Heli coil or 3/8 tap

[Keepi time]

Ok before I do this want some input from those who have gone before. Looking at the 3/8 tap, it's not that much bigger than the 5/16 hole that's in the block now. I would think having an entire thread it would be stronger and last. Looks to be enough meat in there to do the 3/8. Fire away guys. Please only comment if you have done this before. Thanks!

 

[Tim Frank]

Re: Heli coil or 3/8 tap

Ok before I do this want some input from those who have gone before. Looking at the 3/8 tap, it's not that much bigger than the 5/16 hole that's in the block now. I would think having an entire thread it would be stronger and last. Looks to be enough meat in there to do the 3/8. Fire away guys. Please only comment if you have done this before. Thanks!

It would depend on the thread that was there.... 5/16 fine or coarse?.... what you were wanting to go to 3/8 fine or coarse?....and what the bolt is holding.

 

[boobie]

Re: Heli coil or 3/8 tap

Go the heli-coil route. A heli-coiled thread is actually stronger than the orginial.

 

[the machinist]

Re: Heli coil or 3/8 tap

My questions, is it a bolt hole that has a lot of stress, like the only bolt holding on a part or one of another 4 or 5 bolts? Just what size bolt is it? 5/16" ? Does the attached part have enough room to up the size to 3/8" without compromising structural strength?

You are right 5/16" is the drill size to tap to 3/8 course.

Heli-coil kits are not cheap so to do only one may be prohibative.

Another thing you can do if it is not a real strength issue, is to clean the hole/threads GOOD, smear just enough Vaseline on a new 5/16" course bolt to prohibit it being glued in, mix some J-B Weld & put it in the hole, then carefully screw the bolt in. Just before it sets up, unscrew the bolt.

 

[Keepi time]

Re: Heli coil or 3/8 tap

My questions, is it a bolt hole that has a lot of stress, like the only bolt holding on a part or one of another 4 or 5 bolts? Just what size bolt is it? 5/16" ? Does the attached part have enough room to up the size to 3/8" without compromising structural strength?

You are right 5/16" is the drill size to tap to 3/8 course.

Heli-coil kits are not cheap so to do only one may be prohibative.

Another thing you can do if it is not a real strength issue, is to clean the hole/threads GOOD, smear just enough Vaseline on a new 5/16" course bolt to prohibit it being glued in, mix some J-B Weld & put it in the hole, then carefully screw the bolt in. Just before it sets up, unscrew the bolt.

This is a hole where the head is bolted on, it is next to the cylinder wall yes, but there seems to be enough threads for either the helicopter coil or the 3/8. I am thinking to try the heli coil first and if it does not hold the 20 ft pound torque, I can always go to the 3/8. If I start with the 3/8, I cannot go backwards.

 

[Roberthill]

Re: Heli coil or 3/8 tap

To put a heli coil in you have to drill and tap hole first which will probably be as large or larger than 3/8".I have repaired many holes drilling and tapping next size up in aluminum which seemed to work out best .40 years experience machine shop .

 

[Keepi time]

Re: Heli coil or 3/8 tap

To put a heli coil in you have to drill and tap hole first which will probably be as large or larger than 3/8".I have repaired many holes drilling and tapping next size up in aluminum which seemed to work out best .40 years experience machine shop .

To put in the 5/16 heli coil, the bit size is 24/64 which is .375. 3/8 bolt i assume would be a little larger after the tap. I guess I would rather tap half of the depth to put a heli coil in vs. having to tap out the entire hole. More chance of messing up something. Giving it a shot today. Any tips on this is appreciated. Plan on taking my time, drilling straight, using a brand new bit. I plan on blowing out any excess material, using a magnet to be sure, just hope all goes well. If it does not hold the torque rating, I'm screwed! I see enough guys on here have done it, so I should be ok.

 

[Tim Frank]

Re: Heli coil or 3/8 tap

This is a hole where the head is bolted on, it is next to the cylinder wall yes, but there seems to be enough threads for either the helicopter coil or the 3/8. I am thinking to try the heli coil first and if it does not hold the 20 ft pound torque, I can always go to the 3/8. If I start with the 3/8, I cannot go backwards.

No. That won't work.
The helicoil requires a letter "Q" drill. That's too big to allow tapping a 3/8-16 thread afterwards.
If you want to allow yourself the most "strikes", (you never actually have said what thread series you are talking about, but I'll assume that the original thread was a 5/16-18) you could make your first kick at the cat using a 5/16-24 roll-form tap and use that to reform the thread. Use an Allen head cap screw to alert the next person, or remind yourself, that there is something "different" about that head bolt.
If that does not work, you have not re-dimensioned the hole beyond the limit to still tap a 3/8 thread.

