THE BIG­GEST THING peo­ple do to hurt their ex­ter­nal belts is over-ten­sion them. There are two main types in use these days: a ser­pen­tine belt that looks flat with a row of grooves in its un­der­side; And the old-style V-belt you’d find in your typical 70s car.

The V-belts in par­tic­u­lar are prone to dam­age from over­tight­en­ing, and the wrong ten­sion of­ten man­i­fests as a squeal­ing noise. Over time they get shiny on the sides, which is a sign they’re on the way out. The next stage is pieces com­ing off.

If you catch squeals early in the belt’s life you can take it off, give it a clean, ten­sion it right and all should be well. If in any case you can’t stop the squeal­ing, you’re of­ten bet­ter off re­plac­ing it.

They all carry num­bers on the side – 10A, 12A and the like – which is to do with the width and an­gle of the vee, that af­fects the grip it gets on the pul­leys. The wider and less acute the an­gle, the more grip they have.

You’ll find on big­ger and older cars run­ning lots of an­cil­lar­ies – like an old Amer­i­can car with power steer­ing and air – they’ll run a larger belt. It’s crit­i­cal you get the right pro­file, oth­er­wise it will al­ways strug­gle to do the job.

The mod­ern ser­pen­tine belts hold their ten­sion bet­ter than the older style and grip bet­ter. They wear out, too, get­ting a se­ries of fine lit­tle cracks along the in­ner sur­face as they start to fail.

They will take more power be­fore they start to slip and, on a mod­ern car, will be run through and over quite a com­plex path. Just to be safe, be­fore you re­move one, draw your­self a lit­tle di­a­gram of how it runs, or maybe grab a photo with your mo­bile phone. As­sum­ing you for­get to do ei­ther of things, you can of­ten get the info off the web.

You can now buy ser­pen­tine belts for just about ev­ery ve­hi­cle on the road. So if hav­ing the en­gine bay look­ing ab­so­lutely stock isn’t all that im­por­tant to you, it’s likely you can get a ser­pen­tine kit that in­cludes the ap­pro­pri­ate pul­leys, to suit your toy. Worth con­sid­er­ing if you’re do­ing an en­gine build.

They can cover ev­ery­thing from your six-cylin­der en­gine through to a small block Chev. The re­sult is pretty trou­ble-free.

Like all things there are good qual­ity ver­sions and so-so. If in doubt, stick with known brands like Dayco or Gates, or the gen­uine part. In many cases you’re talk­ing $60 ver­sus $30, which is a false sav­ing.

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