Reverse gear on 4.2 'box

[Nick]

Reverse gear on my standard 4.2 gearbox engages OK when the box is cold, but after a run it does not seem to engage cleanly without a grating of gears. I'm assuming the difference is caused by the reduced viscosity of the hot oil, but I'm puzzled why there should be any gears rotating when the clutch is depressed and the car is stationary. The clutch disengages OK in the forward (synchro) gears. The car has had a new clutch and slave cyl. fitted.

Nick L.

[Larry Wade]

Hi Nick,

I believe there is some drag so the transmission might be rotating if it is in neutral. Also you might double check that you've got the correct transmission oil. I don't think there ought to be a difference in behavior between warm and cold conditions.

Larry

[Nick]

Thanks Larry,

If I take your meaning correctly, you are saying there is usually some drag through the clutch which would rotate the layshaft, but the oil drag slows or stops it. Hence my thought that hot oil in the box won't have as much drag.

Is there a usual culprit for the source of the drag in the clutch ? In the driven plate I guess.

Nick L.

[1954Etype]

Thanks Larry,

If I take your meaning correctly, you are saying there is usually some drag through the clutch which would rotate the layshaft, but the oil drag slows or stops it. Hence my thought that hot oil in the box won't have as much drag.

Is there a usual culprit for the source of the drag in the clutch ? In the driven plate I guess.

Nick L.

Nick, is the clutch adjusted properly at the slave cylinder end?

[christopher storey]

I think it needs to be remembered that all rotating masses have inertia. When the box is cold, the added viscosity of a linear viscosity oil stops the rotating mass more quickly. When it is hot, the components take longer to stop . Reverse gear is unsychronised on all the Jaguar boxes of this era, and so there are no rings/cones to stop the rotation on reverse . Thus, a wait of perhaps 2/3 of a second before attempting to engage reverse pays dividends

[Nick]

I have just had the adjustment checked on the slave travel and it was no more than 1 mm out from the 19mm setting.

I guess 1 mm in travel might make a difference with a new clutch. I'll see what it's like on the next long drive. I was just intrigued by the effect of temperature.

Thanks for the comments.

Nick L.