thread and torque advice

[wesalexleft]

I have a 5/16"-18 bolt for an intake manifold on a ford 5.0 V8 with cast iron head. The manifold is aluminum. One bolt failed in place due to corrosion/coolant damage at one corner of the manifold (front, driver side). I've drilled the bolt out, and during tapping, broke the tap about 80% down. After much trial and error, a HF hammer drill and a 3/16" masonry bit broke up the tap, and it's out. I've re-tapped again to about the same 80% spot, but the threads are not in great shape at all. I'm wondering how much torque the tapped threads will stand up to. The required torque according to the manual is a final torque of 23-25 ft/lb. I screwed a good bolt in place finger tight, marked it and pulled it back out. Looks like about 1/2" of threads will have a fair hold in the hole...about 10 full threads. I'm hoping there is a torque expert with a simple diameter/length/size table for a ballpark idea on the question below.
My question. I can try to screw a 5/16"-18 threaded rod in, with thread locker...position the manifold, and then nut it down hoping it will hold the torque, or luckily, for this one hole, I could through drill oversized,and using the same rod, try to nut the underside, and then the top.

Any votes as to which? I'm hesitant to drill through and destroy the threads intact, but I'm NOT breaking off another tap in this thing.

The bottom of the flange from the head does allow me access to the bottom, but it will not be a flat surface for the underside nut. it's angled at the same angle as the head coming from the block. No welder in this shop, and the truck is in pieces, so no trip to the machine shop.

 

[metaleltr]

beyond 7 thread there is little effect. think you should be ok

 

[larry_g]

beyond 7 thread there is little effect. think you should be ok

7 is plenty and if you look at a 3/8" nut you will probably see 5 threads. So as long as you don't bottom out the bole on the bad threads then your good, count the turns (10?) to the bad threads and then when installing the manifold make sure that you don't go more than 10 turns.

Just for grins, how many turns do you have in the other good holes?

lg
no neat sig line

 

[Rookie2]

heli coil

 

[wesalexleft]

I finger tightened to the bottom of the hole, marked, and then counted the 10 threads that weren't exposed. Good news though. A quick run to the hardware store for a "F" bit, and I was able to drill through and preserve the threads. Ran a tap through it again 1/4 turn at a time, then backing it off, and I've been able to thread all the way through. Now I can run an overlength bolt, no bottom, and still nut it from below if needed. Thanks for all the advice.
Just to answer the question above, there are 18 threads from factory. Now I have a few more than that, and the nut option.
Not sure which thread it was in, but a huge thank you to the person who said to use the hammer drill and masonry bit to remove the tap. Worked where a tap extractor, and about 5 hours of other attempts didn't. I just loaded a 3/16 masonry bit into the HF hammer drill. Switched it to "hammer only" and within a couple minutes, the tap was crumbs.

 

[gearhead1]

A good rule of thumb for thread engagement is:
1 x diameter in steel
1.5 x diameter in cast iron
2 x diameter in aluminum

So for example a 1/2-13 fastener would need a .5" in steel, .75" in cast iron, and 1" in aluminum.