One of the things that initially sold me on the OSSA was its cassette gearbox.  It later became a source of aggravation.  I got used to the cassette feature on Yamaha's GP road racers.  You could literally have the Yamaha gearbox out and be changing internal ratios in under an hour.  OSSA's implementation was a disappointment in every way.  Firstly, none of the individual gears had part numbers - you had to buy a complete assembly.  And, even if spare parts were available, the gearbox is not at all easy to disassemble.  It appeared the sole design criteria was low weight (and possibly low manufacturing cost).  Bushings were used where bearings would be in a typical gearbox.  The shift drum is soft aluminum and finding neutral with the engine running becomes nearly impossible due to minor wear.   I eventually learned how to disassemble the gearbox and will pass that knowledge along here.

Gear Ratios

Specification sheets for trials bikes rarely list internal gearbox ratios so I have usually been forced to count gear teeth.  Here are the results:

Primary (27 drives 75) 2.777  (This is the same as GasGas Pro)

1st: 3.167

2nd: 2.571

3rd: 2.125

4th: 1.778

5th: 1.217

6th: 0.724

Secondary (11 drives 42) 3.818

Secondary (10 drives 42) 4.200

Secondary (9 drives 42) 4.666

With an 11T front, 6th gear is good for almost exactly 10 MPH per 1000 rpm with a standard trials tire.  My testing seems to indicate at least one ECU map has a rev-limiting function at about 5600 rpm.  This would predict a theoretical top speed of about 56 mph. 

A different ECU map gave a maximum engine speed of about 7000 rpm.

Gear-shaft annotated with gear positions


Before you can work on the cassette gearbox, you must get it out of the engine.  My first time was a learning experience.  It is best to remove the rear brake pedal before removing the clutch cover. 

Once the clutch cover is off, five special “shoulder screws” must be removed.  These screws require a 3mm hex key.  There is little room to work, and both the clutch hub and clutch basket must be rotated slightly to access all five screws.   Once the clutch spring has been removed, the clutch inner hub is free to rotate independently by hand.  I remove the flywheel cover so I can turn the flywheel by hand to align cutouts in the clutch basket and hub.

The screws have a head diameter of 10 mm, a shoulder diameter of 6 mm, and a shoulder length of 10 mm.  The thread is M5 x 0.8.  They are available from McMaster-Carr as their part number 92981A100.

I considered heating the screwheads with my butane soldering tool but was afraid of starting an oil fire.  Instead, I made a guided drift from a 5/16" diameter chunk of drill rod with a 3mm pilot machined at one end (shown below).  Each screw took a dozen whacks with a 24-ounce ball-peen hammer.  You can see significant mushrooming at both ends of the drift.

Subsequent removals of the gearbox were much easier.  I think a gorilla put the screws in before me, possibly with a strong locking agent.  I use a dab of Blue (242) Loctite. 

To remove the gearbox from the engine casting, it must not be in neutral.

If you have a late 2014 or 2015 bike, see the subsection on the Locker Shaft.  Its left-hand nut must also be removed.

My cassette gearbox removal tools

Primary Locker Shaft Used on 2103+ models, Left-Hand Thread

Long 3mm hex key for gearbox retaining screws.

Long 3/8" drive, 3mm Allen Wrench

I recently discovered this tool for under $5 on AliExpress.  It fits perfectly and allows you to use a torque wrench.  I don't know how strong it is, so if your gearbox has never been removed, you may want to buy a spare.  It seems many gearboxes were installed by a gorilla.  I've stopped using a locking agent on the threads as it seems unnecessary.   

The complete title of the item was 1pc 3/8 Inch Drive Wrench Socket Adapter Hex Screwdriver Bit Sockets Hand Tool Socket Joint Hexagonal Screwdriver H3-H10.”  But I think the key search criteria are “H3” for a 3mm hex, and also “long 3/8 inch drive

Multi-bit sets are available inexpensively via eBay too.

Early versus Late Gerarbox

OSSA had two suppliers of gearboxes.  The initial and final gearboxes were made in Italy by Lamfos.  In the middle years (I am not sure how long) gearboxes came from a Spanish supplier that was not as good.  My notes say the base of the gear teeth is shaped either like a “V” or a “U” and this identifies the country of origin (but I do not know which is which).  One of  OSSA's stated improvements for 2013 is that the gearbox was “built with more precision and better tolerance (fewer sharp edges, better material, and hardening treatment, and a new system to fix the gearbox.”  In the final year or so of production, all the gears were black.

2011-model gearcase

2014-model gearcase, note machining to drain oil via case screw

2nd Gear Retrofit

This only occurred to me after looking at the early and late gearcases side by side.  Notice the ID of the bearing that supports the clutch mainshaft is smaller on the early gearcase.  This corresponds with the slightly smaller OD of the mating shaft.

