Crunching Into 4th

[hossboss]

Hey guys.

I've got a 1990 GT-R and have a problem. Unless I change at lowish revs (around 3000rpm or lower), or really baby the gear change, I get grinding going into 4th gear. Every other gear seems ok, although I haven't driven any other GT-R's so can't compare the feeling. Seems odd to have a prob going into 4th, since it's probably not an overly tortured selection - 1-2 for example would be more stressed I would think.

Maybe it needs a bleed, or some adjusting? Is this overly difficult? I like to have a go at most things if possible to save on the wallet...

If anyone can shed some light I would be most appreciative

[JETGTR]

Seems to a bit of a problem in a few GTR's mate R32 and R33 models. Alot of people have had success gearbox oil with something heavier. There's also an additive people have had success with. And after a few klm 50-200 allowing oil to work its way through the box, the problem seems to go away.

If you try a search you will find alot more information about the problem.

cheers Paul.

[Birds]

If it is fine and not making any sounds once it is in 4th, then it is most likely a worn 4th gear synchro. 4th gear itself rarely has issues with it because of the direct drive ratio.

[hossboss]

It seemed to really appear AFTER I did a gearbox oil change, although I used to have to change in a particular way so that the gear-change was nice and smooth before the oil swap. Any idea why 4th synchro would be worn in-particular?

[bozodos]

yeah mine does it too, and occasionally third now isn't it something to do with 1st - 3rd having twin synchros and being less susceptible or something? I know that RB20 boxes always seem to start grinding in second...

[JETGTR]

It seemed to really appear AFTER I did a gearbox oil change, although I used to have to change in a particular way so that the gear-change was nice and smooth before the oil swap. Any idea why 4th synchro would be worn in-particular?

If it started after you changed box oil... then mayb it was the new oil you put into it ??

[Birds]

It seemed to really appear AFTER I did a gearbox oil change, although I used to have to change in a particular way so that the gear-change was nice and smooth before the oil swap. Any idea why 4th synchro would be worn in-particular?

Unless you altered the viscosity of the oil, this can also happen after a long service interval. What happens, is the old oil builds up a protective sludge (so to speak) around worn or faulty parts. When the new oil is put in, it cleans the parts free of this sludge...revealing component wear through sounds that are no longer masked. This also happens in old engines when people run motor flushes or change the oil after a long long time.

There's a variety of reasons for 4th gear synchro...given it's a straight down selection from 3rd it's common for people to just slam it into gear. People also frequently change between 4th and 5th on high speed roads, with the downshift taking it's toll on the former. Someone also mentioned double synchros on lower gears...which rings a bell for some reason. Think of it as, 4th isn't weak but 1st/2nd/3rd are strong.

[Moodles2]

if you are in third gear and are changing at peak power (6800rpm) into fourth, r32 gtr boxes all crunch.. only thing you can do is delay the shift by a second.

[hossboss]

Kind of ironic that trying to do good (changing oil), can actually cause more bad! Kind of wish that I didn't f**k with it now... I used Nulon 75W - 85 Smooth Shift (!) manual gearbox and transaxle oil for the change. I did some research and this seemed appropriate. Ah well...

[Duncan]

if you are in third gear and are changing at peak power (6800rpm) into fourth, r32 gtr boxes all crunch.. only thing you can do is delay the shift by a second.

nah they're just all getting old at the same time....i put a new gearbox in the race car and even at full bore changes it doesn't crunch. these poor old boxes just have 20 years of abuse on them by now

[Swiper the Fox]

nah they're just all getting old at the same time....i put a new gearbox in the race car and even at full bore changes it doesn't crunch. these poor old boxes just have 20 years of abuse on them by now

Terry tells me its sometimes a spigot bearing worn that makes 4th crunch. He has done rebuilds for people only to have the new box crunch also...only to find a worn spigot when box is removed.

Guys when the box is out replace the spigot bearing.

