Stripped Columbia house

glennpm said:
Hi Tubman,

First, the bolts are 5/16". I just checked on my Columbia. I don't know if they are coarse or fine thread. My hunch is that they are fine so for this re-tap fix my suggestion may by mute.

Fine threads have a larger tap drill size since the minor diameter is larger for fine than coarse. Fine threads have more surface area and the threads are closer to the thread major diameter which is equal to 5/16" in this case. This means that there is more meat of undamaged base material tha can be tapped.

If inspection of the damaged threads is mostly on the damaged tips of the coarse threads, than there is still a significant amount of base metal that can be used for a fine thread tap. If most of the hole is wallowed out to 5/16", this re-threading to fine won't work. In many case this is not the situation.

No harm in trying the re-tap to 5/16-24. If it doesn't work out, drill and tap to 3/8. Due to the application here, there is no need to use a thread insert unless you're bothered by the larger head of a 3/8".

Fine threads have greater tensile strength than coarse thread in the same diameter.

I have used this technique a number of times up to 3/8".

Glenn

514365
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Good detailed explanation, Glenn. However, I still don't buy it. In short, since the outside diameter of both pitches is the same, but the pitches significantly different, metal will be removed re-tapping the hole. That just can't be good.

Someone above mentioned Loctite Thread Repair. I had totally forgotten, but I used this to repair a stripped thermostat housing bolt hole in the intake manifold of my Corvette about 35 years ago. I followed the instructions closely and it must have worked, because when completed, the bolt took and held the specified torque when I re-installed the thermostat housing. I still have the car, and that repair has held through a valve job, a total engine rebuild, and at least two thermostat changes. It is the least I would try initially.

The repair was quite similar, being a through hole in a casting using a 5/16" course bolt. The manifold was aluminum, but I would expect it to work even better in cast iron. If that didn't work, I would use a Heli-Coil. I would think that welding would be a last resort, given the difficulty of welding castings and would probably require removing the unit from the car.
 
put a bolt inside and thread it through the hole making it a “stud”
the metal is a little thin for a helicoil, it can get displaced easily.
another fix i have done is to put a nut inside holding it in place with a bolt and tack weld the nut to the housing
 
Kurt in NJ said:
put a bolt inside and thread it through the hole making it a “stud”
the metal is a little thin for a helicoil, it can get displaced easily.
another fix i have done is to put a nut inside holding it in place with a bolt and tack weld the nut to the housing
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Considering how thin the housing is in this area, I'd go for your proposal.
The only difference? I'd use an elevator bolt. They have a very low profile head and will not cause an interference issue in this area.

You state the material is a "little thin" in this area. It is in fact way too thin for a Helicoil as well as too thin for tapping to a fine thread with any confidence of success.
 
Kurt in NJ said:
put a bolt inside and thread it through the hole making it a “stud”
the metal is a little thin for a helicoil, it can get displaced easily.
another fix i have done is to put a nut inside holding it in place with a bolt and tack weld the nut to the housing
Click to expand...

I suggested this method in my Post #8, except I suggested epoxy to hold the nut in place. I don't see how you could "tack" the nut in place unless you disassembled the whole thing (which I would want to avoid). Also, castings are difficult to weld, so you may want to try brazing it if you do disassemble it. Also "Kube's" suggestion of an elevator bolt is a good one. I have found them handy in several situations.

If it were mine, I'd try the "Loctite Thread Restorer" first, as it is the one method that doesn't alter the original casting. From my experience, it does work and work well, and if it doesn't, you have lost nothing.
 
I've seen other types of housings that have fine thread fasteners originally but someone decided to cram a course thread fastener into. Or one that gets loosey goosey had someone tap it out to course threads. A lot depends on how much effort they went through to Fubar the housing. I've installed a lot of heli coils with good success. It might be worth a try to get the original size tap into it but I wouldn't expect it to be a good fit any longer.

I loctite the heli-coil with the grean stuff (609) or the HD thread locker (271) to insure it never comes loose. Cast iron can be difficult to work with so a person has to take a lot of care when negotiating any sort of repair to it. In many of the aluminum cases I've worked on over the years, a lot of the threaded bores were already heli-coiled. I've put heli-coils into thin stuff before but the coil may need to be ground off if it protrudes too far. This would likely require disassembly so all this has to be considered.

The other option is to braze the hole shut and redrill & tap. This would be a lot more difficult for some but it could be done as long as the case was preheated and cooled slowly. Cast iron likes to crack.
 
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I wasn't thinking about the Columbia half being a deep drawn steel part. A thickness of 3/16" won't have much thread in it. If three or four threads fit in then that might be enough for a coil. Since it's not cast, it could be welded up, drilled, & tapped. I've tried gluing bolts into holes using sealer. It's always a temporary fix but it might get by till the next overhaul.
 
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