Balance Shaft Sprocket Installation and Alignment Check

Introduction

If a 944 is experiencing vibration problems, the cause could be a balance shaft misalignment. This can occur if the balance shaft belt slips a tooth, the balance shaft belt is broken, or if the balance shaft sprockets are installed incorrectly. Balance shaft sprocket misalignment frequently occurs after water pump, crankshaft oil seal, or balance shaft oil seal replacement.

Other Procedures Needed

• ENG-13: Locating and Setting Engine to Top Dead Center (TDC), Cylinder 1

Installation

Each balance shaft sprocket has two slotted grooves on the inside diameter. One of the grooves on the sprocket will slide onto a woodruff key inserted into a slot near the front end of the balance shaft.

1983–1984 model 944s: Each balance shaft sprocket was stamped on the front with an "O" (Ober/Over) beside one of the slotted grooves and a "U" (Unter/Under) beside the other. On the upper balance shaft, the groove marked "O" is installed onto the woodruff key. On the lower balance shaft, the groove marked "U" is installed onto the woodruff key.

1985 and newer models: Only the groove with the "O" is stamped; the groove which previously had the "U" now has no marking. This is a common source of balance shaft misalignment. Mechanics unfamiliar with 944s may assume the "O" goes on the woodruff key on both shafts — this is incorrect for the lower shaft and causes it to be 180° out of alignment, producing vibration worse than having no balance shafts at all.

Critical: On post-1984 cars, the groove with the "O" goes on the woodruff key on the upper balance shaft, and the groove with no marking goes on the woodruff key on the lower balance shaft.

Checking Balance Shaft Alignment

Proper installation is confirmed visually through the sprocket covers:

1. With the balance shaft sprocket covers installed, it is impossible to see the woodruff key directly.
2. Each balance sprocket cover has a number of round openings and one rectangular opening. At the rectangular opening, there is a tab on the back of the cover which is inserted into the groove that does not have the woodruff key.
3. If the sprocket is properly installed on the upper balance shaft, the "O" stamped on the sprocket will be visible in one of the round openings on the cover.
4. If the sprocket is properly installed on the lower balance shaft, the "O" stamped on the sprocket will be visible in the rectangular opening on the cover.

Note: If the tabs on the back of the sprocket covers are broken off, the only way to positively confirm proper installation is to remove the sprocket retaining bolts, remove the cover plate, and slide a heavy piece of wire or small probe into each groove to locate the woodruff key.

5. Once correct installation is confirmed, verify balance shaft alignment as follows: a. Using ENG-13, set the engine to TDC on cylinder #1. b. With the engine at TDC, the notch on the back lip of the upper sprocket should align with the notch on the rear timing belt cover (approximately 1 o'clock position). c. For the lower balance shaft, the notch on the back lip of the sprocket should align with the raised tab on the inside of the rear timing belt cover (approximately 7 o'clock position). d. Some misalignment is not uncommon — approximately half of all 944s show a slight variance. With one sprocket exactly on its alignment mark, the other should be no more than about 1/2 tooth off. If either shaft is out by a full tooth or more, it must be realigned.

Why Alignment Matters — Secondary Damage from Vibration

The balance shafts exist to cancel the secondary vibration inherent to a large-displacement inline-four. A car running with misaligned balance shafts — or with the shafts deleted — vibrates continuously at a level the chassis and its accessories were never intended to endure. Because the excitation is constant, the damage it causes is cumulative and often appears in places that are not obviously connected to the balance shafts. The items below are the ones most commonly traced back to sustained vibration; several are far more expensive to fix than the alignment check itself.

Oil pump pickup tube

This is the failure most directly tied to balance-shaft timing. Sustained vibration works the oil pump pickup tube back and forth until it cracks or fractures at the pump. A cracked pickup lets the pump draw air instead of oil, starving the bearings — a failure that can end the engine. On a car with sound engine mounts, a failed pickup tube is a strong indication that the balance shafts are mistimed (or that incorrect solid mounts are transmitting vibration the hydraulic mounts would otherwise absorb). Confirming balance-shaft alignment is cheap insurance against a far costlier bottom-end failure.

Engine mounts

The 944 uses hydraulic (fluid-filled) engine mounts that damp vibration far better than solid rubber. Constant vibration shortens their life, and a collapsed or leaking mount then transmits still more vibration into the body — a self-reinforcing cycle. Heat plays into it: the exhaust-side mount relies on a heat shield, and if the shield or its bracket is cracked or missing, the mount cooks, fails sooner, and the vibration it no longer absorbs accelerates the other damage below. Symptoms of a failed mount include a noticeable increase in cabin vibration, clunks on gear engagement, and the engine sitting visibly low on that side.

Fuel rail and fuel lines

Prolonged vibration can fatigue the aluminium fuel rail and its fittings, producing hairline cracks — most often at the unions and where rigid lines are clamped. Because the fuel system is pressurised, even a small crack is a fuel-leak and fire hazard. Any fuel weep or fuel smell in the engine bay should be investigated immediately rather than monitored.

Torque tube

On the transaxle 944 the engine and transmission are joined by a torque tube carrying the drive shaft. Sustained resonance contributes to front torque-tube bearing wear (which in turn wears a groove in the drive shaft) and, on solid tubes under high load, can promote stress cracking. The later 968/S2 slit torque tube was a factory measure to damp exactly this kind of torsional vibration.

Accessory brackets and fasteners

Vibration works fasteners loose over time. The alternator and other accessory brackets are commonly reported to loosen — and in some cases to crack or shear a mounting bolt — on hard-vibrating cars, so their fasteners are worth checking periodically. Exhaust manifold studs also fail on these engines; that is primarily heat-cycle fatigue, but a collapsed mount that lets the manifold contact the chassis, together with vibration, can contribute.