With all the technical wizardry involved in servicing cars these days it might seem odd that a little block of wood should be such a useful tool – but it's true.
The factory jacking points on the BMW E46 are these rectangular boxes (right) fitted at the front and rear of the sills. The head of the jack supplied with the car fits neatly inside the box, locating the jack securely while the car is lifted.
That's if a jack is supplied. On the E46 M3 like mine no wheel-changing kit is supplied, which is fair enough because there's no spare wheel either.
When it comes to DIY maintenance like swapping between winter and summer tyres, you can of course use a trolley jack. But the saddle on the trolley jack doesn't mate well with the jacking points on the car, and after a while they get chewed up.
What the 56 x 29 x 12mm wooden block does is act as a buffer between the trolley jack and the jacking point, spreading the load into the jacking point while providing just enough compliance to ensure the jack doesn't slip.
Ideally the block is made from a dense wood, and cut with the grain vertically. If the grain is horizontal there's a chance the block can split if any side load is applied. The corners and edges need to be sanded down because it's difficult to get every last bit of dirt out of the corners of the jacking point, and if the wooden block had sharp edges it wouldn't fit properly.
It's such an easy and cheap tool to make, every E46 owner should have one.