head thread repair on aluminum block

We are working on a Honda 90 Accord, F22A4 open deck aluminum block. One of
the head bolts pulled the threads out on removal. This was not noticed until
the head was repaired and reassembly was attempted.

The threads are recessed approximately 2 inches below the deck surface. The
head bolts are not TTY and are 12mmx1.5. The gasket is MLS type. Torque is 3
stage, 26/52/78.

(Note: ARP makes head studs that will fit. Timesert makes a repair kit [the
kit for the Odyssey on their site], but charges $400 for the kit since it is
intended for pro rebuilders.)

The best we can come up with is to tap and helicoil the damaged hole. But the
recessed threads make this difficult to manage, both in driving the tap and in
breaking off the installation tang. Anyone with experience there?

I can get a bare machined block for $100. What's the chances it will correctly
seal up with the existing head, and what should I do about the rings?

A new (used) motor is $500+freight, but I'm $250 in on the valve job and wasted
head gasket already.

Can anyone recommend a course of action? What was intended to be a quick valve
& gasket job has quickly disintegrated.

 

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.
--
Dave Baker
www.pumaracing.co.uk
"Why," said Ford squatting down beside him and shivering, "are you lying
face down in the dust?"
"It's a very effective way of being wretched," said Marvin.

 

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 in '00
88 Cherokee 235 BFG AT's
Canadian Off Road Trips Photos: Non members can still view!
Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590
(More Off Road album links at bottom of the view page)

 

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.

http://www.futek.com/boltcalc.aspx?mode=metric

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.

 

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 http://www.imagestation.com/album/pictures.html?id=2115147590
(More Off Road album links at bottom of the view page)

 

Is it possible that the bolt really was a 1.25 but was able to thread
into the 1.5 gauge anyway? Of course, maybe their gauge was cross
threaded by another customer.

So what can be done about it jumping around? It'd be nice to get
something approximating the correct torque on it.