Series Land Rovers
By Steve Reddock and Alan Richer
Double (De)Clutching - sounds mysterious, doesn't it?
Unfortunately, with a Land-Rover before 1970 or so it's also a necessity. Before the series of updates that produced the Series III the transmissions in Land-Rovers were not fully synchromesh - they were only so equipped on the higher gears (3/4). Because of this, the transmission isn't "smart" enough to synchronize the gears when upshifting (and more importantly) downshifting.
Fear not, however. The process of synchronizing these gears so that you can smoothly upshift and downshift is known as double clutching (or double declutching on the UK side of the pond). It's not hared to learn, and once you have this skill down you can stop making horrid grinding noises when driving - and your transmission will thank you in the long run.
The actual process is simple. When shifting either up or down into a non-synchromesh gear you need to make all of the internal gearing spin at the same speed to get he gears to mesh quietly and cleanly. This involves:
Shift from gear into neutral
Clutch in and change engine speed to match ground speed in gear
Shift to gear
Not all that complex, but it does take a while for it to become second nature.
Steve Reddock in the UK recently typed this explanation for what you're doing and why. He explains the reason for doing this complex dance in a concise and quite readable manner - and far better than I ever could.
All the answers so far have explained what to do, not what you are trying to do.
The numbers here are all made up, but the idea is right.
Imagine you are driving along in 3rd gear at 25MPH and this had the engine at 2000 rpm.
Now try 2nd gear and 25MPH. You will find the engine doing a lot more - say 3500 rpm.
When you change from 3rd to 2nd you need to make the parts all change to 3500 RPM so that the gears and engage smoothly. This is what a syncro would normally do for you. As you don't have one you need to do the work yourself.
In 3rd gear push your foot on the clutch and pull the lever back to neutral. Let the clutch up (as quickly as you like as you are in neutral). Now you need to rev the engine to slightly above where it would be in 2nd - say 3750 in this case. Push the clutch down (again, fast is OK). Keep the engine revs about where they are (3500). The gears are now slowing due to friction and you need to move the gear lever back into 2nd before the speed drops too far. But you need to move the lever carefully to feel for resistance. Once in gear cleanly lift the clutch up as usual.
When you change up at the same speed the parts need to change from 3500 to 2000 rpm, friction will make them do this without needing to raise the clutch in most cases. So a gently change will normally work.
Practice is the answer - I made awful noises the first few miles in a II box.
If you have trouble you may find it's worth practicing going from 4th to 3rd by double declutching. Or even 3rd to 3rd. There are syncros on these gears so it will cover up your mistakes and avoid most of the grinding noises. When it feels smooth in these gears try the non-syncro ones. Also practice double declutching in a full syncro car. You will not hurt it and it will be a lot easier. When you get this right you will be able to make lightning fast changes in the syncro equipped box without the lever sticking. Sticking in a syncro box is when the syncros are increasing the speed of some parts.
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As Steve states above, practice will make this second nature after a very short time, and your poor Series vehicle will thank you for it.