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"lock tight" on flywheel attach fasteners?

"lock tight" on flywheel attach fasteners?

"lock tight" on flywheel attach fasteners?

Greetings Gents, a couple questions if you don't mind:

Auto hobbyist here; the Chevrolet Chevelles enthusiasts are discussing loctite red for flywheel and pressure plate fasteners. Of course there are differing opinions. Last time I installed a Chevy flywheel or clutch was 1969, so I don't even remember if the factory used "lock washers"

Also, some of the folks believe that the torque from the crankshaft is transmitted as a shearing force on the fasteners. I doubted that is the whole story, so I thought I'd consult those who know...

Thanks, Jim  

RE: "lock tight" on flywheel attach fasteners?

it's more typical for torque to be transmitted via friction in the joint than by shear in the fasteners (unless you want it to break).  The fasteners smoosh the two parts together quite hard, and friction does the rest.

RE: "lock tight" on flywheel attach fasteners?

Lockwashers are way too soft for holding flywheels.
So are French locks.

What's universally used instead is a high- strength bolt with radial grooves in the distal face of the head, which is relatively large and relatively thin.  I forget what it's called, but it has a specific name, and is normally self- locking.

I use Loctite(r) on them anyway.


Mike Halloran
Pembroke Pines, FL, USA

RE: "lock tight" on flywheel attach fasteners?

I mostly use Loctite on SBC flywheel bolts. On the ones that still work loose and show fretting on the crank flange to flywheel interface, I use high strength Araldite.

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RE: "lock tight" on flywheel attach fasteners?

When using loctite, does it act a thread lube? And if so do you recalculate the published torque values?  

RE: "lock tight" on flywheel attach fasteners?

I guess I do not follow the norm as y'all seem to do.  I have never used a chem lock on flywheel bolts or pressure plate bolts.  I do, however, use the best hardened dowels and NAS spec bolts tqd to proper specs for each application.  No failures in racing in over 50 years.  It's just the way I was taught to do it and I have never had any good reason to change.

As an aside, I once saw a flywheel/crank failure that DID have a locking agent present on a Ford 1800 BDH that the builder had neglected to install the hardened dowel and only used the six retaining bolts.  All six (OEM Cosworth) sheared off but, the interface showed NO fretting ! I did find that odd as I would have expected to see at least a little evidence of improper tq or stretching.

I cannot say that the reason the bolts sheared was due to the locking compound, that is a bit of a stretch.  I just noted the compound because it was a very difficult to get the little bits out!!!


RE: "lock tight" on flywheel attach fasteners?

Loctite does not seem to make thread friction any different from a light film of oil, which is what's usually recommended by the OEM.

I don't adjust the torque spec for it.


Mike Halloran
Pembroke Pines, FL, USA

RE: "lock tight" on flywheel attach fasteners?

ARP used to (and may still) talk about typical flywheel>crank bolts being subjected to shear.  Even if I wanted to believe that, the fit of the bolt shank to flywheel and the way the threads are in the "shear plane" make it unlikely. Using a tapped hole for any kind of a reference is

Chevy small and big block 1960s and 70s - 6 non fitted bolts.
Dodge 440 - 6 non-fitted bolts
426 Hemi - 8 non-fitted bolts
Duramax cutaway engine in the lobby 2004 - looks like a few dozen non-fitted bolts ( probably just 8)  

RE: "lock tight" on flywheel attach fasteners?

I generally do not use Loctite on anything but fasteners that are troublesome to deal with should they loosen.

Flywheel bolts rank near the top of the list.  (THE top is inner tie rod threads)

RE: "lock tight" on flywheel attach fasteners?

I vote on thread locker blue "Loctite"

prevents loose bolts from vibration.

RE: "lock tight" on flywheel attach fasteners?

I never cease to be amazed at people who refer to Loctite as "red" or "blue".  There must be 1000 grades of Loctite, quite a few of which are the same colors.

RE: "lock tight" on flywheel attach fasteners?

Yes, there are many types of chemical locking compounds.  Here in Socal the most common, at least in Locktite brand, is "Blue" (242), "Red" (271 and 262) and, "green" (290).
Like I said, just the most commonly used in my shop. There are many other types of Locktite brand that are for specific jobs, not all necessary for even racing applications.  However, I'm not at all sure about "1000 grades", though.

I use blue in the two compounds because one is a "wicking" sealer for installed parts.  Red for most general purpose and green for studs or bearing mounts. Lots of blue and red, very seldom use green as it is difficult to remove.


RE: "lock tight" on flywheel attach fasteners?

Herky Jim,

ivymike is correct.  The torsional moment transmitted between the crank end and flywheel joint is designed to be taken out purely through friction created by the preload clamping force of your flywheel bolts.  This is the only practical way to do it.  

Taking the torque through shear in the bolts would require that all of the bolts have an interference fit with their mating holes in both the crank flange and flywheel.  If there is any bolt hole clearance, the shear load will only be reacted at two of the bolts (at least until they yield and the other bolts begin to pick up load).  And more importantly, any sliding between surfaces at the crank/flywheel interface will quickly result in fretting/galling failures.

Loctite can be an effective form of thread locking if used correctly, and would be perfectly acceptable for locking flywheel bolts.  It creates its locking effect by increasing the friction at the thread interfaces.  However, Loctite should only be considered as a secondary thread locking device.  With threaded fasteners loaded in tension, the primary locking function is the thread friction created by the fastener's installed preload.  

A properly designed fastener installation should always ensure that the fastener will not lose its preload, under any conditions such as temperature changes, creep, corrosion or vibration.  If the fastener maintains its preload, it will remain locked.  The Loctite is just an extra layer of safety.


RE: "lock tight" on flywheel attach fasteners?

excuse me!
thread locker blue 242 medium

and yes & it under stood all bolts must be torqued.

RE: "lock tight" on flywheel attach fasteners?

the idea is to stretch the bolt to such an extend that the friction between flywheel and crankshaft is sufficient to withstand the torque generated by the engine. whether the bolts are coated with something does not matter really, it's pure the elastic elongation of the bolds that generate the necessary force whereas the dowels take care of the exact location of the flywheel.

what is important though, is to take new (non-prestretched) bolt whenever the flyhweel has been taken of the crankshaft.

if using already used bolts, they may become loose and that can result in very nasty accidents. i once saw a flywheel that had come loose and penetrated through the bellhousing into the vehicle interior. that is not what you would like to happen if you are the driver!


RE: "lock tight" on flywheel attach fasteners?

Hmmmm . . . .  There was a time, long ago, when we removed and replaced flywheels, many times reinstalling the same one as part of a lower end rebuild, and using the same old bolts over again was SOP.  This was especially true for bucks down (just about all) drag racers back when there was no real money being doled out in prizes and corporate sponsors like today were extremely rare.

It never occurred to us (who knew?) that bolt stretch could be an issue.  We were concerned with the clutch assembly, mainly disc and pressure plate, staying together.  As they say, ignorance is bliss.  Sometimes.  There were times someone payed dearly for pushing the envelope or being cheap.


RE: "lock tight" on flywheel attach fasteners?

"...Diligent use of Loctite thread locking compounds does more to improve vehicle reliability than anything else, at any price..."

Really!? Who'd have thought it...

RE: "lock tight" on flywheel attach fasteners?

By the way, thanks (belatedly) to all who replied to the original post. Jim

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