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+--- Thread: Crunching between third and top (/showthread.php?tid=9648)
Crunching between third and top - Ivor Hawkins - 20-10-2024
Evening fellas. I’ve been driving all sorts of Sevens for many, many years but the (wrong) four speed crash box in the RP always crunches between third and top unless I double de clutch.
The change between second and third is lovely and smooth and going down the box is no problem either.
Should I really have to double de clutch up to top or while I have the box out of the car should I be looking for anything obvious.
The box is nice and quiet and the internal condition appears to be excellent, so I don’t really want to change the gearbox if I don’t have to.
Does anyone with a penchant for gearboxes have any sage advice?
RE: Crunching between third and top - David Stepney - 20-10-2024
The reason you have to double declutch on the crash 4 speed box is because 3rd and 4th are actually engaged via dog clutches, rather than by sliding pinions (as is 1st and 2nd) This was to give a pair of constant mesh gears that slide together on both the mainshaft and the layshaft, giving what is euphemistically called a 'silent third'. In my experience, dog clutches tend to be less forgiving when changing gear than sliding pinion boxes. My Landrover has dog clutch engagement of 1st and 2nd and that can be a pig when changing down into second.
My own car has a crash 4 speed and is as robust as hell. My only criticism is that changes from 3rd to top and down again can be a bit heavy as one is sliding two gears along two shafts. Otherwise is is a super box to use.
Sage advice: Stick with it.
RE: Crunching between third and top - Howard Wright - 20-10-2024
Hi Both
Get Andy Bird to ‘ramp the dogs’. Cutting the dog with a slope on the back face allows the dog to engage much easier and allows easier changes. I really notice the difference between the Special and the Van. The former has ramped dogs the latter conventional dogs.
Cheers
Howard
Just to show the difference.
On the left standard on the right ramped dogs. These also differ in that the left set is standard the right has Andes ratios. It means less of a gap between third and fourth for three speed users.
Cheers
Howard
RE: Crunching between third and top - Ivor Hawkins - 21-10-2024
Ah, now I know the reason why, thanks David!
I will take you sage advice and stick with it, though Howard’s recommendation makes perfect sense, the illustration clearly showing how ramping the dogs works.
Thanks chaps, I’ll complete the few minor repair I have to do and start nailing it back together!
RE: Crunching between third and top - David Stepney - 21-10-2024
Ivor, if you look up a recent thread, “Clutch Release Bearing Knackered”, you will find that Reckless Rat has posted a super cutaway view of the 4speed crash box, showing exactly how it works.