After spending a couple of hours looking for a set of one-time used flywheel
bolts that I put safely away so that they wouldn't get lost... I gave up and
ordered a set of ARP bolts. This got me thinking about what sort of guidelines
should be applied when deciding whether or not to upgrade the more important
bolts that hold an engine together.

For example, the Toyota OEM flywheel bolts are a well made quality item, and
I had fully intended cleaning and reusing them. However, since I couldn't find
them, I did a bit of reading to see if I could find any techincal articles on just
how increasing the torque/power transmitted by the clutch affects the stress
on the flywheel bolts.

Not unsurprisingly, I found a lot of psuedo-technical BS from people selling
fancy bolts; a ton of opinions from people who wouldn't know a high tensile
bolt from a tuning fork; the usual hand wringing from the merchants of doom
who think reusing a bolt is certain to allow some part of your vehicle to fly
apart at high speed causing death and destruction. But no real tech stuff

It seems that this is a subject not much researched, and one where there is
more misinformation than hard fact. Flywheel bolts can fail for a variety of
reasons... over tightened, under tightened, wrong bolt for the job, too short,
too long, used a washer, didn't use a washer etc. But more interesting is the
possibility that the number of cylinders can have a major effect... 4-cylinder
engines being the worst for vibration.

So to the point Who has had a problem with flywheel bolts, and did the
problem relate to using OEM bolts on a high revving NA or turbo engine ?
Let me hear the horror stories from all you one-legged guys

Cheers... jondee86

Flywheelbolts can be reuesd with no actual risk of a failure. Doing this fotlr years without a problem. I Recently reused the flywheelbolts in my NA B16 daily Civic and this motor sees alot of abuse with 8000+ RPM and the 24year old bolts dosen't care at all

Sure I have no problem with reusing stock bolts on a stock engine no matter
how hard you beat on it !! What I am more interested in are modified engines
that push more torque/power than stock... say 50 to100% more than stock.

The question then becomes... Have I now exceeded the loading that the bolted
flywheel to crankshaft joint was designed for ?? Will the clamping force be high
enough to prevent the flywheel moving on the crank ??

Once the flywheel begins to move on the crank, a bunch of things start happening,
and none of them are good. Given enough time, one or more of the bolts will fail
and the flywheel will come loose. Due to the domino effect, once the flywheel has
become loose, there is a good chance of total failure of the bolted joint, and the
flywheel will no longer be attached to the crankshaft

Now hit me with the success/horror stories

Cheers... jondee86

What it looks like...





Not 4AGE pictures... just random pics from the interwebz to show how it ends
up when things go wrong. May just cost a new set of bolts, but can cost a new
flywheel plus an engine teardown and new crankshaft as well

Cheers... jondee86

I have had an experience with broken flywheel bolts on the 7A.



I do believe there is quite a high chance that they were not tightened correctly though.
Cannot remember if they were used, new genuine or new ARP.
They let go while dyno tuning the engine for the first time,
no other damage apart from what you can see, it just lost drive.

The next failure was definatly not bolts though,
more likely was a weak flywheel from a 5AFE engine or something.
Notice in this picture we also added some dowels to it.

interesting ..... can report that I've not seen an Atlantic broken in this manner... but Atlantics use different clutches...

Here is Billy's set up....

Thanks for the input Matt. I'm guessing that this was one of your turbo engines ? Can
you say what kind of power it was making, and what maximum revs you ran ?

In the first pic there is not enough resolution to clearly see the method of failure of the
bolts. But it does look a bit like they have failed in shear, and the most common cause
of shear failure in flywheel bolts is insufficient preload (= not tight enough). Thanks to
ARP for that bit of information

The 2nd and 3rd pics are really interesting !! Here is a FEM pic showing the deformation
(roughly analagous to stress distribution) in a simple cast iron flywheel...



Basically, as the flywheel rotates at high speed, it wants to fly apart, but the material
of construction holds it together. The highest stress concentration is near the centre,
and you can see a band of yellow at the edge of the hub. This is the point where the
thickness of the flywheel reduces, so there is less material to resist the stress loading.

