Generally in most cars if a cambelt fails it's a very bad thing.
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Valves collide with pistons and the engine is reduced to scrap.
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For this reason the manufacturers specify cambelt replacement intervals that err massively on the cautious side to ensure the likelyhood of failure in service in absolutely minimal.
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Belts are generally replaced as a precaution against them failing rather than because they have reached the end of their useful life.
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On a mk1 the engine is what's known as a
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'non-interference' design.
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In this case should the belt fail then the pistons and valves will not come into contact, no harm will be done and you'll just find yourself stranded at the side of the road, it'd be inconvenient but no big deal assuming you have RAC/AA cover to get you home.
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Combine these 2 factors and you've got a situation where the belt is very unlikely to fail in the next few thousand miles plus if it does fail it's not exactly going to be the end of the world.
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Based on that I'd take the risk and get it done when it's convenient rather than worrying about being bang on schedule.
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