21 replies to this topic

[The Matt]

Oh, and why are we seeing so many loose flywheels nowadays?

They are often lapped nowadays, and I think the reason is because they have mostly been off a few times at least now.

I'm no expert, but I wonder if you've answered your own question there. I've never lapped one personally, but then I've never had one come loose either. Reading through though, I can't see how lapping on a taper can help something grip to it, it seems counter-intuitive to me. One would think that by lapping the taper to be smooth by radially lapping it, you would promote the chance of it losing grip, not reduce the likelihood of it.

Again, I'm no expert, just someone that's interested in seeing the outcome of the debate.

[tiger99]

The taper is supposed to be smooth, in fact it needs to be so it can slip a bit as you tighten the bolt. A small angle taper is effectively self-locking (think how hard it is to remove ball pins or track rod ends sometimes) except under extreme stress and vibration.

However in the case of the flywheel, the dynamic stresses may get close to the frictional force caused by the elastic deformation of the flywheel bore, so it would come adrift if not retained. Temperature cycling due to clutch heat will tend to make the flywheel creep along the shaft in the direction which makes the joint tighter, which is good, provided that tension remains in the bolt to keep it there.

I don't like the thought of heating it prior to assembly, because as I said, you may be creating a larger problem in the future. But in this particularly difficult case it may be the best thing to do.

But how about simply loosening and retorquing the bolt every few hundred miles for a while, so that the bolt is kept under tension reliably? Say 3 or 4 times, in much the same way as you might retorque a cylinder head. (Yes I know, the flywheel bolt is not as easy, but with practice those clutch cover bolts will be coming out blindfolded.) Or, at least torque it up and don't reassemble the cover, leave for at least a few hours, loosen the bolt, retorque again, and repeat as often as you want. The other alternative, which might achieve the same result, is to use a more elastic bolt, but it would have to go deeper into the crankshaft, with a plain shank, or counterbored crankshaft, to give a decent stretchable length between head and the engaged part of the thread.

There is almost certainly enough metal to be able to deepen the crankshaft bore safely, as far as the back of the primary gear. There already is a small bore hole, and a brass plug, which will be drilled out and a new plug fitted to seal the oilway. I am wondering if it might be feasible to go for a higher grade bolt such as 12.9, and a higher torque. (Does anyone know the grade of the existing bolt?) The existing thread would be bored out, and a new, deeper thread with about twice the engagement length tapped in the deeper bore. We don't want to re-ignite the debate about crankshaft thread size again, so I would suggest that a metric thread be used, to be able to get a 12.9 bolt more easily, and the bolt itself could be a very slightly larger diameter, so you would aim for a torque which would give 50 to 60% more tension than the original.

I have been doing some research, and it seems that maximum tension will be achieved by oiling the bolt thread before torquing, which is possibly one reason why these things are failing, if the threads were too dry when assembled.

[foggy]

This is very interesting for me.as the exact same thing happened to my 1275gt i bought off the auction site. although unfortunatley completely destroyed the key way and the taper so i am due to start stripping the engine to replace the crank
however when it comes to re assembly i willmuch apreciate the guidance above...keep it coming!!

[minidaves]

i recon its reving the nuts of something that aint balanced that couled be the cause of some problems.

[tiger99]

If it is well off balance it could indeed make the problem worse. Doesn't do any other parts much good either.

[tjllanrug]

will reeving my engine, and timming abit out cause this to happen?

[tiger99]

No, it should not loosen the flywheel, but it is best to avoid needless revving of engines, and have the timing and carburettor in correct adjustment, as it saves expensive fuel and reduces environmental damage.

It is best to avoid revving engines when not in gear, as they can in many cases reach a speed which will do serious damage. It is ok to go flat out on the road, in top gear, when the speed is limited by aerodynamics, and you will not be able to over-rev the engine.

If you have a tacho, red lined at the appropriate speed for your engine, respect what it is telling you.

I have seen garage mechanics revving engines furiously. They should know better, and nowadays most do. Also, most modern engines have management comouters which will enforce rev limits and sometimes even torque limits to prevent damage.