This is the point where your flywheel let go, and I'd put money on that flywheel having
seen some seriously high rpm's before it let go. Incidentally, the above pic also shows
why taking material off the outside of a flywheel (done correctly) is not dangerous, but
removing material near the centre is indeed a recipe for disaster.

Cheers... jondee86

oldeskewltoy wrote:interesting ..... can report that I've not seen an Atlantic broken in this manner...
but Atlantics use different clutches...

I'd bet that they use billet flywheels and the best fasteners that they can buy
That actually looks like an alloy flywheel for even more reduction in inertia, so
I guess there is a steel facing under there for the clutch. And of course, the small
diameter of the clutch helps with those lightning fast shifts.

Beautiful bit of kit for the serious racer.

Cheers... jondee86

For anyone who hasn't seen it yet, ARP has some good basic information on
failure modes for bolts...http://arp-bolts.com/p/technical.php#p7TPMc1_6

Also, if you are looking for something to read over the weekend, browse
the ARP site for some other basic information on bolted joints and installation
technique for ARP products. ARP are not the only game in town as there are
other specialised manufacturers. But the engineering information provided
by ARP is valid for all high performance fasteners, so just ignore the hype
and digest the facts

Cheers... jondee86

Lots'o fretting on all'o them, moving around A LOT before failure. Just too loose/bad install IMO.
Also pretty unevenly on the first one, nothing symmetrical about its wear.
They have to be tightened in STEPS, criss-cross! Also a perfect surface, lapping in might help.

OK... since it is a given that the flywheel moving on the crank is the root cause
of flywheel bolt failure, I've been doing a bit of digging to see WHY the flywheel
wants to move around.

And it seems this diagram...



... that you might have seen in your physics class, pinpoints the underlying cause
of bolt failure. The diagram shows the crankshaft of a four cylinder engine modeled
as a series of rotating masses on a common shaft. As each cylinder fires an impulse
is transmitted to the shaft, which then tries to rotate faster than the other masses
attached to the shaft. This causes torsional vibration.

Refer to this article for a detailed explanation of the physics involved
http://performancedevelopments.com/porsche6cylharmonicenginefailure/

Cutting a long story short, torsional vibration creates a situation where the crank
is trying to accelerate forward and the flywheel is trying to stop it from accelerating.
The flywheel bolts are applying a clamping load to the join to prevent the flywheel
to crank interface from moving. The clamping load transmits torque between
he two parts by friction alone when the flywheel bolts are correctly torqued.
The bolts are not there to locate the flywheel; that is the job of the centre spigot.

When the torque applied to the join between the flywheel and the crank exceeds
the torque that can be transmitted by friction force alone, the flywheel will try to
move and impacts on the bolts. Repeat the process often enough and with enough
force, and failure will occur either through fatigue cracking or shearing.

So without getting too involved, if you refer to the above diagram you will see an
arrow pointing to Node 1. This is the point of maximum stress due to torsional
vibration, which may under certain circumstances (torsional resonance), be several
times greater than the torque generated by the engine. When the NZ Toyota dealer
team ran AE82GT's with 4AGE engines back in time, there was a known problem
with broken cranks when the engines were run to 8000 rpm. Service life for the
cranks was as little as 2 hours

So yeah... when you start modifying your engine in ways that change the resonant
frequency of the crank/flywheel assembly, and run it at higher than the factory
recommended redline, use some decent flywheel bolts and make sure that they are
installed/lubed/torqued to the bolt manufacturers specifications.

Cheers... jondee86

allencr wrote:Lots'o fretting on all of them, moving around A LOT before failure.

I have read the story behind this one... early model aluminum flywheel on
a high revving modified 4AGE. Appears that due to a chapter of errors, it was
inadvertently installed with ARP bolts but without hardened washers under the
heads; with loctite on the threads but without ARP lube under the bolt heads:
and then tightened to the factory torque rather than the ARP spec.

