How to put two heli-coils back to back?

[TheUser]

I'm heli-coiling one of my alternator bracket bolt holes in the front of my pass. head. The hole is deep enough to require 2 heli-coils back to back. Really you could only help if you've used heli-coils before. I put the one in first and now I'm trying to put the second one in. My problem is that I need to break the little tab thing off of the bottom to let the bolt go through it, but that tab (tang) needs to be there for the mandel (driver) to drive it in. I tried breaking the tang off after I put the insert in the hole, but it wouldn't break off, so I had to pull it back out. I don't know what to do.

It's an awesome day and sure would be nice to get the car running.

I went to autozone and O'Reilly's and neither knew what to do. The guy at autozone said just use the one, but I don't like that idea. I want to do this right so I don't have to worry about it.

any ideas?

 

[TheUser]

nevermind, I got it. I was just getting frusterated and decided to go for broke. I put it in there normally and used a chisel to break the tang off. That's what the instructions alluded to, but I was afraid I would tear something up.

 

[85 Capri]

I would have just puy in the longest one until it's flush, a thread isn't doing much after it gets past that first coil, thread depth should be 1.5 times the diameter. After that the diameter of the bolt is weakest.

 

[TheUser]

I would have just puy in the longest one until it's flush, a thread isn't doing much after it gets past that first coil, thread depth should be 1.5 times the diameter. After that the diameter of the bolt is weakest.

I'm not so sure I understand what you're talking about. The hole is about an inch deep or so and the heli-coil kit I got only had one size, which were about 1/2 an inch a piece. I had to put them back to back or it wouldn't work. I did and put the studs in, it's fine.

 

[Ranchero5.0]

Helicoil kits come in different depths. I'm not a big fan of helicoils because if you crossthread them they ruin everything they touch.

A much better fix is a thing called a Key-sert. Basicly overdrill and tap the hole, then screw this threaded plug into the hole and finally tap the four prongs on the outside with a hammer and they lock the key sert into the base metal. Dang near indestructible. Hard to find for a home owner, but any industrial mechanic has used them before.

Jamie

 

[1993SSP]

You can get the proper replacement heli coil type insert from your ford dealer for that bolt. I ran into the same problem before. I got a set of heads and they were missing, they are called thread reducers.

 

[85 Capri]

i was just giving a little thread theory. if you have a 3/8" bolt, the depth should be 1.5 times the diameter (.5625" is 1.5 times .375) Therefore a 3/8" bolt would have an approx depth of .5625", this is not always true but a general rule. The reason is that a 3/8" bolt has a strength rating and given that strength a 3/8" bolt that was 3" long thread wouldn't make a difference because the bolt would break before the threads gave out. Again a general rule as bolts come in diffenent grades and have different purposes, shear force, clamp force etc.

Also this is why a 3/8" X 3", 4" and 5" inch bolts have the same amount of thread!

Just throwing some thought into it!

 

[46defense]

Basically what he is trying to say is this. The greatest strength of any thread comes at 1.5 times the diameter. When you are a machinist this becomes very important. If I have a thru hole that needs to be tapped that is deeper than the cutting length of the tap, I have to get a special tap to get them all the way through. It's called a pulley tap. After the cutting portion of the tap ends, the shank is then reduced to provide clearance for the tap to go basically as deep as you want. Needless to say this reduces the strength of the tap and makes it a lot easier for the tap to break as it cuts. This is the reason the formula comes in handy. I never want to make the threads any deeper than I have to. If I have a good 3/4" of threads for a 1/2" diameter bolt, I know I have achieved the maximum strength of that thread. Engineers don't have any problem sending out a print that calls for a 10-32 tap,
1" deep. They just throw it on there without even thinking about it. At that time I know I have to look into the actual application so I can decide what the best thing to do is. In your situation one helicoil would have probably been fine in there. Just because the greatest strength comes at 1.5 times the diameter doesn't mean every situation will allow for that. Your alternator would probably still stay put if that thread length was just 1 times the diameter. Also, the best way to break off the tang is with a pin that just fits in the hole after the insert is in there. Then it breaks off the tang but will not allow the end of the heli-coil to pull out of the threads. Also if you are putting two of them in there I wouldn't but them up to each other. Leave a little room in between them.

 

[TheUser]

wow interesting. That's good to know. So basically I could have used a bolt that threaded .5625" into the hole and it would have been just as strong? What I did was I got some threaded rod and cut each piece to about 2.75/3.00" inches long and used them as studs.