So, 5th won't engage...

[Driven97]

Had the car out for an autocross test and tune today and put a LOT of runs on it, maybe 30 which is almost a full season in some parts. Of course in autocross you typically only use 1st and 2nd, actually pretty much just 2nd.

Heading home car would not go into 5th. Reverse is no issue. It mostly feels like the synchros won't align properly. You can sometimes kinda get it into fifth but it pops out. All other gears seem fine, so I don't think there is a catastrophic failure. I don't suspect a shifter or coupler issue in this case, though of course I'll try that first.

I assume the only good answer is "crack it open and have a look" but is it plausible that the 5th gear synchro can fail without the actual use of 5th? This thread is the closest I can find to a matching issue, and there's no closure in it.

Final question - I've pulled a motor before and done a clutch, how fiddly is trans work? The exploded diagram is pretty intimidating. But it does look fun.

[Peter M]

Matt,
First, just check the grub screw on the shift coupler hasn't loosened off.

I had exactly the same issue on my 915 until I noticed the coupler wasn't tight on the gearbox shift shaft. A tweek of an allen key fixed it!

[DRACO A5OG]

Check the Shift Coupler then under shifter on those bushings, last would be gear box mounts ( unlikely but if they wear out it would mis-align the input shaft. )

[Driven97]

So allen key / coupler adjustment didn't help so I dropped the motor out today.

The dog teeth look pretty good I think:


Tapping on the shift thingie with a mallet gets it that far. Is that fully engaged? Can't get the darn camera to focus on the right spot. In the car it feels like it doesn't fully get into 5th and it pops out.

[Driven97]

Better shot, this feels like a hard stop on engagement. Should the slider get all the way to the step on that dog ring?

[Driven97]

Inside of slider looks munched:



Synchro:



Without really knowing how it's supposed to work looks like the deformation in the teeth on the slider prevent the gear from fully engaging. How the slider teeth got that way I don't know. Trans was full and gear oil still looked new.

[Matt Monson]

I'm not seeing anything wrong with the components themselves. I think you have an issue in your shift rods and detents. Something isn't letting that slider drop all the way onto the gear, and it's not at the gear itself. Now that it's apart I bet it pops right onto the gear.

[Driven97]

Is the slider supposed to make it all the way to the "lip" on the dog ring? Because it doesn't feel like it really snaps into place fully either with or without the shift fork on.

[Trackrash]

Engage one of the other gears to see how it should work and look when fully engaged.

[Matt Monson]

I've got to go to the shop tomorrow. I'll snap some reference photos.

[Driven97]

Thanks guys. I don't plan on cracking the center portion of the gearbox at this time, so can't really compare to the other gears. I also noticed a very slight amount of play in the lower gear. Not a lot, but I would expect it should be rock solid on the shaft?

[Trackrash]

You can remove the bottom plate. You will have access to the gears. You will also be able to check to see if the guide fork is secure.

[gomezoneill]

Take the gear apart and check the brake bands and anchor blocks. Saw a post a couple of days ago the op could not engage a gear and his anchor block was jammed into a locking position. Don't know why or the outcome.

[ischmitz]

I am far from an expert here but this looks like that slider was killed with a bad shift. The teeth show heat damage and are flattened and that's why it doesn't go into gear. This happens when the synchro bands can't match speeds during the shift and instead the friction super-hearts the teeth. They become soft and mushroom when trying to engage the gear:




I did this to my third gear with a badly missed shift (clutch pedal not fully depressed while trying to force it into third) and had the exact same effect. Replaced the slider and the syncho bands and all was good.

Ingo

[Driven97]

That's what I was thinking. My Dad took the car to lunch and said it wouldn't go into 5th. Who knows how hard he tried but I know I tried a few times to get it in there too. Already ordered a replacement slider. I guess my big concern is what caused it in the first place.

So I assume I should also replace the synchro band, if it got hot enough to cook the teeth it cooked the band.

[ischmitz]

If the clutch wasn't fully depressed or he speed-shifted into 5th that'll do it. The friction heats up the teeth and makes them soft. These transmissions need to be shifted with a bit of care to give the synchro time to do its job.

[Driven97]

New gear/slider popped all the way in. Plan on replacing:

Needle bearing and whatever associated "bushing" is
Brake band
Nut
Gasket

Sound good?

My Dad is pretty gentle with the car. Any chance a little bit of play from a failing bearing could've lead to the failure? I'd rather not nuke another slider...

[Plays with cars]

Matt, I will reinforce what others have suggested: check all of the bushings, set screws and adjustment throughout the shift linkage. Its shocking how a little bit of slop or misalignment is amplified through the linkage and can compromise complete engagement.

[Driven97]

Yup will do, have already had a good look at the coupler and there's no play in that. Thanks for all the help pelicans.

[Driven97]

Just for closure: replaced the slider, also put a fresh roller bearing and bearing sleeve, and a new synchro band. Broke my cheap c-clip pliers in the process.

Got the motor / trans back in the car in just under 2.5 hours today and drive the car around. All is well again.

Thanks pelican for all the help and most importantly the courage to dig into the trans myself. Special hat tip to Pete Z for his great article series.