Locker Shaft

I've decided the locker shaft needs a section all its own.  As I learn more about these bikes, I see interrelated issues in which the locker shaft is involved. 

A 7mm hole runs through the primary (clutch) gear shaft.  This allows air, and possibly fluid, to pass through the hole.  The presence of a locker shaft makes draining the oil more difficult.  

The model years I associate with early, middle, and late are only approximate.  They may vary by geographic region as well.  According to an OSSA press release, a lot changed in 2013.  Some early bikes in the USA were updated before being sold.

NB: I've recently been made aware of a 2012 Explorer that has a captive locker shaft with a through hole and a 14mm ID bearing.  I'm beginning to think no two bikes are identical!

 2nd Gear Pinion

Along with (or perhaps before) the addition of the locker shaft, the second gear pinion was changed to provide a longer engagement region.  

Notice the adjacent photos.  Instead of just being the gear alone, a bearing surface was incorporated at the end.  This bearing surface has an outside diameter of 20mm.  The inside diameter of the gear and bearing surface is nominally 14mm.  It is an interference fit (approximately 0.00075 inch) on the 14mm shaft.  


This necessitated changing the shaft's support bearing in the crankcase to one with a bigger ID.

Early 2nd gear pinion, fits into 14mm ID bearing

Late 2nd gear pinion, fits into 20mm ID bearing

Primary Shaft Support Bearing

Models with the updated 2nd gear also have a different shaft support bearing.  The early bearing is a 6201 which measures 14 x 32 x 10mm and lacks seals.  In later bikes, two different bearings were used - both of which measure 20 x 32 x 10mm.

The bearing in my late 2014 (2015 specs?) bike is marked “FAG 3804-B-2RSR-TVH Japan.”  This is a double-row, angular contact bearing.  The writing is very small and its seals are marked 6804 (which is a bearing measuring 20 x 32 x 7).  This was initially a source of confusion for me.

The bearing in my early 2014 bike is a 63804.  This is a deep-groove bearing. 


Both bearings are an interference fit in the crankcase (more on this later).  A snap-ring is also used to keep the bearing from moving towards the gearbox.  When installing the snap ring, the sharp edge side must face toward the gearbox.

Method to measure axial play

Measuring Axial Play

A dial indicator can be used to measure the axial play of the clutch inner hub.  Because the inner hub is steel, a magnetic base indicator stand provides a quick setup. 

My investigation is incomplete, but there appears to be about 1mm of axial play in bikes with no locker shaft.  The early locker shaft helps, but there still may be 0.75mm of axial play remaining.

The captive locker shaft does the best job and eliminates nearly all axial play at the clutch hub.

Final (Captive) Locker Shaft Design

I was surprised by an undocumented change to the locker shaft.  Upon attempting to remove my late 2014 gearbox for the first time, it would not budge.  Usually after removing the five shoulder screws and assuring the gearbox is not in neutral, the gearbox will come out with a light tap from a soft hammer on the countershaft.  

Upon further inspection, I noticed the locker shaft was solid (there's no 3mm hole down the center).  Removing the locker shaft's left-hand nut allowed the gearbox to come out easily.

This later locker shaft is very different from the one in my early 2014 bike.  The locker shaft has the same basic shape as a 4T valve.  The head of the early locker shaft is 19.9mm in diameter and can pass through the center of a bearing with a 20mm ID.   Whereas the head of the final locker shaft is 23.9mm in diameter and cannot.  The final locker shaft also lacks the cross-drilled holes found in the earlier version.

I'm not sure when the change occurred.  The 2015 parts book does not list a different locker shaft.  In fact, the only item in the 2015 parts book that ends in “15” is the crankcase itself.

Note absence of through-hole in final locker shaft.

LH nut must be removed to remove gearbox!

I  found this nut installed backward, the curved side goes towards the hub.

Final locker shaft stays in crankcase when gearbox is removed.  I also refer to this as the captive locker shaft.

I drilled a 2mm hole down the length of this shaft in an attempt to mitigate the oil puking problem. 

Gearbox Puking Oil

The fact that the final locker shaft design has no through-hole may explain why my late 2014 bike pukes oil out of its gearbox breather.  Neither my 2011 TR280i (no locker shaft) nor my early 2014 TR250i (locker shaft with through-hole) do that. 

Vented oil filler cap

Venting the Oil Filler Cap

My first attempt at fixing the oil-puking problem was to vent the clutch cover.  I machined a new oil filler cap and installed a sintered bronze muffler intended for pneumatic systems.  Unfortunately, it was unsuccessful.  Oil just came out of this area too.

I'm now wondering if it would have been beneficial to install a second breather in place of the screw that plugs the hole near the countershaft sprocket?  

The early bikes had their gearbox breather in this location.  Later bikes had the breather at the kickstart shaft.

The official fixes for the oil-puking problem are to route the breather so that it lubricates the chain or to extend the breather up near the headstock.  I used the latter method with a piece of clear plastic tubing so I could see the oil percolating.  It is described below.

Locker Shaft Trouble

The clutch in one of my 250s began misbehaving.  Symptoms included slightly lighter clutch lever pull, reduced clutch modulation range, and less vigorous engagement.

Upon disassembly, I discovered about 1mm of axial play on the clutch basket.  Any axial play on the clutch basket must first be taken up before the plates separate or come back together.  The locker shaft is supposed to prevent this wasted motion.  The source of the trouble can be seen in the adjacent photo. 

