Drilling Head Bolt Holes......

[nickmyron]

As some of you may know I am building a 351W....

The original 351 heads that were on the block were cracked so i needed to get a new set. I was informed that 302 heads are the same as long as you get the same cumbustion chamber size, and all you need to do is drill the bolt holes out to 1/2"

Well I got a set of 302 heads and i drilled the bolt holes out to 1/2" a while ago, but it wasnt until yesterday that i was able to try them on the block....

And what i found was that it is very hard to tighen the bolts down. So i took a dial caliper and measured the diameter of the old heads....I found that they are 30/100's bigger then the ones i drilled. So that is why it is so hard to tighten them down because the hole & the bolt are the same size (1/2").

I still had the old cracked heads, so i put them on the block to test how hard the bolts were to thread in and they are much easier, while the new drilled heads are very hard (but still not impossible)

So my question is do i need to drill them a little bigger to allow for expansion or is it fine?

Thanks,

Nick~

 

[2badrotties]

The holes NEED to be bigger than the bolts. The first problem you will have are inaccurate torque readings when you tighten the bolts. And that is just the start. They CAN be as much as 1/8 bigger than the bolts and since you did them by hand and they MIGHT not be completly straight I'd go at least 1/16th bigger. Oh, and don't try reusing any head gaskets.

 

[nickmyron]

Thank you VERY much for informing me of that.....

What bit size do you recomend i use? You are right about the straightness, it aint perfectly straight.....

Maybe 9/16? Or should i go bigger?

Thanks,

Nick~

 

[Jeepster]

Wouldn't this be something that you would want to do on a drill press?

Dan

 

[nickmyron]

Wouldn't this be something that you would want to do on a drill press?

Dan

Yes, I used a drill press, but i meant the hole didnt always get 100% in the middle of the existing hole (but i was very close)

Nick~

 

[2badrotties]

Just try a couple of bits into the holes in the 351 heads. Find something close. I don't have a 351 head here or I would tell you.