I just changed the gear train on the SC4.
As well as the ubiquitous 5mm allen key, this also requires a 2.5mm one. Annoyingly it wasn't in the box of regularly used spanners and allen keys I keep near the lathe and mill. There was an imperial one that was just too big.
I didn't want to take one out of my proper sets so hunted around for one that fitted in my 'odds and sods' allen key box.
This was moderately annoying so I had the brilliant idea of leaving the allen key in the bottom of the gearbox cover 
Fitting the middle cluster, the slotted washer dropped off and rolled along drip tray and ended up under the gear cover. I swang the gear cover wide open and what did I see tucked up at the far end… a little 2.5mm allen key, a perfect match for the one at the other end.
Someone must have been planning ahead!
I've decided to leave both a 5mm and 2.5mm allen key there now, in the hope that I can't miss the bigger one!
Neil