Timing is everything with belts - Letting the timing belt decide when it's time for replacement can have devastating results. - Motor Age - Automotive training, certification & parts info
The #3 cylinder intake valves (center bottom) on this 1994 Dodge Stealth (Mitsubishi 3.0 DOHC V6) didn't survive its timing
belt failure – the intake valves are usually the ones that bend because of their larger diameter. A 2000 Mitsubishi Gallant
4 cylinder we did bent all 8 of its intake valves.
Unless your customer has a rotary engine under the hood, the power-producing mill that delivers power to the transmission
or transaxle is outfitted with pistons and valves, and a spinning shaft with carefully indexed egg-shaped lobes operates those
valves. An engine is basically a big breathing machine, and everything has to be timed or it won't work right. One of the
most crucially timed elements is spark delivery, and spark originally was controlled mechanically. Model T vintage cars had
a lever on the steering wheel for this. You used the lever to retard the timing to start the engine, then advanced or retarded
it as needed while driving. From levers to centrifugal weights and vacuum operated breaker plates, timing control has since
become more computerized. The distributor has disappeared and now every spark plug has its own coil.
Cam timing, on the other hand, remained fairly consistent for almost a century. And while there are a myriad of ways to drive
the camshaft(s), all those ways pretty much have one thing in common: The camshaft(s) always spin(s) at 50 percent crankshaft
speed. Further, camshafts aren't an easy spin – they're operating heavily spring-loaded valves. Anybody who has turned a camshaft
with a ratchet knows that there are numerous short "bump" cycles when the camshaft picks up speed and drives the ratchet.
Those springs tend to push back against the retreating cam lobe as they fight the camshaft and close the valve. The point
is that any mechanism that drives a camshaft has to endure a lot of pounding.
Principles and Precepts
Camshafts with constant mesh helical gears tend to be extremely noisy if there happens to be any lash between the gears at
all, thus the old VW bug camshafts with their minus 5 to plus 5 gear angles. There were 11 different camshafts, depending
on which helical gear angle you needed to zero the lash. Some manufacturers solved this noise problem by using composite material
to make their cam gears, but those gears tended to fail at high mileages. Even when carefully matched steel helical gears
were used, lash tended to increase with wear until they became noisy and the cam gear had to be replaced to make the engine
sound normal again.
Asian designs like Toyota and Isuzu have dual overhead cams but with a single external belt cog that drives both camshafts.
The belt-driven gear usually drives the exhaust camshaft and a special scissor-type helical gear under the valve cover that
drives the intake camshaft. So what is a scissor gear? Think of a two-piece gear that has been sliced like you'd slice bread,
but with a special spring between the two slices to keep the slices loaded in opposite directions, consistently pinching the
teeth on the gear they drive, effectively cushioning the pounding and lash to minimize wear and noise. It's a brilliant idea
and works flawlessly, but there's a special procedure to keep those gears loaded if you have to remove that shaft.
One way or another, cam drive components have to be designed to handle this relentlessly grueling job. In the case of gears
or chains, well, it's obvious that they have to be lubricated. Older laminated style timing chains were tight when installed,
but tend to stretch as time and mileage accumulate. Some manufacturers added dampers and stiff spring loaded tensioners to
mitigate the retarded cam timing that naturally occurs whenever the crankshaft begins to spin a few degrees ahead of its companion.
Today's timing chains are typically long with some colored teeth for indexing and have nylon guides on each side, one of which
will be held snugly against the fast-moving chain with an oil pressure-driven tensioner.
Richard McCuistian is an ASE-certified Master Auto Technician and was a professional mechanic for more than 25 years. He is now an auto mechanics instructor at LBW Community College/MacArthur Campus in Opp, Ala.
Articles by Richard McCuistian
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