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MG TD TF 1500 - Flywheel dowels

Hi,
can anybody tell me the point of the flywheel/crank dowel pins. The fly wheel is centred by the machined recess and can only fit one way because of the asymmetric bolt holes.
I ask because I have never seen a flywheel swapped to another crank where the dowel holes matched up exactly.
Ray TF 2884
Ray Lee

The dowels are there to take the rotational load - the bolts are not a precise fit in the holes and without the dowels the flywheel could fret on the bolts.
Chris at Octarine Services

Hi Chris,
As an engineer I know that those pins would not take the drive line shunt. Dowel pins of that size are usually only used for location.
Plus as I said I have never seen unmatched Flywheel/cranks line up.
Ray TF 2884
Ray Lee


As discussed, dowel pins are strictly intended to accurately locate the flywheel. The clamping force of the threaded bolts carries the load.

Correctly designed keyways and wire wheel splines serve a similar function in that they are also only designed to be locating devices and are NOT intended to carry loads (which are carried by tapered fits or tapered wheel drive cones).
WHT

WHT,
I understand that clamping forces take the load.
My question was why are they needed as the flywheel is located by a spigot and has asymmetric bolt holes. It can only go on in the correct orientation.
I have seen cranks/flywheels where the dowel holes have been drilled and tapped for two more bolts.
This only came up because of a lip seal conversion and the dowel holes are distorted and may have to be drilled and bigger pins fitted. This is needed to stop oil leaking through and on to the clutch.
Further to this a dummy seal was made of steel and used to allow us blue in and remove metal from the sump/timing case channel to set the new front seal. This engine has been line bored and these were no longer central to the crank.
Ray
Ray Lee

Fine - leave the dowels out and see how long it takes for the flywheel to work loose - 4 cylinder engines are inherently unbalanced, it isn't just the shock forces from the drivetrain but the constant firing strokes.

I very much doubt that the factory would have installed dowels unless they were needed and as you pointed out the location of the flywheel is fixed by the mating surface and the bolts.
Chris at Octarine Services

Chris,
as I said, the dowel pins will be replaced with larger diameter ones because of distortion.
Just to muddy the water my 1.8 MX5 does not have pins,only bolts.
Ray.
Ray Lee

There are two dowels 6.8 mm in diameter.

Thus the cross section area for both dowels = 72.7 sq mm.

Converting to imperial = 0.112685 sq ins.

The shear strength of mild steel = 50000 lb/sq in.

Thus the force needed to shear both dowels = 0.112685 x 50000 = 5634 lbs.

The maximum torque of the XPEG engine = 78 lbs.ft or 936 lbs.in .

The dowels are at a radius of 1.375 ins.

Thus the force applied to the dowels at maximum torque = 936/1.375 = 681 lbs.

But 5634 lbs are needed to shear the dowels, which is 5634/681 = 8.27 times as much as is being applied at maximum torque.

So the dowels are much more than adequate, with a safety factor of over 8.

Thus the dowels take all the torque load of the engine, and no other forces are involved (obviously the four bolts securing the flywheel must be tightened to the correct torque to prevent the flywheel moving away from the crankshaft flange). If the dowels are stronger than mild steel, then the safety factor is higher.
R WILSON

getting a bit technical now, but inertia loading will far exceed the puny torque of a XPAG.
As I said modern engines do not seem to require dowels
Ray
Ray Lee

Also note that the dowel pins are tapered. So a tapered reamer is needed to install correctly.

Butch
R Taras

Probably wrong , but if the dowels are out , isn't the procedure for refitting --
bolt the flywheel to the crank hand tight then drive the dowels through the f/wheel into the crank, then tension up the studs----------??
and Ray,
The MX5 bolts have machined shanks that are a neat fit in the flywheel and act as a set of dowels as well as being bolts, and ,I have had them sheer off as well, so suggest a set of ARPs if you're poking out some decent power
willy

It always amazes me that flywheel bolts can break, yet the tiny tailshaft bolts stand up to it
William Revit

This thread was discussed between 07/12/2019 and 08/12/2019

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