Gearbox crunches into 4th and 5th

[1_eyed_jim]

Hello the vans gearbox has been crunching into 4th and 5th for sometime now and was wondering what would cause this? Is it lack of oil or some serviceable part? It doesn't crunch in any other gear just 4th and 5th?? Can sometimes get it not to crunch if the revs are really low.

Ta

[California Dreamin]

In a constant mesh gearbox (as fitted to the T25) the pairs of gears representing each gear ratio are in 'constant mesh'.
One of each pair is 'free wheeling' (not attached to the shaft it sits on)
In order to get the power to follow a certain 'gear path' the free wheeling gear has to be locked to the shaft it sits on, this is where the 'syncro hub' comes into action.
The syncro hubs (usually each one operates two gears) have the basic function of syncronising the gear to the speed of the shaft for smooth and quiet engagement of the gear. The main component responsible for this action is the syncro ring (a tapered cone) slows and syncronises the 'engaging' parts in a 'wedging action' before the baulk ring locks everything together.

The reason for your particular fault is a syncroniser ring has worn over many thousands of engagements or it may have cracked which means the action of the components slowing down and syncronising 'before' engagement has failed, typically producing that 'crunching' or 'baulking' action.
One or more of your syncro hubs needs attension.
Aidan on this forum is the person you need to speak to.

Martin

[Ian Hulley]

Like Martin say's Aidan is the man

[kevtherev]

Nice description Martin

[1_eyed_jim]

In a constant mesh gearbox (as fitted to the T25) the pairs of gears representing each gear ratio are in 'constant mesh'.
One of each pair is 'free wheeling' (not attached to the shaft it sits on)
In order to get the power to follow a certain 'gear path' the free wheeling gear has to be locked to the shaft it sits on, this is where the 'syncro hub' comes into action.
The syncro hubs (usually each one operates two gears) have the basic function of syncronising the gear to the speed of the shaft for smooth and quiet engagement of the gear. The main component responsible for this action is the syncro ring (a tapered cone) slows and syncronises the 'engaging' parts in a 'wedging action' before the baulk ring locks everything together.

The reason for your particular fault is a syncroniser ring has worn over many thousands of engagements or it may have cracked which means the action of the components slowing down and syncronising 'before' engagement has failed, typically producing that 'crunching' or 'baulking' action.
One or more of your syncro hubs needs attension.
Aidan on this forum is the person you need to speak to.

Martin

Thanks for the fantastic description. It's things like this that make this hobby worthwhile and satisfying.

[Plasticman]

Martin[/quote]

Thanks for the fantastic description. It's things like this that make this hobby worthwhile and satisfying.[/quote]
Nd potentialy expensive
m

[California Dreamin]

He took the news well.....

Options are: a known, good second hand box or Aidan sprinkling fairy dust over your existing unit (that fairy dust sure is expensive mind) parts costly, repair time substantial and profit low, his expertise unquestionable.

I'll have me tenner next time I see ye Aidan....

Martin

[Niander]

for sometime now and was wondering what would cause this? Is it lack of oil?

I simply cannot believe you haven't checked !