Looking for more advice.
I'm still running stock 6 speed with a 5X shifter rebuild kit. Finding it really hard to downshift when I'm going hard under brakes, I'll bring the revs up to match the downshift, but I really struggle to engage the gear. When going slower / less rpm there is no issue at all.
I've experienced this with last two 6 speeds I've had, I damaged the synchro on the previous box as I forced it in under pressure 🤦🏻-♀️

 

What fluid are you running? Have you checked for proper clutch disengagement?

 

Are you using the metal 5x shifter bushing?

 

Yeah the bronze one, when I swapped my gearbox, I swapped my shifter too, bit of a bitch to line that bushing up and get it in, so it's very snug

As for fluid, using Motul 300 Gear... No never checked clutch disengagement, I'll go and have a look what the clutch fork travel is like.

 

 

Sweet will give that a go, I do have a new'ish plastic bushing sitting around.

 

Clutch disengagement and fluid choice are both good guesses.

 

Is it in all gears or just select one?

I only ask because I read somewhere some forks are aluminium which expands making shifting a pain. Unfortunately i cannot remember off the top of my head whether it was 6 or 5 speed that did this.

 

What power are you making? I partially separated 4th gear, which is a pressed together gear and it became hard to shift into many gears but that was at all speeds and up shifting and downshifting both.

I'm going to guess it's fluid related since it happens at higher RPMs and more often on downshifting. If the oil is too slick the synchros won't have enough drag to match gear speeds quickly to allow engagement.

 

That is a 6 speed thing, but its a power+ambient temperature+session duration issue, not RPM related.

 

Im not sure if related but change fluid is a good test if it is this. Being Motul I wonder if it is similar....

Our MX team used to use the high end Motul Motorcycle gearbox oil (I forget the name now Pro or expert somethign liek that...) in the National champs one year and we all suffered missed shifts, jumping out of gear issues. Scarey on a motocross track.

It was only when I had used up my stock of sponsored supply and used the standard gearbox oil for a practice session that it suddenly fixed the problem. We were all blaming that years team Honda Crf450's as being a poor design until then.

The high end Motul oil was just to slippery for the synchros. As soon as everyone changed back to the standard stuff everyone was happy again.

 

I've seen a big improvement switching from mobile 1 to mt-90. More than i was expecting...

 

That's usually what happens when you use the correct fluid in the car. M1 is a GL-5, MT-90 is a GL-4. Our transmissions call for GL-4 oils.

 

Oh, yeah. If it is GL5 that is probably it.

 

I had issues with both Redline and Amsoil in my 6 speed. When hot on track {Sebring summertime} would miss upshifts to 5th and downshifts to 3rd.

Went to the oft cited Ford fluid and all is happy. Came to the conclusion the arms just don't like super slippery fluids?

 

If downshifting is still difficult from high revs after you switch to Amsoil MTG than the synchros might be going. We have used the delrin Miata Roadster shifter bushing the success. OEM plastic also fine but the Delrin is stiffer and lasts longer.