Change head bolts to grade 8?

A friend of mine is in the middle of having his Cub engine rebuilt. "Leo" is from the class of 1950, and its time for a rebuild. Lots of Kroil but, the machine shop has reported back to him that one of the head bolts is broken off in the block. His understanding is that broken head bolts are a common occurance on Cub engines.

The question is this: Is it ok, or a bad idea to change these bolts from the standard grade 5 to a grade 8 bolt? I'd like to hear comments both good and bad. Have you had a broken head bolt? Tell me about it friend.

Also, can anyone tell me the proper length of the bolts ( or length of threads, or anything the makes the bolt special ) that go on the cub head? The engine came in a million pieces when purchased.
Thanks for your time.

the way to go is to use the grade 8 bolts, the only probblem is that the bolts are slightly longer than the factory bolt and extend in to the cooling system of the block, they should be cut off about a 1/4" of the bolt and use thread sealers so the antifreeze does not creep past the bolt threads
Tim

Here is my 2 cents, I use anti-sieze when I replace the head bolts, and I would use the factory recomended bolt I'm not sure of the length, I don't think it is a good idea to use grade 8 bolt as a replacement.

I would use grade 9 bolts. You can get them from McMaster Carr. 45 lbs torque spec exceeds grade 8. http://www.mcmaster.com/#standard-cap-screws/=kjxx9n Trim them to the proper length so they don' stick out into the coolant chamber. Since coolant can weep up the threads, use some type of sealer. Lots of opinions here, none are "wrong". My personal preference is Rectorseal No 5. Used by NASA- that is good enough for me.

Grade 8 or better and I use C5A on the threads.

When I replace them, I always use grade 8 bolts, the torque spec. exceeds the capacity of grade 5 bolts. I don't know the lengths off the top of my head, but the long bolts are available in the correct length, the short ones are an odd length so they'll need to be trimmed about an 1/8" or so. I made a little tubing jig that I slide over the bolt and grind the bolt, on my belt sander, till it reaches the jig. I can do a set of bolts in about 5 min. this way, with nice uniform results. When installing, I seal the bolts with PTFE pipe thread sealant. If you buy head bolt thread sealant from ARP, the most respected maker of performance fasteners, PTFE is what they supply.

The other thing to check is where the bolts were made. Grade 5or8 in this country may not be the same in another country. It' s about supply and demand and quality control.

I don't remember things too well as it has been 20 years since I was a mechanic, but it seems that drilling out a broken grade 8 bollt was a lot harder than a grade 5 correct me if I'm wrong

Something to be aware of when torquing bolts coated with thread sealants. Torque comes from the friction between the moving parts of the threaded components as they tighten. Thread sealant acts as a lubricant, 45 lb of torque on a dry bolt is far less pressure than 45 lb on a bolt with sealant.

Do not use grade 5 of any manufacturer. Made in USA grade 8 or 9. I used 8's and cheated the torque a little at 43 lbs. Seems to be working just fine.
If you buy IHC bolts, you'll be out about $100. If you buy McMaster or Fastenal, you'll be out about $20.

As far as the drilling - it will be tough going whichever you use as the broken bolts couldn't be tighter if they were welded in the block. I spent about 3 hours and multiple drill sizes, taps, prayer, kroil, deep creep, you name it I tried it. Finally got the bolt out in little leftover curls of threads.

When drilling broken head bolts use the head as a guide for centering. Use a piece of tubing that your bit fits through so as to center and not damage the hole in the block.

All good info guys. Many thanks. Now my friend needs to dig in boxes and boxes of Cub parts that we hauled to his farm. Its was quite a haul. Finals, blocks, hoods, grills, 4 plows, platforms, seats,...you name it. Should be enough to built 2.3 Cubs from his best guess.

Anyone know the correct length of the bolts? Finding some in those boxes will be tough. I checked the C-60 engine parts manual (page 15) and found P/N 251-230 and -231, but not a length.
http://www.cleancomputes.com/Cub/Cub%20 ... index.html

If all else fails, I guess a dental pick down the hole, and measure how far it sticks up with the sharp end hooked around the bottom in to the water jacket.

If you don't have measurements by tomorrow I'll get them for you. Have some old ones in the garage.

3/8-16 2-3/8" 9 head bolt
3/8-16 3-1/4" 6 head bolt

Thanks!