Tim­ing Belts

Treated cor­rectly, tim­ing belts can be bet­ter than chains. Here’s how.

Car Mechanics (UK) - - Contents -

Things were much sim­pler in the old days. At one time, vi­tal parts of the en­gine were pre­vented from smash­ing to­gether, by be­ing syn­chro­nised with a de­pend­able chain. Buried within the bow­els of the en­gine, it was in­tended to last the life of the car and, even if worn, the driver was alerted by a tell-tale rat­tle long be­fore chain fail­ure could cause the en­gine’s vi­tal in­ter­nals to be­come vi­o­lently and ex­pen­sively in­ter­twined.

Progress – back & forth

An ad­van­tage of early side-valve and pre-1960s over­head-valve (OHV) en­gine de­signs was that their cam and crankshafts sat fairly close to­gether within the en­gine block, mean­ing that the in­ter­link­ing chain’s length was kept rel­a­tively short. On some en­gines, such as Ford’s Cologne V4 and V6 units, solid gears were used in­stead of tim­ing chains, but even they could suf­fer from stripped teeth as the en­gine bear­ings be­came worn. Most OHV en­gines linked the camshaft and valves to­gether through a se­ries of rods and rock­ers, hence their de­scrip­tion of ‘push-rod’, which re­quired tap­pet ad­just­ment at ev­ery ma­jor ser­vice. Push-rod en­gines lasted un­til fairly re­cently, one of the most fa­mous be­ing Ford’s En­dora-e; its lin­eage can be traced to the 1950s Kent unit, which ended its days pow­er­ing cer­tain 1300cc vari­ants of the first-gen­er­a­tion Ford Ka.

With the de­sire for more power, im­proved re­fine­ment and greater ef­fi­ciency, over­head camshaft (OHC) en­gines re­placed the push-rod’s valveg­ear, by plac­ing the camshaft over the valves. This cre­ated a prob­lem for the hum­ble tim­ing chain. With the camshaft po­si­tioned fur­ther away from the crank­shaft, not only did it have to be longer, which dic­tated more elab­o­rate ten­sion­ing medi­ums, but also the risk of elon­ga­tion from wear (chains do not ‘stretch’) in­creases the chance of not only fail­ure but the en­gine’s tim­ing also be­ing af­fected. The so­lu­tion was to mount a toothed belt out­side the en­gine

block, where it could not de­grade from oil con­tact. The tim­ing belt, or cam­belt, had ar­rived. Com­pared to a chain, the belts were more ef­fi­cient, qui­eter and cheaper for car-mak­ers to in­stall.

As many read­ers will tes­tify, the main dis­ad­van­tage with tim­ing belts is the need for rou­tine re­place­ment, for which the pro­fes­sional labour cost alone can run eas­ily into many hun­dreds of pounds. When cam­belts ap­peared first on main­stream cars, man­u­fac­tur­ers tended not to be both­ered too much about is­sues that would af­fect a car that was likely to be out of war­ranty by the time re­place­ment was re­quired. Nev­er­the­less, tim­ing belts still gave them more than a few headaches. Ex­ag­ger­ated re­newal in­ter­vals on the ser­vice sched­ules meant that fail­ing belts caused cat­a­strophic en­gine dam­age. In­ter­est­ingly, this is­sue ap­pears not to have gone away en­tirely. In the 1980s, belt change in­ter­vals tended to be quoted at be­tween 40,000-60,000 miles, but break­age could hap­pen well be­fore then. To­day, of­fi­cial belt change quo­ta­tions tend to be be­tween 100,000150,000 miles, or 10-15 years, but many ex­pe­ri­enced me­chan­ics ad­vise that 80,000 miles, or eight years, is a far more re­al­is­tic in­ter­val. Even so, ac­cord­ing to An­drew Vaux, Tech­ni­cal Train­ing and Sup­port Team Spe­cial­ist for the Gates Cor­po­ra­tion, a ma­jor belt man­u­fac­turer, mod­ern tim­ing belts may look very sim­i­lar to those made in the 1960s but many un­seen ma­te­rial changes have been made, in­clud­ing to the cord, jacket and tooth pro­file.

