head thread repair on aluminum block

I have seen and used helicoils in an aluminum block, I don't think they will pose a problem with your Accord 2.2 block. The holes are blind so that no coolant is present in the bolt hole which could potentially leak past the threads.

Breaking off the tang should be easy. Strike it with a long flatblade screwdriver and hammer in one quick tap. Pick up the tang with a greased stick of some sort. I can't remember if helicoils are "magnetic" since they are some stainless type of compound. I seem to remember getting tangs out with a pocket magnet in the past, but my memory sucks.

You can thread further down into the block by modifying your 14 X 1.5 mm Helicoil tap. Cut off half of the square drive end of the tap and cut a matching half out of an old 3/8" extension or similar tool steel shaft. Assemble the two parts together, butting the lap joints you just made as tightly as possible, and weld the assembly together (or have a welding shop do this. Now grind down the thickness of the extension shank as necessary to get it to fit in the threaded holes. If welded properly, it will hold up and allow you to reach farther than the tap would normally allow. You will probably need to install two helicoils (stacked). Make sure the highest point of the top helicoil is slightly below the uppermost thread -- this is where the most stretching load is concentrated so this area needs to be sound.

Toyota MDT in MO

Helicoil is the best way. A 12mm helicoil tap ought to be long enough for your needs. If not you can buy tap extensions, weld a bit more shaft on the end, cobble something together with a 1/4" drive female socket bar extension and grind the square on the tap down to fit. All sorts of ways with a bit of ingenuity. The tang is hardly an issue. You can knock that off with anything or just leave it in place if the bolt doesn't go that deep. The bit you ought to be worrying about is using the right length insert. Standard in most kits is 1.5 x D and they are usually fine. You might need a 2 x D for sufficient strength in aluminium but you can work that out by seeing how much thread is engaged on one of the other head bolts.

Heli coil if done right may do it. They are sometimes a pain in places like that. A professionl install may be better on it You might see if your machine shop will make a house call or tow it by machine shop. Also see if machine shop can get a stud repair kit for it. And also check the other head bolt threads to. Good luck MT

That sounds like a reasonable plan. How do I deal with the situation where the two stacked helicoils end up being higher than the uppermost thread? Use one instead? The helicoils are 18mm and the threaded part of the bolt is 1 inch (25.4mm). We do not know how deep the threads in the block are yet.

1" is more than enough to hold the bolt. [for stress analysis purposes, the load is only carried by the first 3 threads if any threaded bolt anyway.] as long as you can overcome the issue of the recessed hole to drill, tap and coil the new thread, it'll hold just fine.

So a 1" bolt threads inside a single 18mm helicoil should be sufficient not only to hold the load but also to keep the bolt from backing out?

Is there anything different about a helicoil tap from a normal tap? We are thinking of just buying a long normal tap of the correct dimension. If so, what tap goes with a 12mmx1.5 helicoil? I emailed the manufacturer to ask them but they didn't answer...

no, a helicoil tap is NOT the same as a regular tap - you need the tap that comes with the specific inserts that you want to use. The helicoil tap is larger in diameter but still has a 1.5mm thread pitch because the outside of the insert is by necessity a mirror image of the inside. The same is true of pretty much any type of threaded insert such as Time-sert etc.

nate

True on Helicoils and other coiled spring inserts. However most of the sleeved style inserts use a different exterior thread. On the inserts I have here the thread profile is also different, it is almost a round profile.

All of them require the specific tap for the insert though. If you buy an insert kit it will have the tap and various length inserts in it.

As an aside helicoil inserts in aluminum make stronger threads than the bare threads in aluminum. Take a look at most military hardware and you will find inserts in the soft metal parts.

OK, so we helicoiled it. Used the suggestions of welding the tap to an extension it worked great. Then noticed the Honda factory book said the bolts were 12mm x1.25 not x1.5. had a WTF moment but then determined that the book is wrong. Used some zinc assembly lube (all I could find). Did a test torque with the old gasket. All of the holes are good up to 80 ft lbs but the bolt is jumping around in the helicoil hole past 50 lbs. Removed it, and relubed it a few times but no improvement. Think moly lube would be better? I'm not sure if it's jumping in the helicoil hole because of something we did wrong, or because it is steel on steel so it needs different/better lube.

I haven't been following this thread, but I'm going to make some presumptions: you are talking about steel and aluminum. The coefficients of thermal expansion are different, depending on the alloy.

What you want to aim for is a fastener TIGHTENING in a hole as the assembly heats up. IOW, the fastener should have a larger coefficient of thermal expansion than the pieces being assembled with that fastener, to prevent the fastener from loosening in the hole as the assembly heats up. The hole gets bigger, the fastener better be able to keep up with it. I suspect in your case...the hole in the aluminum is getting larger, and the helicoil is -not- expanding along with it to keep a tight bond. Of course it is expanding somewhat, but it has to overcome or at least match the rate and dimensions of the expansion of the assembly metals.