Although I found locker shafts available in Italy for about 30 euros, I decided to manufacture a better one.  The problem is that a 3mm hole runs through the center of the M6 x 1.0 thread.  This does not leave much metal at the root of the thread.  The shaft itself measures 6.8mm in diameter, and I figured an M7 x 1.0 thread would be better.

I ordered the correct left-hand tap and die from China via eBay.  I machined a new shaft from the strong, easy-machining steel alloy 12L14.  Instead of making the matching nut from scratch, I just drilled and taped the original nut for M7 x 1.0 LH.

Torn locker shaft thread

My M7 thread above, OE M6 thread below

My Improved Locker Shaft

This is what OSSA should have built in the first place.  Although the nominally 7mm thread only measures 6.8mm in diameter, it is still stronger than the M6 thread.

I bought a long 3mm drill bit but did not need it.   I was able to drill the shaft from both ends, starting with a 7/64" drill bit.  I finished with a #32 drill bit which measures 2.95mm.

The position of the cross-drilled oiling holes is probably not critical as there is nothing inside the clutch mainshaft for them to meet.

Bearing Removal

The mainshaft bearing is an interference fit (about 0.001 inch or 0.025mm) in the cases.  It is also held in place with a snap ring.   Removing the bearing involves heating the crankcase area near the bearing (both inside and outside) to about 200° F (93° C).

On models with the captive locker shaft, it is a simple matter to push the locker shaft/bearing combination out using an M5 screw.  Be gentle!  If it does not push easily, more heating is required.

On models with the removable locker shaft, a blind bearing puller will be required.

M5 screw to push against locker shaft aids bearing removal

Bearing Fits and CTE

Firstly, I apologize for not doing this calculation in metric but I am far more familiar with thousandths of an inch for bearing fits.  A 32mm OD bearing nominally measures 1.2598 inches.  I measured my bearing OD at 1.2594 inches.  My case bore measured 1.2585 inches.  Both measurements were made at the same ambient temperature of around 60° F. 

1.2585 - 1.2594 = -0.0009 (or slightly less than 1 thousand of an inch interference).

This is considered a light press fit.  To easily install the bearing, there must be clearance between the bearing and the cases.  This involves shrinking (by cooling) the bearing and expanding (by heating) the crankcase.


The linear coefficient of thermal expansion (CTE) for steel varies from 6 to 7 micro inches per inch per degree F depending on the alloy.  Let's call it 6.5. 

My freezer is about 10° F.  This provides a 50° delta from room temperature.  Thus, the bearing will shrink about 0.0004 inches.

1.26 * -50 * 6.5e-6 = -0.0004

The linear CTE for aluminum varies from  11.7 to 13.3 micro-inches per inch per degree F depending on the alloy.  Let's call it 12.

Assuming the case is locally heated to 200° F provides a 140° delta from room temperature.  Thus, the case will locally expand by about 0.0021 inch.

1.26 * 140 * 12e-6 = 0.0021


Taking the shrinkage of the bearing and the expansion of the case together provides about 0.0025 inches of clearance.  This will be adequate if we can keep that much clearance for the duration of the bearing installation.  Unfortunately, the cases will rapidly conduct heat to the rest of the engine, thereby cooling and shrinking.  Similarly, as soon as the bearing contacts the hot case it will warm and expand.    

So the key is to work quickly and have an alignment/installation tool that can be struck with a hammer to seat the bearing.  Ideally, the case would be heated in an oven but because the work was performed in situ, I used an electric paint stripping gun.  Where fire is not a concern (e.g., a wheel bearing) I use a propane torch for heating.  I also literally keep the bearing on ice until the cases are hot.  An infrared thermometer is practically a necessity.  If in doubt, strive for a greater temperature differential /  larger clearance. 

For those who want to perform the calculation in metric, the CTE for aluminum is 21 to 24  micrometers per meter per degree C.  The CTE for steel is 10.8 to 12.5 micrometers per meter per degree C.

Finally, for completeness, I will mention what happens when using heat for bearing removal.  When you heat the aluminum crankcase with a bearing installed, both the crankcase and the bearing expand.  Because the CTE for aluminum is roughly double that of steel, the aluminum will expand more than the steel for each degree of temperature rise.  Because the aluminum is on the outside, clearance will form between the crankcase and the bearing making disassembly easier.  If however, an aluminum shaft is inside a steel bearing, heating will only make the fit tighter and more difficult to separate.

Bearing & Locker Shaft Installation

Installing the bearing alone in the crankcase may be accomplished with a standard bearing driver.  However, installing the locker shaft at the same time complicates the matter.  The locker shaft must be held in the center of the bearing. 

To accomplish this, I made the special tool shown in the adjacent photo.  It consists of two pieces of aluminum and a 6.5" length of 1" PVC plumbing pipe. 

Both pieces of aluminum have a 6.8mm through-hole.  One requires an OD of just under 20mm to center the locker shaft in the bearing.  The other requires an OD that is a push fit in the ID of the PVC tubing.  

The PCV tubing needs the end near the bearing turned to an OD just under 32mm.  When the locker shaft nut is tightened, it must hold the bearing firmly against the PVC.  This allows you to rapidly position the bearing and seat it with a light hammer blow.  Make sure the bearing goes in far enough to reinstall the snap ring.

Late bearing/locker shaft installation tool