I have a vague recollection that the 2012 models received a field upgrade (by the US importer) which included a better 2nd gear.  This would imply that you can't just swap an early gearbox with a late gearbox without also changing the bearing in the gearcase.

Locker Shaft and Idler Gear

The Primary Locker Shaft goes through the clutch basket.  Its stated purpose is to keep the clutch basket from having axial play.  Note that it has a left-hand thread.  Below are photos of what you will find once the gearbox has been removed.

Note the location of the idler gear.  This gear is behind the primary gear on the clutch basket.  It is driven by the kickstart sliding quadrant gear.  If the diameter of the idler gear measures 50.80mm or less, it indicates a Spanish (poor) gearbox.  If it measures 51.30mm, it indicates the Italian (good) gearbox.   

Location of Idler Gear annotated in red

Note: complete removal of all retaining screws is not required

Shift Drum Replacement

The most common OSSA gearbox problem is wear on the shift drum and shift shaft mechanism.  Besides the shift forks, these are really the only user serviceable parts as intended by OSSA.  Replacement is pretty straightforward but I do not have notes or a gearbox needing repair right now to write a guide.  It is possible the shift shaft assembly may be the same as a GasGas part.  The shift drum is most certainly not, as GasGas had a patented 4 gear-pairs provides 6 ratios system.

Worn neutral detent

Worn gear selector engagement areas

Shift Shaft Assembly

Chopsticks for spring installation.  Small hole drilled down the center of each rod to help install the tensioned shift return spring.

Gearset Disassembly

In the photo below, there are three non-obvious circlips (locations annotated in red).  Gearset disassembly starts by sliding circlip #2  along the shaft a bit at a time which allows the secondary shaft to wiggle out as you fiddle with the primary shaft.  Moving the primary shaft gears allows the secondary shaft to be removed.

In the next photo, the gray-handled screwdrivers were found to work for circlip removal.  The red and green screwdrivers were too small.  It helps to have the gearbox held rigidly (I lightly clamped it in a vise) when first moving circlip #2.  I had to progressively move the circlip along the shaft as more and more wiggle room became available.  Eventually, everything just fell apart.

I tried using the pallet knife shown between the circlip and the adjacent gear, but it was not helpful.  Although not necessary, I removed the circlip completely as I did not want to keep it stretched while waiting for parts to arrive.

With the secondary shaft removed, you then have access to the kickstart idler gear stub-shaft which is held in by a circlip and a shim behind that.  With careful manipulation of the primary shaft (you do not need to remove it to get the stub-shaft off) you can gain enough clearance to remove the stub-shaft.  I am told that even when new, the primary shaft clutch basket is pretty sloppy.

Locations of "hidden" circlips

Only the gray-handled screwdrivers were useful in sliding the circlips along.

Minimum parts removal needed to begin disassembly.

Clutch Primary roller bearing and Idler Gear

Clutch Bushing Replacement

Disassembling the gearbox to this point is not for the faint of heart.  The pinion at the end of the shaft is a very tight interference fit, and may only be disassembled three or four times before it becomes too loose.

As can be seen in the next photo, I am using a 2-arm puller.  I tried a 3-arm puller, but the number of gear dog slots is not evenly divisible by 3, so the 2-arm puller fits better.

I also used a sacrificial aluminum rod (must be less than 14mm in diameter) so the puller does not bear directly on the gear shaft.  I heated the end gear to about 200º C.

The bushing for the clutch primary shaft is a DU type measuring 28 x 32 x 25.  I purchased this from American Sleeve Bearing as well as the 14 x 20 x 10 DU bushing for the idler gear shaft.  While everything was apart, I also replaced the clutch primary roller bearing.  This is a 35 x 45 x 12mm GasGas MT280236158 bearing. 

Pinion that requires heat and a gear puller for disassembly.

2-arm puller preferable to a 3-arm puller.  Photo shows placement of sacrificial aluminum rod.

Clutch basket shown with DU bushing that was sloppy in my 2014 250.

Mainshaft shown disassembled

1st Gear Seizure in Early Bikes

Early bikes experienced 1st gear seizures when running at high road speed in 6th gear.  This is because 1st gear also rotates on its shaft when 6th gear is engaged.  A lack of oil caused the seizure.  To fix this, some early bikes were modified by grinding a spiral groove across the shaft bearing surface to allow an oil feed path.  Later bikes had the spiral directly machined into the ID of 1st gear and also had a larger oil hole in the shaft.

Eventually, it was discovered that simply over-filling the gearbox to 450cc remedied the issue too.