5-Speed 5th Gear Issues

[Big McLargeHuge]

Hey Prime-mates,

I'm having an issue with my 5-speed transmission 2-3 years after I rebuilt it. At the moment, while shifting into 5th gear above ~40mph, my synchro doesn't seem to engage properly at first and I get a gear grind or buzz for a second going into gear. Regardless it always ends up going into gear and has never popped out of gear, just makes me cringe. There are zero issues when I double-clutch going into 5th.

It seems to have come and gone, where before I was using Honda MTF and it was on rare occasions and would disappear for a while, now I changed to Torco MTF for the NSXPO track day this month and after a few days of driving it happens every time I don't double-clutch into 5th above 40mph. Shifting after double-clutching feels perfectly normal and no other issues in any other gear.

Because of the feedback through the shifter it feels like the synchro cone might not be fully clamped onto the gear when I push the shifter and the shift sleeve is trying to engage with a still-moving synchro, but after a second it provides enough force to clamp the synchro and gear cone together and engage the gear. I rebuilt the trans with all new synchros, shift hubs/sleeves, and I think a new entire 5th gear back in 2020. The shift fork also looked like the updated stronger version already but not 100% sure, I can provide pics of the rebuild if necessary.

Any ideas for potential causes and solutions? I'm planning on doing a drivetrain-out service in the next year and can unwillingly split the trans case again, or hopefully I could use a friction modifier to help. Thanks in advance.

 

[Big McLargeHuge]

 

[MotorMouth93]

Not sure how I missed this thread.

I've had nothing but problems in the several times over the years I've tried different transmission fluids in my Hondas, I always end up back with OEM MTF.

5th is also a tiny and weak synchronizer on the 5 speed boxes, so you have to be careful with it even in the best of cases.

 

[Honcho]

I don't know how I missed this one either!

It sounds to me like maybe the sleeve is getting stuck on the internal hub gear or not smoothly engaging. Could be maybe a metal shaving or something got into the hub and is preventing it from moving freely onto the blocking ring??

I managed to find a pic of my 5th gear hub and synchro during install.



If you are able to shift into 5th using double-clutching, you're removing the brass blocking ring from the equation and manually synchronizing the shaft speed with the free spinning 5th gear. That means the blocking ring is not always able to match speed with the gear. Either the sleeve is not able to put enough pressure on the blocking ring to make it engage with the cone on the free-spinning 5th gear, or the blocking ring is unable to "grab" the cone itself. This suggests either a problem with the hub/sleeve engagement or the blocking ring.

I'm not sure that the change in fluid would cause such a drastic and specific failure. My 5-speed has been running on Torco MTF for almost 9 months and I have no issues with 5th (also using a new synchro and 5th gear) in ambient temps from 15F to 100F. Also, if the MTF was unable to provide the correct friction for the brass blocking rings, it's likely this problem would occur with all synchros on the transmission, not just 5th. Especially 2nd, which is a dual-cone synchro. You would be getting grinding in all shifts because the blocking rings would all be slipping on the engagement cones. Thus, I think the switch to the MTF may be revealing a problem versus causing it (you mentioned it happened with the Honda MTF as well).

Thus, I suspect there is some kind of interference in the free movement of the hub/sleeve and/or the blocking ring that is preventing it from fully engaging the cone. It could also be a linkage issue- as John mentioned, the 5th gear selector is weak and a problem with either the selector cable position or the selector itself could also explain why you can't get enough force on the blocking ring to get it to grab the cone in 5th, but all other gears are fine.

Finally I suppose it could be a clearance issue. Kaz mentioned that the OEM parts quality- especially transmission components- has gone down recently. It could be that your 5th gear, hub and/or blocking ring is slightly out of spec.

You could switch back to Honda MTF and see if the problem goes away, but I think that is a band-aid. I'd crack open the case and inspect the 5th synchro and hub.

 

[Big McLargeHuge]

Thanks guys. What I'll most likely do is take apart the transmission case during my drivetrain/general refresh I keep delaying while living away in CO, now early '24.

Pretty much all the 5th gear parts besides the shift fork were new but I'll check to see if there's anything obviously wrong and recheck all the tolerances. The explanation about the synchro and hub/ring engagement with double-clutching makes sense to me. Hopefully just some shim adjustment or maybe a new hub and/or synchro at worst. I just cleaned out the trans filter and there were some small bits but nothing overly concerning yet.

 

[Honcho]

Thanks guys. What I'll most likely do is take apart the transmission case during my drivetrain/general refresh I keep delaying while living away in CO, now early '24.

Pretty much all the 5th gear parts besides the shift fork were new but I'll check to see if there's anything obviously wrong and recheck all the tolerances. The explanation about the synchro and hub/ring engagement with double-clutching makes sense to me. Hopefully just some shim adjustment or maybe a new hub and/or synchro at worst. I just cleaned out the trans filter and there were some small bits but nothing overly concerning yet.

Good thing is that you don't need 5th for the NSXPO track day.