Timing belt and balance shaft belt on the 944 Turbo (951)

In short: The 951 engine is timed by a toothed belt and smoothed by two balance shafts that run on a second belt. If the timing belt fails, serious engine damage is likely. Replacing and tensioning the belts is therefore one of the most important maintenance jobs on this car – and it only works with the workshop manual and the correct gauge.

Why two belts?

A large four-cylinder engine is inherently less smooth than a six: second-order inertial forces are felt as vibration. On the 944 engine, two counter-rotating balance shafts cancel these forces. They are driven from the crankshaft by a belt of their own.

The second, wider belt is the actual timing belt. It links crankshaft and camshaft and keeps valves and pistons in step. This belt also drives the water pump.

What happens if the timing belt breaks?

The engine is an interference design: if the camshaft stops while the crankshaft keeps turning, pistons and valves collide. Bent valves and cylinder-head damage are typical results. A broken balance shaft belt is usually less dramatic – but it can get caught in the timing drive and cause damage as well.

When to replace?

The manufacturer’s maintenance schedule is authoritative – by distance and by age. Rubber ages even when the car is hardly driven. We have not yet checked the binding intervals against factory documents; they appear as “pending” in the table of values below.

For cars with an unknown history, the rule of thumb in the scene is: without proof of the last belt change, replace first and drive afterwards.

Tensioning: feel is not enough

Both belts must run at a specific tension. Too loose, and the timing belt may jump teeth. Too tight, and bearings, rollers and the water pump wear faster; an over-tight balance shaft belt often announces itself with a whine.

Tension is measured with a dedicated gauge, usually referred to by the tool number 9201.^[2] Target values depend on measuring point, engine temperature and belt version, among other things.

Why no numbers here? Various tension values circulate in forums. Until we have checked them value by value against the workshop manual, we do not publish them. A wrong value here can cost an engine.

What belongs to a belt job?

If the belt drive is apart anyway, the parts that would otherwise need a second teardown are usually inspected or replaced too:

Tensioner and idler rollers
Water pump (see above)
Shaft seals on cam, crank and balance shafts if they leak oil

Exactly which parts apply to the 951, and whether the procedure differs from the naturally aspirated engine, will be added during the technical review.

Checklist before you start

Have the workshop manual for exactly this model and model year at hand.
Obtain or borrow a belt tension gauge.
Check the top-dead-centre marks on crank and camshaft before loosening anything.
Only turn the engine in its normal direction of rotation.
After assembly, turn the engine by hand through several revolutions and check the marks again.

Value	Figure	Condition	Status	Source
Timing belt replacement interval	— pending	See the manufacturer’s maintenance schedule (distance and age)	pending	[1]
Balance shaft belt replacement interval	— pending	See the manufacturer’s maintenance schedule	pending	[1]
Timing belt tension	— pending	Gauge, engine temperature and measuring point – see workshop manual	pending	[1]
Balance shaft belt tension	— pending	Differs by belt version – see workshop manual	pending	[1]
Balance shaft belt idler clearance	— pending	See workshop manual	pending	[1]
Tension gauge (tool number)	9201	Not officially confirmed; successor tools possible	unverified	[2]