You need 1x dia. for thread engagement. Use blue Loctite.
Murphy, that's a nice rule of thumb; I like it enough that I often use it myself. My thought is that rule came into popular use because it is a nice approximation of the thickness of the standard nut for each bolt....and how could that be wrong?
But it isn't the last word.
If you want to go into more detail, check out:
trwww.engineersedge.com/calculators/iso-minimum-thread-engagement.htm
There calculations come closer, but there's problems with that approach too.
Scott, it's a nasty problem. There might be "helicoil type" inserts that would work; I don't have any experience with them. But they all require hand-tapping into the cast engine block material and that would be a problem all by itself. Last resort.
I like the fix that Coyote came up with. Somehow you need to get a bolt out and measure the depth of unused thread in the block. There will be some.....hopefully there will be enough. Then decide what thread you have and go to McMaster Carr to get some.
Now the choice is whether to use a new tap and carefully chase out the damaged thread. As to how to do the thread restoration.....
I've actually had better luck restoring damaged threads by not using a commercial tap at all - commercial taps are too sharp with too much cut and not enough thread support. Too easy to get them started wrong and they will cut regardless of right or wrong.
What I do is better and costs less. Take a hardened bolt of the right thread and size and cut half a dozen slots with a hacksaw perpendicular to the threads. Cut the slots about an inch long and somewhat deeper than the minor thread diameter. Now carefully taper the bolt for the first couple of threads. If you want, I can probably find an old bolt I've done this to and post a picture. This becomes my handmade thread restoration tap. It is way better for the purpose than a tap. Start it straight, go very slow with lots of light lube, and the real trick is to back it out to clean the particles with a hand wire brush quite often. Sometimes I back out every 1/8 turn I gain....or even more often. Constantly cleaning the particles out of the hand-made restoration tap is critical to doing this job. Each time you back the hand-made tap out, Use WD-40 and a nozzle to foam out the particles left behind in the thread bore.
When done, try to thread in the longer bolts you bought - best source for those might be at McMaster. With luck, there will be enough good thread to hold the torque. That's not quite as good as new, but I can't think of a better solution. Be careful not to use too long of a bolt. You'll probably have to cut them to special length. Use hardened washers. Measure to leave a little free space at the end. You sure don't want to go too deep and crack the casting.
What a pisser. Yes, the original bolts were more than likely poorly installed. This is as good as I can come up with. Don't give up on the threads you have until you've tried every way to get them into use again. Anyone else?
luck, rScotty