Fixing the valves a matter of timing

By Barry Lake

A KEY element of most modern car engines is the opening and closing of the inlet and exhaust valves.

Crucial to this basic need is a drive system from the engine's crankshaft to the camshafts, which have the cams that control the opening and closing of the valves.

If the camshafts do not rotate, the cams can no longer do their job. If the relative positioning of the crankshaft and the camshafts is incorrect, the result can range from poor running to a disastrous engine blow-up.

Cog-belt drive is the most recent method of driving camshafts, although it has been in common use for almost three decades. The cog belt was initially rubber-based but its growing acceptance is a result of technological advances in composite construction using plastics and fiber materials.

While durability has been improved on a regular basis, there has been a trend by some motor vehicle manufacturers to become more cautious in their recommendations on replacement intervals for cog belts.

This is because if the belt breaks -- particularly when the engine is running at high speed -- a piston can slam into valves that remain open. The valves will be bent and the piston damaged.

The piston may collapse and the head break off a valve. The subsequent carnage caused by pieces of metal flying around within the engine can be catastrophic.

Some manufacturers have avoided this possibility by machining deep cut-aways into the piston tops to clear the valves should a belt breakage occur.

But this can have adverse effects on compression ratios and gas flow and, consequently, on power, economy and exhaust emissions. So most engine manufacturers have not taken this route.

Belts should, therefore, be replaced at recommended intervals, which can be from 40,000km upwards.

Gear-driven camshafts are a rarity in modern road-going engines. For overhead camshaft designs, they are expensive and noisy and it is difficult to maintain correct meshing of the gears after engine repairs which involve shaving of the head and/or block.

Timing chains are still used in many road engines, notably in prestige and high-performance vehicles. Chains have much longer service lives and rarely break, but they are more costly than belts.

They are potentially noisier and can become more so as they wear, which can also result in a small variation in the timing of the camshafts.

Replacing a chain, which is contained within a timing cover on the front of the engine and included in the engine's lubrication system, can be more difficult than changing a timing belt.

Sometimes, it will also require the replacement of the sprockets. It is not an impossible task for an accomplished do-it-yourself addict, but should be done with care.

There are several traps for the unwary.

Being located outside of the engine's oil system, at the front of the engine and usually behind easily removed covers, a cog belt offers a more straight-forward replacement operation for a backyard Mr. Fixit. The main thing to watch is maintaining correct timing.

If the old belt is removed and a new one fitted in its place without any movement of the crankshaft or camshaft(s) during the operation, then the timing will not have changed.

But it is safer to follow instructions in the workshop manual on lining up various timing marks to ensure correct timing.

For those not inclined toward this type of work, it is important to have the car serviced by a quality service center, preferably one with staff knowledgeable on your make and model of car, and to stick to the manufacturer's recommendations on replacement intervals.