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Sheared Flywheel Bolts


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...what is wrong with the original design...

A matter of taking available of best-available technology of the day, Roger, not faulting the old. Yes, the original bolts were good. My car's flywheel bolts lasted almost 60k miles (I won't say 52 years 'cause it was off the road for at least thirty of those!). We replaced the originals with ARP just as we did cylinder head studs. The same best o'modern approach led to replacing the old bias-ply tires with radials, the old sealed beam headlamps with H4 halogens, and the old brake linings with non-asbestos modern grades.

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Also hand-held pistol drilling is unlikely to leave a hole perpendicular to to face of the flywheel.

 

This will mean the bolt is being bent when you tighten it and the head may not sit flat.

 

All this will impair tightening and instigate future failure.

 

If you dont have time to get a fluted extractor I'd bash an old screwdriver in and try to unscrew the bolt.

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Busy day in the sheared flywheel bolt saga, despite heavy rain and thunderstorm (working outside - not enough room in garage). Managed to replace crankshaft rear oil seal, repaired damaged bolt hole with thread repair insert, refitted flywheel with new ARP bolts loctited and torqued to 85 foot lbs. see attached photos

 

post-1193-0-72929600-1403996848_thumb.jpg post-1193-0-50569300-1403996896_thumb.jpg

 

New clutch fitted and gearbox replaced, Just got to refit propshaft and refill gearbox with oil and road test, might even make tomorrow's Thames Valley Group lunchtime meet! Could not have achieved all this without the help of my good TR owning friend Peter and my friend and neighbour Lewis, Thanks Peter and Lewis I am very grateful

 

I have noted the comments re thread repair, before making my decision I consulted several engineer friends both automotive and aviation, their opinion is that correctly made insert thread repairs are stronger than the original. I also Googled the subject and their opinion is widely confirmed. I used a Recoil thread repair kit see link to inf. In order to accurately drill out the old thread I used the hole in the flywheel itself as a guide.

 

http://www.afsrecoil.net/uploads/pdfs/Recoil%206pp%20Bro-ENG-WEB.pdf

 

Many thanks for the replies

 

Bill

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Hi Don,

thank you for the your answer on the 'why use modern stuff' comment. Most acceptable. But your using radial tyres - OMG :o

 

 

Hi Bill,

well done in battling against time to get it sorted. I too have been thinking about the thread insert problem/answer. In my opinion I believe you will be fine.

 

The nature of the insert is to make the thread/hole system stronger - the insert is stronger material and is then acting at a larger radius.

The tightening process makes the insert grip more and more as the torque is applied.

 

The bolts are acting in shear so there are no forces pulling on the bolt/insert.

 

I bet your ears will be aching after CLM having listened all the way there and back.

 

Roger

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/anorak mode on

 

"The nature of the insert is to make the thread/hole system stronger - the insert is stronger material and is then acting at a larger radius."
"The bolts are acting in shear so there are no forces pulling on the bolt/insert."

True under static conditions Roger but not once the engine is running. Its amazing how flexible any seemingly solid structure becomes as frequency increases - above a hundred Hertz or so everything turns to jelly to some degree. When the engine is revving there will be vibration frequencies up into the hundreds of KiloHertz range and at some of those the flywheel will be bending like a Pringles crisp (known as a resonant mode). That exerts all sorts of forces on fixings that are not there under static conditions. Don't worry- the displacements are in the micron range so you would not even see them, however the forces developed are real.
By using a thread insert there are now two interfaces (bolt-to-insert and insert-to-crank) where before there was only one. That makes the bolt fixing less 'stiff' and will lower the resonant frequencies which could give rise to cyclic fretting if the joint is subjected to vibration at that frequency.
Don't get me wrong- I'm not saying the fix will fail, it will probably be fine for the life of the engine - but things are not as simple as they seem on the surface and its not just static tensile stress which needs to be considered.

 

/anorak mode off

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I'm all for Kevlar socks.

 

Mick Richards

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... using radial tyres - OMG :o...

 

Yes, alas, I'm selling out to the modern world with radials on my car. Authenticity has its limits, eh?

