boring 258 block for a 4.0 head

[carnuck]

I use an open end wrench to see what size the bolts are.

7/16" is the tap size of the threads, not the nut hex head size. Both 1/2" and 7/16" head bolts use the same size hex head.

Guess I didn't completely explain that (blame it on the pain drugs!) Use the open end wrench on the SHAFT of the headbolt to determine it's side.

[Canoe]

Sounds to me like you think you have 1/2" bolts but actually have 7/16" bolts, especially since your Eagle is a 1982.
...its 1/2" bottom and 7/16" top. Those are the only two possible combinations since 4.0 head bolts still used 7/16" threads at the top compatible with all the old 4.2 hardware. A 4.2 would have at least one of those double ended head bolts at the front of the driver side for attaching a Vbelt pulley...

I don't know how mhy '86 ended up that way, but the bolts are 1/2", with three of them having the 7/16" top threaded. Two are for the belt idler bracket; I didn't notice what the third was for but it's sitting on the bench.

Now I have to get back to digesting all the rest.

Thanks all!

[captspillane]

No worries, 1/2" bolts started with the 86 model. Same as the one that I have which is original engine !

Interesting. I was the one who confused the two then. I had a 1987 head off and I thought I was holding 7/16" bolts. That means 1/2" head bolts are available that are not "torque to yield." I wonder if I could use a low mileage set of Eagle head bolts in the high mileage 4.0 I'm about to put new rings into.

[profeagle]

The short answer is "yes" you can, at least in my opinion and that of my machine shop/engine re-builder. We had some discusion about that and he said it would be OK and indeed the old bolts worked just fine.
I put a 4L. head on 86 258 block. Engine totally gone over and modified.

[carnuck]

4.0L headbolts and 1/2" 4.2L headbolts are the same EXCEPT for the accessory studs on some 4.2L bolts.

[captspillane]

4.0L headbolts and 1/2" 4.2L headbolts are the same EXCEPT for the accessory studs on some 4.2L bolts.

They are dimensionally exactly the same but they might not be the same. Torque to yield bolts are designed to have a very specific Yield Point because of the way the metal is treated. The change in metal properties of the bolts should correspond with the change in recommended torque values.