Came loose during a race, but the car was pulled up when it started vibrating,
and actually made it back to the pits with two bolts still holding the flywheel.

Cheers... jondee86

don't forget.....

you also have the load from the driveline... so additional stresses add to the equation. One reason I went with Blacktop flywheel on my car was it was designed for the rpm levels, and power out put I was anticipating making, and my car was a bit lighter then an AE111

oldeskewltoy wrote:You also have the load from the driveline... so additional stresses
add to the equation.

Correct. However, since the clutch acts as a "fuse" in the driveline, the torque
loading from the driveline is limited to the torque rating of the clutch in normal
usage. By design the flywheel to crank joint would be capable of handling this load
with a decent safety margin to take into account wear and less than perfect
operating conditions.

Shock loads on the driveline such as side stepping the clutch, shift locks etc will
impose momentary peak loads on all of the driveline components. If the flywheel
joint has already lost some of its preload, then this could shear some bolts

Fatigue failures are caused (IIRC) by the repeated application of loads between
the elastic and plastic limits. If the engine happens to hit a destructive resonant
frequency at high rpm, a very high number of load cycles can occur in a very short
time. When the engine is driving the car, I expect that the inertia of the driveline
is added to the inertia of the flywheel. So there will be some change to the total
inertia of the rotating components.

The degree of angular movement induced by crankshaft harmonics is very small,
but the forces involved can be very high. So they can be very destructive** to a tight
fitting joint, but might only rattle the teeth in the gearbox where clearances are
much looser. There are a lot of variables involved, and some people get lucky,
and some people don't !!! Getting the basics right brings the odds in your favour

**EDIT: In some ways comparable to the way in which the key eats into the crank
pulley when the crank pulley bolt has not been torqued correctly and comes loose.
The degree of movement between the two parts is very small to begin, but when
the two bang together repeatedly 5000 or 6000 times a minute, it does not take
long for one part to start destroying the other.

Cheers... jondee86

I skipped through all the technical mumbo jumbo and ill give you an actual scenario that happened to me..

Setup: Turbo GZE pushing 150 lb ft, and 181 rwhp

Bought brand new OEM bolts and installed them {Torqued to Toyota spec) on a billet flywheel. I hated the lightweight flywheel with the turbo setup, so i went with a Silvertop flywheel. Torqued bolts to same spec as before. About 45 minutes into a track day, i downshifted into 2nd gear to break the rear end loose around a turn and it felt like my tansmission exploded from under me, lost all my gears, and limped it off the track. I removed my trans expecting to find a grenaded clutch or a broken input shaft and saw this:



This was taken during extraction, so some bolts had already been taken out.

I will NEVER EVER reuse flywheel bolts in my life again, the flywheel spun on the crank after the bolts sheared, and luckily got seized onto the crankshaft itself. Had to get two prybars to pull the flywheel off. Imagining a 18lb flywheel spinning at 6500 RPM with sharp ass teeth cut on the end is a scary thought, and im lucky it didnt fall off the crank and go through my floorboard.

Dont be cheap, spend the extra 100 bucks and buy some ARP bolts, peace of mind is cheaper than a bent crankshaft and possible engine damage, or hell your legs haha


But also i believe that the flywheel bolts are exactly made like the head bolts are. They are Stretch torque bolts {Technical term: Torque to Yield}, so as soon as you torque them once, they will now be "elastic" and wont be as strong, and wont hold the same amount of torque anymore.

Just came accross this...



Pic of a 4AGE bigport crank that broke at the #5 main bearing. Fcuking
harmonics... ya can't take them anywhere !!! At least the flywheel was
still bolted nice and tight

Cheers... jondee86

ToeKnee805 wrote:Dont be cheap, spend the extra 100 bucks and buy some ARP bolts, peace of mind is cheaper
than a bent crankshaft and possible engine damage, or hell your legs haha

Thanks for the input And you are right... when you are running a modified engine
and beating on it, you need to put some good bolts in there. From a distance it looks
like those were fatigue failures... bolt starts cracking from one side and slowly the
crack works its way about halfway across the bolt making a kind of seashell pattern
in the steel. Then, when there is not much of the bolt still hanging on, you give it a
whack with the clutch and... ping... broken bolts.



