I fitted a 5KG flywheel to my Astra. At the same time I fitted a new clutch, clutch release bearing and clutch cable.

This is a reasonably involving job. I started at about 8pm and had it all back together by 2am. No major disasters occurred on the way.

In order to fit a lightened flywheel you need to remove the gearbox. Since the gearbox supports one end of the engine then you need to support the engine when you remove the gearbox. This isn't difficult - an axle stand or jack underneath the sump and a piece of wood and some rope above the engine will work.

First I jacked the car up quite high and supported it on axle stands. Then I took the end of the gearbox so I could get to work on the infamous input shaft. This shaft is held in place by some tricky circlips - need decent internal circlip pliers for this job. It has a threaded hole in its end which you screw into so that the shaft can be pulled out. It is quite common for the shaft to be stuck which means that a tool of some sort will need fabricating. I've been lucky twice now though, the shaft has always pulled out easily.

Next up I removed the drive shafts. This involved undoing the lower ball joints on the suspension struts so that I could remove the drive shafts from the wheel hubs. A top tip for removing the shafts from the gearbox is to just pull them. At first nothing will happen, but struggle away and they will pull out.

The speedo connection and the gear selector have to be disconnected.

The engine was now firmly supported with an axle stand beneath and rope above. Next the gearbox was removed.  It was supported with a jack underneath. Numerous bolts hold the box onto the engine but nothing startling there. The gearbox was then lowered down to ground level.

With the gearbox gone the clutch and flywheel are left on the end of the engine. It is possible to change the clutch without removing the gearbox. Notice that I removed some of the water pipes to make access better.

The six bolts holding the clutch to the flywheel were undone. Finally the six bolts holding the flywheel to the engine were undone. A top tip here is to put one of the clutch bolts back in backwards so that it catches against the engine block and gives you something to turn against. That's necessary because the flywheel bolts are threadlocked in.

Original and lightened flywheels side by side. You can see where material has been removed from the flywheel - the centre has been dished out and it has been skimmed down thinner on the outside. Since inertia is much more sensitive to the material furthest away from the centre (goes with r^4) and most of the material has been removed from the middle radius area then it would be safe to say that the weight loss would be greater than the inertia loss.

A manky picture of the gearbox. You can see how the clutch cable works here. It rotates the shaft on the right of the picture. This pushes on the release bearing in the centre - pushing it towards the camera. This bearing pushes on the clutch plate when the clutch pedal is pressed. As it wears it becomes more and more noisy - it used to make a range of noises when I was sat at traffic lights waiting to move off.

Putting everything back together was the reverse. Getting the gearbox back into position on my own was, er, entertaining. I used a jack to push it up. Once it was close then it was easy because the box was caged in, but when it was lower it had the chance to fall off the jack. Which it did a few times.

When refitting the clutch I had to hacksaw the clutch bolts a bit shorter because the thinner flywheel meant that they stuck through further and the ends were perilously close to the engine block.

The other entertainment came with reconnecting the gear selector mechanism. I got it right by trial and error. First go gave me 3rd and 4rd where 1st and 2nd should be. There is a better way of doing this. See info here.