Sprag Clutch Failure, Part I - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 106 (permalink) Old 05-04-2009, 09:50 AM Thread Starter
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Sprag Clutch Failure, Part I

Sprag clutch failure is almost certainly due to failure of it's inner workings, in particular the finger retaining spring. The spring of which I write is a tiny, mushy little thing, hardly capable of damaging anything in your engine, in the unlikely event pieces of it wander from its required location.

Why is this unlikely? The spring provides the initial frictional force used by the sprag fingers to bite into the shaft. Therefore, if the spring breaks at start-up, the engine will fail to start.

The mushy spring surrounds the sprag fingers comprising the inner gear. The inner gear is encased by an outer gear assembly of rather hardy construction. Triumph did a very good job of encasing the engagement fingers and inner hex heads. Also, they used "green" loctite on said inner hex head bolts which join the inner and outer gear assemblies.

If any other part of the sprag clutch let loose in your engine, you would most likely find shattered gears(not just teeth) in the oil sump.

The cause of sprag failure is most likely due to low battery starts. Other factors can hasten sprag failure, including: Starting without the choke, Starting in gear, using an oil of too low viscosity(the lower the viscosity = a higher viscosity number /// 80w is less viscous than 20w).

Another possibility:

When disassembling your engine, pay close attention when opening your clutch. Closely inspect your judder-plate. Are all the brass rivets still in place? Brass shards from your judder-plate can destroy a sprag spring, clutch plates, block oil passages...

Crappy thing too, as once those rivets separate from the judder-plate and migrate from the clutch housing, they like to hide in the engine. Then on occassion, they will fire off through the engine creating little phantom events, some of which include: intermittent clutch failure, inability to shift, a random clack-similar to the sound one might hear when dropping a metal tie wrap into a sink's grinder.
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post #2 of 106 (permalink) Old 05-04-2009, 10:35 AM Thread Starter
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Part II: Sprag Clutch Workings

Triumph and Haynes do not address the sprag clutch, except in overall assembly. I will attempt to delve inside and explain the sprag clutch parts, and their simplified function.

A simplified parts list comprising the sprag clutch:
  • Outer Gear: Receiving gear to starter gear.
  • Sprag Fingers: Oblong bearings of the Inner Gear.
  • Inner Gear: A ring of oblong bearings, and Sprag Spring.
  • Sprag Spring: A long narrow gauge spring, affixed at both ends, creating a ring.
  • Sprag Hex Bolts: Hex bolts which mate the outer and inner sprag gears.

When you hit the "Start" button:
  • Affixed to the end of the starter motor is a small gear, which begins to spin. The starter gear is in constant contact with the Outer Gear of the Sprag Clutch.
  • The Outer Sprag gear begins to spin. This motion combined with the Sprag Spring, forces the Sprag Fingers inward toward the Alternator Shaft.
  • This combined outward force, causes the Sprag Fingers "bite" into the Alternator Shaft. If sufficient friction is present, the Sprag Finger's bite will cause the Alternator Shaft to spin.
  • The Alternator Shaft then in turn, spins the crankshaft and ignition begins.
  • Upon ignition, the Alternator Shaft begins to spin faster than the Sprag Fingers. Centripital force causes the Sprag Fingers to recede into their housing and away from the Alternator Shaft.
  • The Alternator Shaft is now allowed to spin freely.

In reality it is a little more complex than this, but you get the idea.

Last edited by Alterna; 05-04-2009 at 10:36 AM. Reason: Clarify
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post #3 of 106 (permalink) Old 05-04-2009, 11:17 AM
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In my post (woe is me) i said i thought my sprag had gone.As it went
clunck and all i got when i pushed the starter after that was a free spinning startermotor.
going by your post you said startermoter goes by alternator and sprag to turn engine.Do you think it could be a brocken alternator shaft bolt that could be my ploblem ??
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post #4 of 106 (permalink) Old 05-04-2009, 11:51 AM Thread Starter
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Part III: Sprag Clutch Repair

Sprag Clutch Replacement will not be provided, as that topic has previously been discussed by others. I will discuss how to fix your busted sprag assembly.

The following will hopefully save you $200.00US in parts, and/or $1500.00US in service fees at your dealership. If you decide this is the way to go for you, please remember you do the following at your own risk.

It should go without explanation, that to repair your sprag clutch, you must first remove your sprag clutch.

Once removed, you will notice that the sprag clutch appears to be in working order. In other words, it will only turn one-way, and bind when attempting to spin the opposite direction. Trust me, it's junk!

Go to the auto parts store and get:
  • An M6 hex-head Allen Wrench.
  • "Green" loctite.

Do not attempt to use the hex-head wrench in a traditional manner, as it will most likely be futile. "Green" loctite creates a bond which is truly a pain to break free.
  1. Notice that there are (6) M6 hex-head bolts on the backside of the sprag clutch.
  2. Put the hex-head wrench into a vice, or small diameter pipe.
  3. Hold the sprag clutch in one hand.
  4. With the other hand, insert the hex-head wrench to one of the bolts.
  5. Now with both hands, push down on the sprag clutch, until the bolt unseizes from the loctite.
  6. Repeat 5x.

