Cracked Block Repair

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Cracked Block Repair

Postby George Willer » Sat Dec 16, 2006 11:56 am

This is what I have done in the past and plan to do again soon to another engine. (On a recent eBay prize). When I get a round tuit I plan to take photos, but for now I'll try to do it with words, as best I can remember from a dozen years ago.

The load that breaks the ears is mostly tensile. Cast iron isn't very strong in tension, and in spite of what many think welded cast iron near a repair is even weaker, although the weld itself may be very strong. For that reason I use bolts.

The 3 lowest front cover bolts (3/8") are removed and the holes drilled out to 7/16" clearance as deep as the crack (and no deeper!) and then the hole is continued as deep as absolutely possible for 7/16-14 tap size without breaking through and tapped with a bottoming tap. 2 more new holes are then drilled between the originals and treated the same way so there are 5 bolts to carry the load.

It's very important to have as many threads as possible, so maybe all but the top one which would break out in the oil pressure relief could be through holes. (I used all closed holes before, but through if possible would allow more threads) Each grade 8 bolt should be bottomed and the amount remaining under the head measured to determine how much to shorten, and each bolt should be marked so it goes in the right hole.

I don't know for sure this repair is as strong as original, but it must be close. Only those with sharp eyes will see the larger bolt heads and the one extra that is visible, so this could even be done to a damaged show tractor.
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Postby Donny M » Sat Dec 16, 2006 12:17 pm

Great idea :!: I wonder if using a 7/16-20 thread would be beneficial, giving almost 50% more threads. I often wondered why IH used a 10-24 screw in the bolster when the depth of the hole is only about .5" and the screw only .38" long. Not much thread bite if using a lock washer. :shock:
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Postby Merlin » Sat Dec 16, 2006 12:28 pm

You described it very well George. Thank you. I am looking forward to the pictures. Sounds better that what I did, even though its been 2 years and still holding up. If I ever do another one, I plan on doing something like you described. I just removed one bolt and drilled through the crack, tapped it, squeezed in some JB Weld and put a longer bolt in its place.
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Postby George Willer » Sat Dec 16, 2006 12:57 pm

Donny M wrote:Great idea :!: I wonder if using a 7/16-20 thread would be beneficial, giving almost 50% more threads. I often wondered why IH used a 10-24 screw in the bolster when the depth of the hole is only about .5" and the screw only .38" long. Not much thread bite if using a lock washer. :shock:
8)


Donny,

There's a good reason fine threads are only very rarely used in cast iron. The coarse threads are much stronger.

I've wondered about the 10-24 dogleg screws too. Rambo's are all 1/4-20 now and I expect to change others over as well.
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Postby Donny M » Sat Dec 16, 2006 2:45 pm

George Willer wrote:
Donny M wrote:Great idea :!: I wonder if using a 7/16-20 thread would be beneficial, giving almost 50% more threads. I often wondered why IH used a 10-24 screw in the bolster when the depth of the hole is only about .5" and the screw only .38" long. Not much thread bite if using a lock washer. :shock:
8)


Donny,

There's a good reason fine threads are only very rarely used in cast iron. The coarse threads are much stronger.

I've wondered about the 10-24 dogleg screws too. Rambo's are all 1/4-20 now and I expect to change others over as well.


Mr. GW,

Is the root dia. the reason a 14 pitch bolt is stronger :?:
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Postby George Willer » Sat Dec 16, 2006 3:26 pm

Donny M wrote:
George Willer wrote:
Donny M wrote:Great idea :!: I wonder if using a 7/16-20 thread would be beneficial, giving almost 50% more threads. I often wondered why IH used a 10-24 screw in the bolster when the depth of the hole is only about .5" and the screw only .38" long. Not much thread bite if using a lock washer. :shock:
8)


Donny,

There's a good reason fine threads are only very rarely used in cast iron. The coarse threads are much stronger.

I've wondered about the 10-24 dogleg screws too. Rambo's are all 1/4-20 now and I expect to change others over as well.


Mr. GW,

Is the root dia. the reason a 14 pitch bolt is stronger :?:
8)


Donny,

It would take a real engineer to say for sure. :lol: I only play one on farmallcub.com. :shock: What I'm pretty sure of is that the crests of the threads add very little strength because of Cast Iron's brittleness. My guess, and it's only a guess, is that the broader roots are more forgiving.
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Postby beaconlight » Sat Dec 16, 2006 5:10 pm

I am no engineer either but I see it the same way as George does for cast iron. Many hand tapped holes in cast iron are ragged to say the least. My old Polish machinest friend used lard as the lubricant for cast. Bill Murowski tapped clean holes in cast. I remembered my old Frenchs Engineering Drawings book and dug it out. for Cast iron, Brass and Bronze the minimum entrance length is 1 1/2 diameter. Steel is diameter and Aluminun 2 times diameter. It is general commercial practice to use 75% of the theoretical depth of thread for tapped holes. This gives about 95 per cent of the strength of a full thread and is much easier to cut. Our commonly used taps and dies are made to provide this 75%.
As a test, a bolt inserted in an experimental nut made with only one-half a full depth of thread will break before the thread will strip. I don't see any reference to wheter this is a grade 2, 5 or 8 bolt. Used with cast bolt head may be a consideration if the head rests on cast. A chamher at the hole entrance for the radius under the head, or a washer faced head. Fo course using washers and lock washers fits in to this too.

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Postby Phillip W. Lenke » Tue Dec 19, 2006 6:29 am

Bill , Exactly on the depths, that is my standard for tapped holes, I learned that from an old Die maker.
As a common practice in the molding industry, our shop used to helicoil all Aluminum casting tapped holes. Now I know these can be weaker than most, This got pricey at times , but not as pricy as having a failed mold during a production run.
I have helicoiled the 5/8 -11 on our cub due to stripped threads. That's the only way I will go now, Can be pricey for one Kit. However My brother gave me the Kit
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Postby g19605 » Tue Dec 19, 2006 10:24 am

The Helicoil idea is real good for cases where repeated insertion and removal is required. This is especially good for aluminum where seizing and galling is to be expected. The Helicoil wire is very smooth and the "squeeze" is high for a given torque.
I don't know how the strength angle works out -- since the circle of contact of the Helicoil wire is considerably larger than that of the bolt/stud that fits into it. I think that the Helicoil people have lots of information on the subject.
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