User Guide

Free Running

No Valve/Piston Interference

Interference

Valve/Piston Interference

Since timing belts are inside the engine, they tend to be an out-of-sight, out-of-mind item and are

seldom thought of as a part of routine maintenance. Here are some questions to ask that will help

sell belts and protect the customer from expensive repairs:

1. Did they buy the car new? If yes, they’ll know if the belt has ever been replaced.

2. Did they buy the car used? If so, they won’t know if the belt has been replaced. This is a ﬂag

for the customer to get the belt checked, replaced or at least be aware that he has a timing belt.

3. How many miles are on the vehicle? Generally belts should be replaced at around 60,000 miles

or less. Our Timing Belt Replacement Recommendations Booklet is an excellent tool for determining

replacement intervals.

4. Have they ever replaced the timing belt? If the answer is yes or no (and the customer knows the

mileage) you can determine where the vehicle is in the replacement cycle.

Here’s a helpful hint for customers with vehicles within a few thousand miles of the recommended

replacement interval: If they are having any engine repair done, have the timing belt checked. If it

needs replacing, save time and money by doing it along with the other repairs.

NOTE: For your convenience, interference engines will be footnoted in

our publications. This lets you easily identify them, and pass that

information on to your customers.

Free Running or Interference Engine?

You and Your Customers Need to Know.

As illustrated below, in the free-running engine with the crankshaft still moving, there is enough

clearance between the valve and piston, even if the cam stops with a valve fully open. However,

interference engines usually sustain damage if synchronization is lost. As illustrated, disrupted

synchronization allows the piston and valve to collide causing damage and very expensive repairs

for the vehicle owner. Damage can occur to valves, pistons, or heads, and in some cases, completely

ruin the engine. Most import (with higher compression engines) and most car and light truck diesel

engines are interference. To avoid costly engine repairs, follow the auto manufacturers’ mileage

replacement recommendations, or we recommend timing belt replacement at around 60,000 miles.

Automobile engines can be classiﬁed as either Free-Running

or Interference, depending on what occurs if piston/valve

synchronization is lost.