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My Cosworth A6 cam runs 18 thou inlet clearance and 20 though exhaust, it seemed like a lot to me, but I've stuck to it with no problem thus far. In theory closing the clearance up will give a slightly longer duration and lift, you can also work it the other way, but I suspect its pretty marginal unless you have the kind of engine thats incredibly highly strung, the reality for most of us is probably that we can't afford that level of tune so it becomes irrelevant anyway, the 4 stroke tuning book I use suggests yo may adjust the clearances in certain circumstances (track dependant) but I've never bothered, I certainly wouldnt open thme up too much because of the risk to the pushrods
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