This is a Sump bike; a 1969, 250cc Triumph TR25W Trophy. This OHV, air-cooled single is basically a badge-engineered BSA B25. But that's okay. We like Beezas too.
Riding back from somewhere South, we came over Tower Bridge in Central London one hot afternoon and suddenly lost the gear change. The engine was running okay, but was stuck in second. The bike was nursed along through heavy traffic to a convenient spot and was shut down. Plenty of oil suddenly over the top end too, and a then completely floppy gear lever. Going nowhere quickly. Breakdown truck job.
Gear lever shafts on pretty much any British bike are apt to strip splines. If the levers are looked after and kept tight, the splines will last a long time. But ultimately, they will fail. There's a lot of pressure and leverage there. Time takes its toll.
▲ Sump's own 250cc Triumph TR25W Trophy. It obviously had a hard life in the hands of its previous owner or owners. But we're nursing it back to good health. Great fun on the move. However, they're a little highly strung and won't suit everyone.
The usual fix on the Triumph TR25W (or unit B-series BSA singles) is to strip the gearbox and clutch, buy a new splined gear shaft and weld the new shaft onto the gear selector quadrant. Not the kind of work every home mechanic wants to do. It needs a good, strong weld, ideally from a TIG set.
In this instance, however, something else was going on. The splines on the gear change shaft were't damaged. They were fine. But the shaft was in two pieces instead of one; one half was sleeved onto the other half and was secured by a split pin which had broken.
▲ Top graphic; the gear shaft showing the shaft sleeving and the hole for the split pin (not shown). Second image; welding the shaft and the hole for the split pin. Third image: the shaft after machining to allow smooth passage through the outer gearbox cover seal.
That split pin was bound to fail sooner or later, and so it did. The repair was to clean up the two parts of the shaft, fit a tight steel pin where the split pin had been, weld the joint, plug the holes (thereby locking the pin in situ), and re-machine the shaft to size. Dick Smith at Baron's Speed Shop handled it. The entire weld/machine job took about thirty minutes. The reassembly took slightly longer, largely because an inexperienced mechanic will need to familiarise himself/herself with the gearbox innards to get it all back together.
▲ The Triumph TR25W gearbox with the outer covers removed. Top right is the camshaft (magenta), Beneath that, the crankshaft pinion (orange). Beneath that, the oil pump (brown). The gearbox lives in a separate compartment (left). Mainshaft gears in blue. Layshaft gears in red. Gear selector cam plate in green. Just study the scene for a while without touching anything. Work out what the designer was trying to achieve. Then probe further.
▲ The gear change quadrant return spring (in orange) is prone to failure. This one was fine. The gear selector cam plate (green) will need to come out. Make sure it goes back the right way up or you'll reverse your gear changing pattern. Eventually you'll get at the cam plate quadrant and remove it. You'll scratch your head the first time you do it, but you'll figure it out. We did. Check all gears carefully (blue). Check for chips and cracks.
While you're down there, it's a good opportunity to deal with any other clutch or gearbox jobs that need doing, such as weeping gaskets or leaking seals or replacing clutch plates or whatever. Check each gear for damage. The gearbox will re-assemble in one way only, so you can't really get it wrong. You might have to loosely refit the gear shaft and twiddle it around to get the gear cluster into the box (don't worry, when you need to do it, you'll see what we mean). We took about a dozen attempts before we understood the sequence. Now we can do it drunk in about a minute.
Make sure you ask your dealer about any necessary gaskets, thrust washers, springs. And particularly check that kickstart return spring. You'll need to lightly re-assemble the kickstarter and wind it up to relocate the gear cluster and hold it in place as you press it home. Will become clear when you're down there.
▲ Triumph TR25W index spring. These are made in three parts and are also prone to failure. They act upon the gearbox cam plate indents. Inexpensive, but expect two to three hours work to change. Maybe more. With practice, you can cut that by half.
Important! See caption immediately below.
▲ Incorrect spring assembly. On this feature, we originally showed the spring assembled this way. But Sumpster Mike Rothwell has since pointed out that this order is wrong and suggested that "disappointment will surely prevail". Well, using Photoshop we've swapped the springs around. But we can't remember if we actually re-assembled the springs in this order (with the longest spring at the bottom of the stack), or if we looked a little closely at our manual before refitting. It's possible that we simply loosely assembled the spring for photographic purposes, then installed it correctly. Either way, the gearbox is in fact working perfectly, and we don't much fancy stripping it to check. But if it fails any time soon, we'll know what went wrong. Thanks to Mike for the input.
Also, note that the usual gear selection failure is the gear selector cam plate spring (image immediately above). This is a three-piece flat spring located on the opposite side of that green quadrant (and acts against that quadrant). The spring is cheap to buy, but a fair amount of mucking about to fit. However, you won't have to take down the gearbox this far. You can get at that spring simply by removing the outer gearbox cover. But that will unwind the kickstarter.
Once again, study the mechanism. Work out what the designer was trying to achieve. Also, study the clutch release rack and pinion device in the outermost cover. That needs to be positioned correctly if you want the actuating lever to work right. There's a small (and easily lost) ball bearing there too (behind the rack and pinion). So watch it. It will also make sense when you're down there. You might have to refit the outer cover a few times before it's understood, so try a few practice attempts before you fit a gasket or use jointing compound.
▲ Triumph TR25W Trophy gearbox diagram. It's a fairly simple transmission, and it's reasonably robust. But it does have its weak spots. Make sure the clutch actuator arm (the lever at the bottom right hand corner) is at 90 degrees to the clutch cable when the clutch is fully disengaged. If not, you might have trouble finding all four gears.
Top end oil loss
But what was the top end oil loss all about? Well, that's a weak spot on the TR25W and BSA B-series unit singles. There's lots of pressure up there at the rocker box. Ours had been weeping for a while and, we figure, decided to let loose the Niagra. Partly coincidence, we figure, and partly due to revving the bike extra hard when the gear change failed. It was a hot day too.
A clean up and replacement gasket solved the top end problem. But a rocker box stud needed replacing. There was no other damage to the bike. But because this engine had had a recent rebore, we had feared a piston nip-up.
Our advice? Get a Triumph 250cc TR25W Trophy manual. Photograph each stage (sooner or later you'll be grateful you did that). Sit with a coffee and/or a fag and just stare at it for a while. That's important. Take it down slowly. Stop. Think. Make notes. Also, make sure you've got a decent impact driver to preclude the ever-present risk of damaging the screw heads on the outer cases. When they get stuck, you're really stuck.
The average home mechanic can sort this one out, and you'll feel a lot happier if you've never looked that deeply into your bike. You'll feel empowered. Expect to spend a good three to four hours on this job. Maybe a little more. An entire day wouldn't be ridiculous the first time around.
Think you know better? Good, drop us a line at Sump and we'll include your thoughts/experience.