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6 Speed Manual Transmission Synchromesh

10 posts in this topic

Posted · Report post

I have a 2009 370Z with about 55K on the clock.  I do regular track days with the MSCA and this year will be my 3rd Phillip Island 6 hour.  The car is competitive in the 5M MSCA Sprint class.  Lap times are Sandown 1:24's, Phillip Island 1:52's, Winton 1:39's.  The transmission has just started to crunch on the upshift from 4th to 5th.  On a downshift from 6th it is fine.  If you shift very slowly from 4th to 5th it's OK but it needs to be a slow shift.  I'd appreciate any suggestions on the best repair approach  Currently I'm considering 4 options:  Buy a new gearbox from Nissan Australia (cost around $3700), Buy a second hand gearbox (cost around $2500 out of a 30,000KM Z), recondition the existing gearbox (maybe around $1500), Import a new gearbox from the States ($1900 + Taxes + shipping).

Has anyone had a similar experience?  What did you decide to do and in hindsight would you have done something different? Any suggestions appreciated

 

Thanks

Stephen Morel

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Posted · Report post

The synchro matching is probably masking the problem on the down shift?

 

I wonder what shipping from the US would cost - that will surely reduce the attractiveness of that option

 

Have you changed the oil recently?

And are you using a premium oil?

 

If you change the oil, throw a tube of Nulon G70 gearbox additive in there as well

 

I would be inclined to fix your original gearbox

While you have it off, switch to a lighter single mass flywheel (and replace the clutch disc)

This will make your up shifts faster and reduce the pressure on the synchro

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Posted · Report post

Stephen, I have been living with this for 3 years ... my 5th to 6th is a right pest and I am facing a similar issue, albeit my solution is a bit left field - it is called the QEB69G from Quaife, but I suspect that will be a bit expensive for you.

 

The syncro's in this box are a bit dodgy (not all that durable) and don't like quick shifts with hot oil.

 

My fix that has let me keep going with the box gradually deteriorating is to use a synthetic gearbox oil for my brothers tractor !!!  The box is a pain until it warms up, but the fix has worked for 3 years, although the box is getting pretty sad now.

 

I'm with Martin - if it is bad enough to warrant replacement, the cheapest solution will be to recondition the box in your car.  For the labour cost (remove/strip/reassembly and refit) and parts (seal kit, bearing kit and syncro's replaced) I still reckon you'll be well under the combined cost of new box out of the US with freight (freight is not likely to be less than USD600, so you are looking at close to A$3K by the time Paypal/the credit card companies take their gouge on the currency exchange).

 

The other point to consider is whether your shift quality is being affected by clutch drag - the Z34 clutch slave cylinder is notorious for being temperamental when clutch fluid is hot, so if the clutch fluid has never been changed, that is the first place to start (together with a heavier transmission oil).

 

In the event you do rebuild the box, then a replacement flywheel and uprated clutch is definitely the way to go.

 

If you need someone who is a gun gearbox man, I can recommend the guy who does most of the Historic Touring Car and Historic Sports Car boxes. 

 

PM me for details if you want them (he is a retired gearbox engineer located in Chadstone).

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Posted · Report post

Thanks for your responses MartinZ and BGTV8.  I'm using Redline MT85 (safe for use with Brass syncros).  Change this twice/annum.  Also change the clutch fluid annually.  Have been using an Exedy OEM clutch with their lighter single mass flywheel for a couple of years now which is a big improvement over the std dual mass flywheel.  The local Nissan dealer even installed the clutch and flywheel for me including a new clutch slave cylinder.  The clamping force with this clutch is about 25% greater than std and the force required to depress the clutch pedal is about 10-15% more than stock.  That clutch has worked very well over the last 2 years.

 

I will give the Nulon additive a go followed by a rebuild to keep the cost down.  Maybe even the tractor oil if the nulon doesn't help.  By the way BGTV8 if you ever decide you are going to buy a new transmission for your 370Z let me know - maybe we can get a better deal for 2.  BGTV8 - Re your suggestion on the heavier transmission oil that's something I hadn't considered - what grade would you suggest as a logical step up.  I am already using Motul 10W40 for the engine (up from 5W30).

 

I had considered 2 options to recondition the gearbox:  Dealer (if they are interested) or Revzone (who have been doing some good work on the car).  BGTV8 would you still recommend your gearbox specialist if I'm going to go with the 'rebuild OEM gearbox' option?

 

Thanks

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Posted · Report post

BGTV8 makes a good point about the 370 slave cylinder - if you pull the box out get the after market CSC

 

Here is another thing you can try, rather than that painfully slow up change

Pull it out of 4th into the neutral position, clutch up for a mere moment, clutch down and into 5th

Double declutching will be quicker

 

Sounds like you're on top of things otherwise

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Revzone are an OK alternative ....

 

I'll bear you in mind when it is time to order the QBE69G (would have been ordered by now, but "her-indoors" has convinced me that I need to build her a new house !!!).

 

MotorSport budget has been decimated !!

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Posted · Report post

Haha - know the feeling - but it's all good - you win both ways

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Posted · Report post

I would vote new box out of the states. I broke 3rd - 4th selector fork in my Zed and had the box rebuilt, and has never felt as tight. Now I have 6th gear synchro ring issues, I have to shift very slowly into 6th, costing a bit of time at PI and Sandown. From memory I paid close to $2K to get a rebuild, a new 350Z box from the states would only be marginally more expensive than that.

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My third to fourth was shot. $2200 shipped to my door, for a brand new box, and a couple other upgraded bits courtesy of Coz over at CZP. 

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That's a pretty good price indeed - still need to add ~$650 for the swap and then ~$100+ for incidentals (oil/gaskets/etc)

 

You can also get an engine/gearbox/diff from HongKong and Japan for $3500~$3800

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