I have a 60 series box out of a late model Nissan Champ (A14) 5 speed in my B110 that I race with, its held up amazingly well besides getting jammed in 4th twice now...

First time was on the track, leasurly down shift and it just wouldnt budge, i tried everything but to no avail, took the box out and took it to my transmission guy, he said one of the selector bolts (10mm head i was told) came loose, he just nipped it up with some loctite, I put box back in and was on my ways, lasted over 18 months now (20 race events including enduro's) and about 10 000km's...

On the way to work last week I was giving a V-tak-yo guy a leason on old school power, took her up to 8400rpm in 4th and as I went to 5th it would not budge, same as last time, drove it home in 4th (real fun with a 2.8kg flywheel) stripped the box out and opened it up, besides the hate of gear oil I could see nothing, the only 10mm head bolt is the one that holds the wedge for the selector, it was tight and in place... after fighting with the box trying to get the tail housing back on for an hour i admited defeat... besides that the parts shop gave me the wrong clutch, this one is 10mm larger disk and doesnt suit my flywheel...

Taking it back to the trans guy on monday, just wondering if anyone else has every seen or heard of this happening? changing clutches every time sux!

One thing im happy about is that the gears look 100% new having only ever ran on Redline light weight shock proof (smurf blood)

Yes this does happen. I had it happen to a 60L 5spd. I think from memory the problem was there was too much lash or float on one of the gears. We had to put a wee packing shim under one of the gears. IIRC the endfloat allowed the gear to cock over slightly and jamb so it couldn't disengage. The packing shim stopped it from being able to do that and it was fixed. Strip you box down and see if the selector hub for 1st gear shows signs of having movement here.

When it's stuck in 4th is the stick suddenly floppy and loose?
If had that happen once. I shifted diagonally from 4th to 5th and it popped.
What I found on mine was a roll pin that holds the 5/R selector thing on the rail had sheared.
(NOTE: in the picture about the 5/R label is backwards. I f'ed up.)
The extra play between 3/4 and 5/R allowed the selector finger to slip between the two and out.

Could be a number of things. I had a 60 series 4 speed jam in 4th once. turned out to be caused by the wire clip that hold the 3 little lugs in the selector hub not being put in properly, one of them popped out and jammed it in gear.

mine actualy went tight, very very firm, I tried rocking it back and forth but nothing,

Is it just me or is there a major trick im missing with getting the tail shaft housing back on, i see the manual says rotate the shift mechanism anti clockwise when you install, i tried everything, wont go on, not realy a massive problem as I plan to have my trans guy that does my race cars box's go over it and measure all the gears and shims etc,,,

The stoper pin bolt and the return spring plug, spring and plunger should be removed. They are located above the speedo drive assy.
With these removed they allow the shifter to rotate more and clear the bits inside.