Re­al­is­ing that many fleet cus­tomers – re­spon­si­ble for more than 80% of new car sales in the UK – were fac­tor­ing main­te­nance costs as part of their pur­chas­ing de­ci­sions, car-mak­ers re­acted by re­design­ing OHC en­gines to ac­cept in­ter­nal chains in­stead of tim­ing belts. Ex­cept in very rare cases, there was no re­place­ment in­ter­val, mean­ing that fleet car man­agers could trim a use­ful main­te­nance cost from their run­ning cal­cu­la­tions. Un­for­tu­nately, the tra­di­tional dis­ad­van­tage with us­ing long chains in an OHC ap­pli­ca­tion re­mained, not helped by mod­ern con­sid­er­a­tions that in­clude the qual­ity of re­cy­cled metal, the need to re­duce weight, very

lengthy oil change in­ter­vals and, of course, man­u­fac­tur­ing costs.

The re­sult was that the sup­posed fit­ted-for-life tim­ing chains were ei­ther rat­tling, or even fail­ing, at very low mileages. As they were not de­signed to be re­placed, some cars (such as those fit­ted with BMW’S N47 and N57 en­gines, which lo­cate the tim­ing chain be­tween the en­gine and trans­mis­sion) re­quire the en­gine to be re­moved just to gain ac­cess. This has be­come such an is­sue that cer­tain car man­u­fac­tur­ers, in­clud­ing the Volk­swa­gen Group, have per­formed yet an­other volte-face, by re­plac­ing cer­tain en­gines’ chains with belts. Thus, it may not be worth pay­ing ex­tra for a car fit­ted with a tim­ing chain, be­cause the un­sched­uled cost of re­plac­ing it can be far more in­volved and ex­pen­sive, when com­pared to a tim­ing belt.

Re­plac­ing tim­ing belts and their as­so­ci­ated parts is a fid­dly task, which is why garage labour rates are so high.

Con­sider other parts that may be re­quired. Pic­tured on the top left of this Mit­subishi en­gine is a bal­ance shaft belt, which must be re­placed at the same time as the main tim­ing belt. Note also the hy­draulic belt ten­sioner, which utilises en­gine oil pres­sure in this dry belt ap­pli­ca­tion (cir­cled).

Never fit the tim­ing belt alone. In­stall a kit that in­cludes, if ap­pro­pri­ate, a ten­sioner and idler roller. De­pend­ing on your car, you may need other parts – such as a wa­ter pump – if it’s driven by the tim­ing belt.

When fit­ting the tim­ing belt for the first time, the belt may need to be placed in a spe­cific po­si­tion. On this Re­nault F-type en­gine, two marks on the belt must align with the notches on both the crank­shaft (pic­tured) and camshaft. The socket, in­ci­den­tally, is hold­ing the belt in place tem­po­rar­ily.

If re­mov­ing the wa­ter pump, its con­di­tion will give you an idea of the cool­ing sys­tem’s sound­ness. This one was clean, due to reg­u­lar coolant changes. In­ter­est­ingly, this pump’s bear­ings were start­ing to stiffen af­ter 80,000 miles of be­ing driven by the belt, the of­fi­cial change in­ter­val of which was 150,000 miles.

As as­sess­ing belt wear is al­most im­pos­si­ble, never re­fit a used belt. This one ex­hibits miss­ing teeth, the most com­mon fail­ure.

Cer­tain ex­am­ples of the lat­est gen­er­a­tion three-cylin­der units are util­is­ing BIO tech­nol­ogy. Pic­tured is Ford’s Fox Eco­boost en­gine, with the tim­ing belt shown clearly as be­ing sit­u­ated within the en­gine block.

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