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Stock setting on a 70 BOSS 302 is 25 hot/cold (the joys of having a solid lifter engine). My question is would happen performance wise (good or bad) if say I set them 20 instead or do do I reset them at 25 and leave them there?
 

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If set at the tighter setting, during high rpm, high temp operation it is possible (but not probable, talking of .05) for the valves to hit the pistons. But more importantly, it does change the valve timing making things open or close sooner or later than spec'd. In most cases, a slight reduction in performance would be expected or the power band slightly changed resulting in lesser performance because nothing else has been re-tuned to compliment the new spec's. Set em .25.
 

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In my experience, setting the lash tighter results in an increase in performance. Valves open sooner and close later resulting in more "area under the curve".

I would still set them at 0.025" hot. You'll never notice a difference.
 
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