I've never seen anything that says the 4AGE bolts are stretch bolts. I know some cars
use them, and they will have a snug down to some torque then 45, 60 or 90 degrees
method of tightening. ARP just say blue loctite on the thread and their special lube
under the head then torque to 58 ft-lbs.

Factory bolts are for stock engines and they work fine. But like you say, once you
start pushing more power, you need to use good hardware and make sure it is torqued
to the bolt makers instructions.

Cheers... jondee86

jondee86 wrote:

Basically, as the flywheel rotates at high speed, it wants to fly apart, but the material
of construction holds it together. The highest stress concentration is near the centre,
and you can see a band of yellow at the edge of the hub. This is the point where the
thickness of the flywheel reduces, so there is less material to resist the stress loading.

This is the point where your flywheel let go, and I'd put money on that flywheel having
seen some seriously high rpm's before it let go. Incidentally, the above pic also shows
why taking material off the outside of a flywheel (done correctly) is not dangerous, but
removing material near the centre is indeed a recipe for disaster.

Cheers... jondee86

I'm guessing this includes installing dowels to the crank & holes in the flywheel? I've read about people doing this before, and it was in one of these failed FW pics.

TODA flywheel 3.7kgs...



Note that while there are plenty of holes around the outside of the flywheel, there
are no holes inside the friction face. I believe that TODA know what they are doing
when the design a flywheel. But I am not so sure about some of the China made
flywheels full of holes inside the friction surface that I see on Ebay.

Dowel pins are for locating, bolts are for generating clamping force. I have never
seen any evidence that fitting dowels will solve a problem caused by lack of sufficient
clamping force. If the bolts are doing their job, the dowel is just along for the ride.
If the flywheel starts to move on the crank, it will shear just the same as the bolts.

Incidentally, when I fitted my flywheel, I used ARP bolts and their recommended
lube and torque instructions. I tightened them in four steps, but the last step was
only around 10ft-lbs and some of the bolts did not even move before the wrench
clicked. I wasn't happy with that, and backed all the bolts out. Then I tightened
them in three steps with the last step around 20ft-lbs. That meant I could get a nice
smooth pull until the wrench clicked. That felt a lot better !!!

Cheers... jondee86

So, I used an Illegal Garage lightweight flywheel/clutch setup when I put my motor together a long time ago. LOTS of track days / Time Attack / autoX, it was basically all I used the car for.

I honestly cannot remember if I just re-used the original flywheel bolts, or if I got new ARP ones, but I've not had any issues. but now I want to yank things apart and look to relieve the paranoia, lol!



actually, looking at the full-size image, here it looks like there's a little "ARP" stamped on some of the 12-point bolts. so I think I went with new ARPs and am no longer paranoid...

Yeah, I know the feeling... should I have loctited those bolts... should I have
used a torque wrench instead of my ratchet Being cautious with matters that
have a direct bearing on your health and safety on the road is not stupid !!!

Rule of thumb... use OEM parts and fasteners on a stock standard car. But if you
are modifying and upgrading parts for more power and performance, use better
fasteners and a torque wrench. It doesn't matter how strong the fastener is if
it is not tightened sufficiently to generate the required clamping force.

OEM fasteners are designed with a decent safety margin built-in, but they are
selected for the engine power and maximum rpm as the car leaves the factory.
When power or rpm are increased from factory, the safety margin is reduced.
Then it is up to the engine builder or modifier to decide at what point the margins
have been reduced to the point where upgraded fasteners are required.

Cheers... jondee86

plus, if that's what you're into, you can get a lot of street cred by saying you have ARP fasteners for everything! lol.