My new PPG dogbox even crunched into 4th...we replaced the clutch and spigot...still did it. Only to replace the shifter the next day and fixed it.

[Birds]

Kind of ironic that trying to do good (changing oil), can actually cause more bad! Kind of wish that I didn't f**k with it now... I used Nulon 75W - 85 Smooth Shift (!) manual gearbox and transaxle oil for the change. I did some research and this seemed appropriate. Ah well...

Yep...if your car has done 100,000km on one lot of engine oil...you're best to leave the stuff in there, that's for sure

Sounds like you picked the correct viscosity, I would say your gearbox just needs a recondition to freshen it up.

nah they're just all getting old at the same time....i put a new gearbox in the race car and even at full bore changes it doesn't crunch. these poor old boxes just have 20 years of abuse on them by now

True that.

[Swiper the Fox]

Yep...if your car has done 100,000km on one lot of engine oil...you're best to leave the stuff in there, that's for sure

Sounds like you picked the correct viscosity, I would say your gearbox just needs a recondition to freshen it up.

True that.

Like the guy at work who didn't service his Pajero (company vehicle) for 70000klm. 2 days after work demanded he get it done imediately the engine seized with the new oil in it.

[Duncan]

yeah good point, always use a new spigot bush, the damn things cost about $2 and are dead easy to do when the clutch is out.

While we were at it....dump a short shifter if you have one too. They do not help you change gears quicker and will wear the synchros more as they are more likely to baulk. Paul's obviously just been through the same thing with his dog box but the same holds for a synchro box.

Finally...shifting technique. you can move the shifter quickly to the centre of the gate but once there you need to allow time for the synchros to work. If you are down shifting give the throttle a quick blip on the way down to help the synchros. Unless its a dog box in which case bang it straight across firmly and quickly.

[GTS4WD]

My 32 GTS4 box does the same..

To slow down my changes.. I put in the clutch, shift into neutral, clutch out, in then change into 4th from 3rd.

5th to 4th is generally a bit of a bitch to change. I try to avoid it, if I know im going to go into 3rd, ill just skip 4th. You'll need to brake earlier into the corner because you dont have the engine brake to assist you in slowing the car down though.

[hossboss]

First of all I just want to say thanks to everyone helping me out here with info and advice - I'm taking it all in!

I'm just wondering what the next step should be. Would a cost effective short term option be to drop the oil and replace with different (thicker viscosity) oil? As for the spigot bearing/bush, do you have to rip the whole 'box apart to get at it, or is it possible to throw a new one in relatively easily?

I really don't want to have to drive around the problem, as it is just too bloody irritating, and ruins the driving experience!

I could always go the frowned upon lucky dip secondhand box option...as I mentioned, cost effectiveness is paramount in whatever I do...

[Duncan]

cheap "fix"? simple...drop the oil and put in redline lightweight. Its been shown time and time again to reduce or eliminate crunching. It does however actually increase the wear on the synchros so in a few years you will still have to do something more permanent (or just sell the car lol)

[Birds]

In the meantime you can learn how to double clutch on downshifts which should eliminate a crunch sound from the synchro...not only is this fun and cool sounding (engine blips ftw), but as long as you do it properly it will preserve the life of your engine and gearbox.

[hossboss]

Well an oil swap it is then! (again). Bugger, it wasn't the most fun task either...

As for double clutching, it's a habit I've had for years, and definately makes for smoother progress (something I like to achieve when driving in general), especially when your givin' it the berries The grinding kinda kills the mood, as far as wanting to press on (I have mechanical sympathy)

[hossboss]

cheap "fix"? simple...drop the oil and put in redline lightweight. Its been shown time and time again to reduce or eliminate crunching. It does however actually increase the wear on the synchros so in a few years you will still have to do something more permanent (or just sell the car lol)

What's the best weight Redline Lightweight Shockproof to use, or is there only one?