 

I had the chance a few months ago to drive TS2797LO, an unrestored long-door TR2 with the tires originally put on it circa 1955. They still hold air, and give an "interesting" ride at any speed. My buddy who owns it drives it far too often in my mind, as in this shot.

i-Xfk5LCw-M.jpg

 

 

Tom has these in his barn, too, just waiting for the right project. Lemme know if you're interested. They look like NOS...

i-bCcNHc3-M.jpg

 

i-dCbjdKd-M.jpg

 

i-5Sw6Lrd-M.jpg

 

(hijack out)

Edited by Don H.
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Hi

The best way to get broken bolts out of is to weld large nut onto the broken bolt my ways of a mig welder

Just hold the nut with a pair of grip over the broken bolt and weld though the center

Welds the nut to the broken bolt and the heat will losen the bolt then unscrew

You might have to do this a couple of time

Works for me al the Time. Good luck

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Hi

The best way to get broken bolts out of is to weld large nut onto the broken bolt my ways of a mig welder

Just hold the nut with a pair of grip over the broken bolt and weld though the center

Welds the nut to the broken bolt and the heat will losen the bolt then unscrew

You might have to do this a couple of time

Works for me al the Time. Good luck

Kev,you have a PM from Me,Top Right of Screen Click on Red Box.

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Gents,

 

Kindly indulge my engineer's perspective on " bolts acting in shear ":

 

This in fact dooms bolts unless they are " body bound " i.e. interference fit bolts designed for shear duty. Flywheel / crankshaft connections are generally effected by the frictional interface between the clamped surfaces, hence the bolts are clamps only and do not experience significant fluctuations in load ( which would only accrue from separation forces, not torque ). If the frictional connection is overcome the bolts are doomed to shear in short order. TRIUMPH's design appears to conform to this convention, and the opinions above about the soundness of thread inserts in this application are consistent as well. All bets are of course off when greatly aggravating the connection by souping without upgrading same.

 

( Why would I know anything about this topic? See my website www.torque-inc.com ).

 

Cheers,

Tom

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Tom,

 

So more torque needs more friction area between flywheel and crank?

The blower will give maybe 40% more torque at same crank rpm.

Is is likely the flywheel crank joint will be suspect?

Or is the Triumph over-designed in that respect.

 

What's the best method to prepare the mating surfaces?

Would adding more dowel pins be better?

 

Extra bolts might be a move in the wrong direction?

 

Peter

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Sliding a bit sideways on the topic but Tom's exposition is the reason why I don't put any lube on the mating tapers of my knock on wheels.

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Tom,

 

So more torque needs more friction area between flywheel and crank?

The blower will give maybe 40% more torque at same crank rpm.

Is is likely the flywheel crank joint will be suspect?

Or is the Triumph over-designed in that respect.

 

What's the best method to prepare the mating surfaces?

Would adding more dowel pins be better?

 

Extra bolts might be a move in the wrong direction?

 

Peter

 

More area isn't readily available, so more clamp force is the next recourse. More bolts may not be accommodated amidst the existing holes, but a racing crank could maybe do with 1 or 2 more with different spacing. They are already at the maximum diameter, so no room for improvement there either.

 

Engine torque is not the highest load imposed on the connection; rather it's the torsional spikes due to resonant frequencies which put it to the test. There must be a correlation but I don't know if it's proportional. Assuming TRIUMPH factored in the variances / range of resulting values of assembly ( dry vs. oiled interface, dry vs. lubricated bolts, min vs. max tightening torque ) I'd say the design should accommodate the 40% more you reference if done optimally.

 

The steps I mentioned before will make the most of the connection. Higher tensile strength bolts with higher tightening torques will give more clamp yet. I don't put much faith in the dowel, nor in adding another.

 

To evaluate the capacity of the connection via friction we take 10,000 lbs/ bolt x 4 bolts x friction coefficient of 0.15 ( dry ) x the bolt circle diameter / 24 = lb-ft. If the bolt circle diameter is 2.5" this yields 625 lb-ft. Using the minimum tightening value rather than the maximum drops it to ~ 425 lb-ft. which illustrates the point above.

 

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Tom,

Thank you.

It looks like problem of tired torsional vibration dampers could appear at the opposite end of the crank with flywheels working loose.!

 

I've long ignored oil leaks from the rear crank oil seal, but that crank-flywheel joint wont like oil-wetting - right ?

 

As usual I fall back on the fact that I drive at high torque for only a small fraction of the time, the primary torsional vibration

is at rpm I rarely reach. And the oil leak has yet to loosen the flywheel. But I shall keep a watching brief, hoping that there are warning signs.

 

Thank you for the insights.

Peter

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Back to the helicoils.

 

Long ago, on two occasions we had to utilise helicoils as a last resort when a flywheel threw wobblies during practice on a race engine - Ford engines, not Triumph. Working life of no more than 50 racing miles, then scrap crank and flywheel. Worth it at the time, better than a non-start and they lasted the race, but a costly solution. And high risk.

 

I wouldn't even think about using a helicoil to hold the flywheel on a road car, not worth the risk of it trashing the whole damn engine when it cuts loose - which it inevitably will, and probably sooner rather than later. Not to mention the risk of a disintegrated flywheel - you won't need to worry about the future lack of feet, you'll have bled to death before that becomes an issue.

 

Cheers

 

Alec

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