Hi Dale,
Earlier TR gearboxes had that groove in the shaft to keep it from moving side-to-side. At some point that was discontinued and the retaining bolt was omitted. This leaves only the fork itself to keep the shaft aligned in the gearbox. IMHO, this might put more side-to-side stress on the dreaded taper pin and/or the two pins in the fork that interact with the throwout bearing carrier. Personallly I'd want the retaining bolt and slotted shaft, if at all possible.
I suggest you return the shaft and ask them to send you one with the groove in it (I know TR4s used it, not sure what other models). You could have the shaft modified locally, but the earlier ones with the locking groove and the even earlier ones with both the locking groove and the grease passages are easily available through the various vendors.
Speaking of which, while you are at it, you might want to specify the clutch cross shaft that's drilled for greasing, too. It has threaded holes in each end that allow grease zerks to be installed. Those holes lead to short drilled passages that route the grease to the bushings in the bell housing. The later cars don't have this. Later cars also use a smaller, approx. 3/8" wide bushing in the bell housing that is plain inside. The earlier ones had an approx. 3/4" wide bushing that gives more support to the shaft *and* is drilled and grooved inside to give the grease a path to work into the area. All it takes is a quick one or two pumps with a grease gun every year or two to keep the greasable-type of assembly fresh and working smoothly. The later, greaseless-style with the narrower bearings wears faster and might get a bit stickier over time, putting more strain on the rest of the linkage mechanism.
At some point I think you said you have an engine from an earlier car (TR250?). Perhaps you have an earlier gearbox, too. It might already have the 3/4" bushings in the bellhousing (although I doubt it since the shaft isn't drilled for greasing). A possible clue... first locate the gearbox serial number. Beginning with TR4 and on all later TRs it's on the boss that sticks out on the LH side, that the cross shaft runs through (Moss shows its location in their catalog). Look of the rearward face of that boss for a stamped serial number which starts with two letters, followed by a short series of numbers.
The two letter prefix of the gearbox serial number can tell you what TR it was originally built for. "CT" is TR4/TR4A, "CD" is TR250/early TR6, "CC" is 1973-74 TR6 and "CD" is 1975 and later TR6. "CP" was also matched up to some 6-cyl., but I'm not sure which model. There are other prefixes such as "MD", "ME", "MG", "MK", "VA"and "VF" indicating the gearbox came out of one of the Triumph sedan car models. "TS" and "UF" indicate a gearbox lacking first gear synchro that was originally fitted to TR2/3/3A. Finally, "GR" was used to indicate a factory rebuilt gearbox and might appear in any of the cars.
There are some other visual clues, too. For example, the flange attaching the gearbox to the engine (yours only had a few metric bolts in it, right?) was beefed up on the later cars. It's roughly 1/2-5/8" thick at the bolt holes, I'd guesstimate. The earlier gearboxes were only about 1/4-3/8" thick at those points. Also, the earliest gearboxes have a long input shaft that sticks out past the edge of the bellhousing. I.e., if you stand the gearbox on end it can't sit flat without a couple blocks of wood under it. The TR4A and later with the newer diaphragm clutch use a shorter input shaft that allows the gearbox to sit flat when on end.
All this is a bit of a moot point, though. Any gearbox could have been rebuilt with different input shaft, internal parts and gearing, and installed in any car. The external housing and the serial number aren't necessarily a guarantee as to what's inside. There were slight changes over the years, including gear ratios, etc. So, hopefully, the innards match up to the serial number if you ever need any internal gearbox work done.
The clutch release shaft you're working on right now was made in several variations, as you can see. But all *are* interchangeable, so no worries there. Again, I recommend using the earlier, greasable version (circa TR4).
Are you installing a new release fork too? I ask because they have two nubs or pins on the inside of the fork tines that interface with the throw out bearing carrier (aka clutch release bearing carrier). Used fork pins tend to get worn flat on two sides. These can be replaced, if needed. Knock the old ones out, press fit in new ones and peen them a little to help keep them in place. A new fork would most likely have new pins installed. A "cheat" here is simply to get a good grip on tje worn old pin and turn it 90 degrees, so that an unworn area is now engaging with the TO bearing carrier. This reduces slop in that area, which can contribute to clutch problems if it gets bad enough.
While you are working on all this, it's also a good idea to replace the clevis pin at the slave cylinder pushrod, if it shows any wear, is a sloppy fit. The hole in the slave cyl. pushrod can get worn oversize, too. Now there are adjustable versions of those pushrods now being made for TR6, too. They originally came with non-adjustable type. The earlier cars had an adjustment there, but Triumph eliminated it on the later cars. Adjustment is a good idea to help insure full and accurate clutch release.
Also, there is no external return spring on the slave cyl of the later cars. They rely on a spring inside the slave cylinder itself I think and the clutch's own return springs to move the linkage back. The external return spring is another easily retrofitted item, if you wish. There's just a hole in the clutch slave bracket for one end, and a special little tab that fits under the clevis pin at the other end. This just makes for some added security that the slave cyl will retract all the way properly.
If this sounds like you are taking giant steps backward by retrofitting TR4-era parts to a TR6, keep in mind that the earlier cars' clutches seemed to last forever, while later cars have had all kinds of problems.
When Triumph went to the diaphragm clutch in the TR4A and later cars, in several steps they changed the setup to reduce servicing and, probably, to save a few bucks. Plus they modified it a bit to make the pedal a little more "user friendly". Unfortunately, this was all done at a cost to reliability! To make matters worse, the 6-cyl. engine developes more torque than the 4-cyl, and thus puts a bit heavier load on the clutch mechanisms.
The later diaphragm clutch you have *is* preferable (and not directly interchangeable due to mounting holes in the respective flywheels). This is because it's a more modern design, smaller and much lighter in weight, plus cheaper and easier to find.
If intereseted, all the "retro" parts can be seen at Moss website. Just look up the exploded views showing the clutch linkage for TR4, instead of TR6. Given a choice, I'd use the earlier clutch master cylinder, too. The pedal will be a bit heavier, giving your left leg slightly more of a workout, but the .75" master cylinder gives a bit more stroke at the slave cyl. and a more certain, complete release of the clutch.