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JCFcuda,
A chunk of any metal with a hole in it when heated will cause the hole to enlarge and increase the clearance. Thus the loss of oil pressure when the pump cannot keep enough flow like at idle. When half that chunk of metal is AL then the hole becomes even larger due to heat.

Damon

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Half gets bigger than the other half, the bottm half I have used the Mopar Ductile iron with really good results To bad there not avalaible anymore Now, how to decide between steel and aluminum caps Probally aluminum