 

[gm280]

Re: Heli coil or 3/8 tap

You could easily use a total threaded insert as well. Those are used everywhere in the aircraft industry and hold extremely well.

 

[Tim Frank]

Re: Heli coil or 3/8 tap

To put in the 5/16 heli coil, the bit size is 24/64 which is .375. 3/8 bolt i assume would be a little larger after the tap. I guess I would rather tap half of the depth to put a heli coil in vs. having to tap out the entire hole. More chance of messing up something. Giving it a shot today. Any tips on this is appreciated. Plan on taking my time, drilling straight, using a brand new bit. I plan on blowing out any excess material, using a magnet to be sure, just hope all goes well. If it does not hold the torque rating, I'm screwed! I see enough guys on here have done it, so I should be ok.

OK, do it your way, but you'll only have the one option, and I'd recheck that drill size if I were you!
Also, are you sure a magnet is going to add anything to the party???? Just saying....

 

[Keepi time]

Re: Heli coil or 3/8 tap

OK, do it your way, but you'll only have the one option, and I'd recheck that drill size if I were you!
Also, are you sure a magnet is going to add anything to the party???? Just saying....

Sorry I meant 21/64ths. Hey, I'm new at this, believe me, I'm taking in all the advise I can on this one.

It is a 5/16-18 thread. I was thinking by going with a 5/16-24 fine thread, it would not allow the same torque, and changing the pitch like that would take more metal out and not allow for a 3/8 hole on the next go around. Again, I'm not the expert and appreciate all of your ideas. Just hard to know which one to go with. I am gambling either way i guess.

 

[Tim Frank]

Re: Heli coil or 3/8 tap

The roll-form tap does not cut....it roll-forms....and so you don't really lose material per se...in theory .
That's the up side, plus you do get that extra chance. Downside is that roll-form taps are a tad pricey; if you don't have one on hand, it will run you anywhere from 20-30 bucks if you shop around. If you don't shop around, you may pay as much as $50-....but compared to a non-DIY solution, in my books that works; plus you have a useful tool for next time.
The fine thread can typically handle more torque than a coarse thread, but you have to be a bit more careful on assembly not to cross-thread.

That's about all I can suggest.

BTW, the 5/16 -24 should require NO drilling, just use the tap directly.
Nice thing about staying with 5/16 is that the head will not have to be re drilled.

 

[Keepi time]

Re: Heli coil or 3/8 tap

The roll-form tap does not cut....it roll-forms....and so you don't really lose material per se...in theory .
That's the up side, plus you do get that extra chance. Downside is that roll-form taps are a tad pricey; if you don't have one on hand, it will run you anywhere from 20-30 bucks if you shop around. If you don't shop around, you may pay as much as $50-....but compared to a non-DIY solution, in my books that works; plus you have a useful tool for next time.
The fine thread can typically handle more torque than a coarse thread, but you have to be a bit more careful on assembly not to cross-thread.

That's about all I can suggest.

BTW, the 5/16 -24 should require NO drilling, just use the tap directly.
Nice thing about staying with 5/16 is that the head will not have to be re drilled.

Tim thanks for all of the advise. I think I am going to try the roll form. You seem pretty confident in your reteric. The other heli coil is 40 bucks so price doesn't really bother me. Not messing up the thread is more important to me. Ill let you guys know how it goes. Thanks again!

 

[Roberthill]

Re: Heli coil or 3/8 tap

You never stated condition of the hole but if the 5/16 18 threads have been stripped out the roll form tap will have no material to form the thread .A 5/16 24 roll form requires hole to be .295 , a 5/16 18 stripped out hole is .312. Hole size is critical in roll form-ing .

 

[Keepi time]

Re: Heli coil or 3/8 tap

You never stated condition of the hole but if the 5/16 18 threads have been stripped out the roll form tap will have no material to form the thread .A 5/16 24 roll form requires hole to be .295 , a 5/16 18 stripped out hole is .312. Hole size is critical in roll form-ing .

Figured this out today, now going with the course thread. Rainy here today. Tomorrow is the day. Fingers are crossed.

 

[Keepi time]

Re: Heli coil or 3/8 tap

Well everything went well. Ended up having to add a second one after I retorqued everything down. Thanks for the advise fellas.