Crash first, synchro box, double de-clutch, pudding spoons...


When you first enter the world of historic vehicles there's a whole load of new jargon to absorb. It doesn't help, of course, that everyone else understands it (or claims to!).

You don't want to appear dim or uninformed so you nod and make vague approving noises at appropriate intervals and leave, still baffled. Recognise the symptoms? Honestly, unless we were amongst those privileged few who took their first baby steps in a workshop, dropping their rattles amongst the spanners, the chances are that we've all been there.

I work on the principle that there are no daft questions, only people who are daft enough not to ask when they want to know more.

At It's My Classic, we positively encourage questions about historic vehicles. So, want to get down and dirty and learn to talk technical? Let's get at it...

Although our title here is crash first, let's actually start with a synchro box - a sychromesh gearbox.


According to a dictionary, it's "a system of gear changing, especially in motor vehicles, in which the driving and driven gearwheels are made to revolve at the same speed during engagement by means of a set of friction clutches, thereby easing the change".

The synchro is essentially a cone-shaped collection of toothed discs with a friction surface. Connected to the bottom of your gearstick is a selector fork. When you change gear, the selector fork moves a part of the drive gear called the pinion (the input shaft from the engine). The pinion moves up or down the synchro cone, changing the pinion's speed to match the speed of the gear that's been selected. In other words, it synchronises the output speed of the engine with the speed of the wheels.

To any driver accustomed solely to modern synchro gearboxes, the older system of gear changing (using what is often referred to as a crash box) is likely to seem far more involuted and time-consuming. As with learning any new skill, that's probably only true until you get used to it.


Synchro may predominate these days, but it's not all sunshine and roses. Repeated fast gear changes will eventually burn out a synchro cone. What might be surprising to many is that a synchro box can actually be slower than a well-driven crash box, which is why crash boxes are often used in racing - especially if the driver is an accomplished heel-and-toe practitioner (we'll talk about that technique on another occasion).

Sliding mesh gearboxes are also a possibility in historic vehicles, but we'll concentrate instead on the more talked-about and oft-maligned crash box.

So, by now you'll have gathered that a crash box is simply one without synchro. They're also sometimes called dog boxes. Not just used in cars and commercial vehicles, these are also very common in racing motorcycles.


Some classic gearboxes (such the A Series MOWOG on the Austin A30) are a combination, having synchromesh on second, third and fourth gear, but not on first. These are often referred to by classic petrolheads as 'crash first'.


Next up: double de-clutching (DDC). This is best described a little bit of a black art, ideally mixed with some knowledge and careful practice. Some claim you need to DDC whether changing up or down the gears, most say only when changing down.


The change down works like this:

 1. Hand/foot off the throttle

 2. Engage the clutch

 3. Change out of gear

 4. Release the clutch

 5. Feather (lightly touch) the brake, letting the revs drop to the appropriate speed for the next gear (this is where a rev counter and/or a working understanding of the speed-gear ranges is useful)

 6. Engage the clutch again. Change into gear

 7. Heave a discreet sigh of relief if there were no crunching, grinding or other painfully expensive metallic sounds from the gearbox

Step seven is probably where the term 'crash' box came from, although the cognoscenti still argue about the etymology.

With crash-first gearboxes, always, always, always bring the vehicle to a halt before changing down into first gear.

Oh, and a pudding-spoon gearbox refers to a worn box where it seems like the driver has to stir it vigorously with the gearstick to find an appropriate gear!

If you have a general technical area to do with classic cars or classic motorcycles that you think classic newcomers might find useful to know more about, do get in touch. We'll look into compiling an article for future inclusion.

It's My Classic - the free online classic car and motorcycle magazine for people with a passion for heritage vehicles.