by ever-increasing engine temps to meet emission

standards) would result in the threads in the holes for

the studs breaking. The result was loose cylinder heads,

compression loss, and severely diminished power.

In 1968, Porsche adopted the magnesium alloy

pressure cast method for case production. This worked

great in the 2.0 L-2.4 L engines. In 1974, Porsche bumped

the displacement to 2.7 liters. Engines were running

hotter to meet emissions (in 1975 they incorporated a

five-blade cooling fan to increase the engine temp).

I was a unit tech (engine and transmission repair) at

30 November

(Maniacal) Mechanical Musings: Make it Stop!

By Ed Mullenix

then Forest Lane Porsche+Audi. I’ll bet I put thread repair

inserts in 50 engines when I worked there. Porsche liked

their alloy pressure cast cases, so their fix was to change

the exhaust-side stud material from steel to “dilvar.”

Dilvar had the expansion coefficient of

aluminum; the theory was that the aluminum

cylinders were expanding more on the lower

(exhaust) side, causing the threads in the case to

fail. Unfortunately the dilvar alloy was so brittle,

the studs would break; so much for the bestlaid

plans of mice and engineers. The alloy was

extremely hard; carbide drill bits were required to

drill out a broken stud.

In 1976, with the introduction of the 911

Turbo, Porsche used aluminum as the case

material. It was more forgiving than the

magnesium alloy. They kept the aluminum cases

through the SC, 3.2 Carrera, 964 3.6, and the

993. Several iterations of head stud were tried.

The problem does not show up on the 964 or

993 cars but is a ticking time bomb on the SC

and 3.2 cars.

If you are thinking about the purchase of a

3.0 or 3.2 911, absolutely have a pre-purchase

inspection done. Include removal of the lower

valve covers (drain the oil out of the engine, to

be refilled later), with a torque wrench set at 12

ft-lbs (half the prescribed head nut torque of 24

ft-lb), turn the wrench, and observe if there is any

movement of the head nut. If there is any movement at

all, that stud is in an early stage of failure.

I have removed lower covers during the course of

an engine maintenance and had sections of broken

head stud with nut still attached fall out on the floor.

Replacing head studs is essentially doing a “top end”

on the engine. Depending on where the stud broke, it

could mean total disassembly - see attached pic of an SC

case with studs broken off inside the case. I no longer

use dilvar studs; I use steel studs for upper (intake side)

and lower (exhaust side). In 20 years, have not had a

single problem using steel studs.

Having fun!!

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