Although Gilmer (that's its real name!) timing belts seem to be a
rather new innovation, they've been used for many decades. That being
said, however, timing chains were generally used by most manufacturers
before the shift to Gilmer timing belts.
Most OHV engine designs used timing chains (most USA manufacturers) or
timing gears (many European manufacturers). Actually, timing gears are
the best, but can be costly to design and manufacture.
In most OHC designs, timing chains were historically used. For
example, the classic Jaguar DOHC inline 3.8L and 4.2L six used timing
chains, as well as most other European cars including the SAAB SOHC and
DOHC inline four. FIAT used Gilmer belts in their 124 series in the
'60s. Mercedes and Porsche used, and still use, timing chains.
Timing chains are more durable than the Gilmer belt. The generally
accepted design spec for replacement of the Gilmer belt is 50,000 to
100,000 miles. Hyundai specifies replacement of the belt at 60,000. On
the other hand, a timing chain - either simlex or duplex - can last
much, much longer. The primary problem with a timing chain design over
time and mileage is chain stretch. This natural wear is compensated by
a timing chain tensioner, either oil pressure or mechanically operated.
One of the primary reasons for the original shift from a timing chain
to the Gilmer belt is one of economics. Although the timing chain is
more durable than the Gilmer belt, it's generally much more expensive
to replace on an OHC engine than a belt. Not only is the timing chain
itself more expensive ($50 to $200), the replacement (labor) can be
very costly. In some OHC designs which use a timing chain, the engine
has to be pulled to effect the repair. Whereas the Gilmer timing belt
is inexpensive ($20 to $50) to purchase, and the replacement cost is
much less than a timing chain. NOTE: Although we Hyundai owners may
complain about this fact, it's nevertheless much less than a timing
Timing chain replacement in the classic American OHV engine design is
also quite inexpensive, both in terms of parts cost and labor.
Although OHC engine designs are much more efficient, the efficiency
comes at a long-term maintenance cost increase over OHV designs.