Carefully slip the outside sprag from the inside sprag. Do not dislodge the needle bearings in the inner sleeve. Now, remove the inner gear. It will look very similar to Losiu's picture @ advrider:


Remove the sprag spring, which surrounds the fingers. DO NOT REMOVE THE FINGERS. THEY GO IN ONE-WAY ONLY!!!

Notes:
  • Take spring to auto parts store, ask the parts counter to show you some shaft seals.
  • Most domestic shaft seal springs are 2 sizes (approx. 1,2mm-1,5mm) in width.
  • You are going to need the thinner spring(approx 1,2mm). The width is the most important measurement, as the spring must ride within the groove surrounding the sprag fingers. The one I used was for a Toyota (Sequoia?).
  • The spring can be too long, just unwind where it is joined, and cut it to fit-Just BE SURE to cut the fat end of the spring and not the skinny end, as you'll need that end to wind back inside.
  • I wound quite a bit extra spring into itself, and used green loctite on the ends to be absolutely sure of a tight fit. I had/have no idea what the original tolerances/lengths are, so I adjusted tighter than what I originally found, but not noticably stretched so that spacing was evident in spring coiling.
  • When done, my spring band initially stretched about 0,25cm over the wider part of the sprag fingers. Rolling it to the middle of the fingers, the spring band seated in it's groove with a snap.
  • Slide the inner gear into the outer gear.
  • "Green" loctite the (6) M6 bolts, and button the two-halves back together. I have no idea of torque spec, so i just tightened them down as tight as I could in a star pattern.
Results:
My Daytona's starting has never been so solid feeling.

(thank you to losiu from advrider for the good pictures)
You can get more pictures from his post here: http://www.advrider.com/forums/showthread.php?t=222515
PhilipG likes this.

Last edited by Alterna; 05-04-2009 at 12:01 PM. Reason: Clarify
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post #5 of 106 (permalink) Old 05-04-2009, 11:58 AM Thread Starter
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Quote:
Originally Posted by bjbiker View Post
In my post (woe is me) i said i thought my sprag had gone.As it went
clunck and all i got when i pushed the starter after that was a free spinning startermotor.
going by your post you said startermoter goes by alternator and sprag to turn engine.Do you think it could be a brocken alternator shaft bolt that could be my ploblem ??
You're backwards. Rather, the starter motor engages the sprag. Then, the Sprag engages the Alternator shaft. To me, it sounds like the spring in the Sprag Clutch has slackened.
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post #6 of 106 (permalink) Old 05-04-2009, 01:49 PM
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Thanks for the replie
Don,t know if i am brave enough to do all that my self .

reguards
bj
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post #7 of 106 (permalink) Old 05-04-2009, 04:25 PM
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Excellent post, Alterna.

Reading the advrider and the relative ease of pulling the KTM sprag, makes you want to tar & feather T3 designers.

Cleaning out my T3 spares - Tokico 6 pot calipers, T3 (Daytona/Trophy/ST/Sprint/Trident) headlight switch, search classifieds for my id
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post #8 of 106 (permalink) Old 05-04-2009, 09:37 PM Thread Starter
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DJW: I see you're in Texas. What part?
I grew up in Big Bend, but now live just North of Galveston.

Yeah, our sprag removal could be on an episode of MP's Flying Circus.

Now I'm off to recharge the battery on my Indian Chief. Loaned it to a friend who forgot to turn the key to "off", and close the fuel petcock...Doh!
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post #9 of 106 (permalink) Old 05-04-2009, 10:04 PM
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Quote:
Originally Posted by Alterna View Post
DJW: I see you're in Texas. What part?
I grew up in Big Bend, but now live just North of Galveston.

Yeah, our sprag removal could be on an episode of MP's Flying Circus.

Now I'm off to recharge the battery on my Indian Chief. Loaned it to a friend who forgot to turn the key to "off", and close the fuel petcock...Doh!
Sadly, I'm the the dfw metroplex, but in a good part close to the exit sign. Was in Galveston a couple times last year, not much Galveston in Galveston now. If I find an excuse to move further south I might take it.

Yep, my TR7RV has real bike manual controls too most don't remember

Cleaning out my T3 spares - Tokico 6 pot calipers, T3 (Daytona/Trophy/ST/Sprint/Trident) headlight switch, search classifieds for my id
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post #10 of 106 (permalink) Old 05-04-2009, 10:32 PM
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Haha.. tar and feathering!

Quote:
Originally Posted by djw View Post
the relative ease of pulling the KTM sprag, makes you want to tar & feather T3 designers.
Yes, great post Alterna. I will soon be in the position to do this. Thanks for the write up.
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