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They leak because they are thin and vibrate breaking the silicone seal. If your ported RPM heads are still stock intake port size just use the stock tin valley pan. I've even ported the valley pans a little with a die grinder and sanding rolls. When I went to a raised port window on the RPM heads I had to fabricate my own valley pan.

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This is what I did also. I slightly opened up the intake ports and just ground the valley tray to fit during mock up and there are no leaks ANYWHERE on my motor for that matter.......