Just a thought from a guy in the peanut gallery, who has come to the game *late,* and doesn't know "who is on 2nd"

Lg

You could easily be describing the results of trying to put a bolt with the wrong threads into a helicoil....

Are you 'sure' the book is wrong???

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail >

Jumping, as in the static friction in the threads is so great that you turn the bolt-head a little, and the threaded part is 'stuck', then the threaded part suddenly turns a bit in the threads?

Or jumps, as in you put torque on the bolt head, and the bolt head doesn't move, then all of a sudden the bolt head turns a bit (large static friction where the bolt head meets the head).

Or, as Mike R suggests, could the bolt actually be slipping out of the threads?

Dave

i don't know what the thread is on a prelude, but i can tell you for sure that the head bolt thread pitch on the civic is 1.25mm - i've just checked a couple i have in my box of bits.

but since you've already helicoiled the block and are committed at this point, and have screwed the bolt in, you'll have effectively re-threaded the bolt. i suggest you screw it in and out a few times to complete the cutting process, then leave it. you'll know better next time. you may want to up the torque a couple of notches too. the finer thread with give a higher bolt tension than the coarser thread at the same torque, so since you have a coarser thread, the torque needs to be a little higher to get the bolt tension. not sure exactly how much without looking it up, but you may be able to find it on the web somewhere.

That sucks, before ordering the Helicoil stuff we checked the bolt against a thread gauge at Napa (fit exactly 12mm x 1.5) but I guess something was wrong since there would be no other reason for the bolt to be jumping now. I thought if the threads were wrong it would start hanging up way before that point....

Well, do you think it would be possible to find a 12mmx1.5 bolt to use in its place? The bolt is 6 3/8 inches and the threaded part is 1 inch. The head is 14mm. I don't know where to look for this kind of thing... the best I could think of is maybe some other Honda uses a similar bolt.

Yeah, we sure will.

I put in the following values for the original bolt: Application Data Major Bolt Diameter: 12.00 Bolt Pitch: 1.25 Bolt Proof Strength: 827 Recepticle Length: 25.4 Recepticle Strength: 414 Applied Tensile Load: 0 k Factor: 0.2

Recommended Torque

113.25 N-m (11.555 kg-m) (83.52 ft-lb) Resultant Tension:4811 Maximum Torque 151 N-m (15.407 kg-m) (111.36 ft-lb) Resultant Tension:6415 83 ft lbs is in the ballpark of the recommended torque from the manual (78 ft lbs)

And then the following values for the current situation:

Application Data Major Bolt Diameter: 12.00 Bolt Pitch: 1.50 Bolt Proof Strength: 827 Recepticle Length: 25.4 Recepticle Strength: 414 Applied Tensile Load: 0 k Factor: 0.2

Recommended Torque

108.19 N-m (11.039 kg-m) (79.79 ft-lb) Resultant Tension:4596 Maximum Torque 144.26 N-m (14.718 kg-m) (106.39 ft-lb) Resultant Tension:6128

What is strange is that it actually recommends less torque for the 1.5 pitch (79 ft lb) than for the 1.25 pitch (83 ft lb) with everything else constant. That doesn't really make any sense, does it?

Actually that does make sense, on a coarse thread you have less surface contact per length of threaded area, so you are losing less of the torque reading to thread friction.

to be honest, i haven't thought it through. i was under the impression that finer threads gave higher tension for a given torque based on geometry. it may be that a lower torque is specified because typically, coarser threads are in lower strength bolts. i'll think about it some more.

well, if you checked the bolt with a thread gauge and it was 1.5mm, then you got it right! that's good news.

best stick with the original bolt regardless.

jim beam wrote in news:ZeudnVNI1M4DkL7YnZ2dnUVZ snipped-for-privacy@speakeasy.net:

Fine threads are used in instances where vibration is a concern.

Consider that, mechanically, a screw thread is just a variation of a wedge. The shallower the slope of the wedge, the less the mechanical advantage of the opposing (loosening) force. And, conversely, the easier they are to strip...

Coarse threads are, in and of themselves, stronger, but are more susceptible to vibration and loosening on account of their steeper slope. There exist M12 bolts with coarse threads, and indeed, they are in certain circumstances easier to find than fine-thread fasteners.

Correct stretch and the use of prevailing torque nuts can satisfactorily allow coarse-thread fasteners to substitute for fine-thread fasteners.

I guess you now know why the hole was stripped.....

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06 (More Off Road album links at bottom of the view page)