Plugging air injection holes

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6

65rbdodge

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I’ve done it many times, tap it with 1/4-20 and put the a bolt or whatever I have in there. I have a set of 308 heads that were on a jasper motor, I tried the 1/4-20 tap and it slides right through the hole, like it’s been drilled out already. Can it be safely taken out to 5/16”?
 
I don't recall the "normal" size and tap to use, which is smaller than 1/4". But I've never seen or heard of any of the holes being that big!
 
Or poor casting. Those heads were not the “best” ma MoPar made.
 
I just ran a 5/16 tap through the holes and it worked just fine. Is it possible 302 heads have a smaller hole for a 1/4” tap and the 308 heads are bigger for a 5/16” tap? That’s my experience
 
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