I was running some expensive Manton 3/8 double tapered pushrods( double tapered to clear pushrod hole on head).

Now since im taking my engine out , the heads will also be disasembled.So while im there, i will ask the machinist to enlarge the pushrod hole to clear a straight 3/8 pushrods .

I will push more boost, change my Cr with a Little thicker cometic gasket and will probable need different lenght pushrods and straight 3/8 are way less expensive.

So what size hole do i need to clear the 3/8.
Please no slightly bigger answer, machinist need number not guess.

Thanks

 

On my 243 heads I drilled them out to 1/2".