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Old 7th November 2019
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Yes, vacuum petcock. I don't think the petcock is a problem but I'll keep it in mind.
I decided to go ahead and reduce the breather bolts and that went OK except my memory went South on the 04-up hole size.
I drilled 1/8" holes instead of 1/16".
But, it's still a restriction. The new assembly doesn't hinder the A/C.

For lack of having fine thread 5/16” bolts, course thread SS (5/16“x18) bolts were re-threaded to 5/16”x24. This works fine and re-threads easily.
But it's easier to go and buy the correct bolts.



The holding block was needed to keep the bolt straight while boring a hole through it.
You could use a block of wood, plastic or steel. The hole drilled was .2720“ and tapped 5/16”x24.



Each bolt was screwed into the hole, corner to corner lines drawn across the head to find center and the center punched to keep the bit from wandering.



An O-ring was added under the head to seal off the original hole.
The factory threads were chased with a 5/16“x24 tap to straighten warpage and clean the threads for the new bolts to be inserted.
Normally, marking center on both ends and sending the drill bit in just over half way from each end will make a more straight thru hole.
But this just being a test, the hole was drilled thru from one side.
The hole ended up off center in the thru end but this is not a critical point in this application.

So now, at least I'll get to see what affect smaller holes has inside.
I may re-drill more bolts to 1/16" if this shows a reduction.







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