Tuesday 10 June 2014

Sprag Clutch - Demystified


To start this thread with the correct mindset about CI engine bulls (like mine), lemme make this very clear:

" Bullets break down. Bullets get back up again. It just gotta be with a person who is patient and knowledgeable. "

Now if you have been following my posts or have seen my bike, it's a no-brainer to guess this baby (forgive me for saying this about the Bull which enjoys a macho status in India) is taken care of well. Well.. just to brag a bit more about our relationship, lemme say we take care of each other well, through thick and thin. It never gave up on me during any big rides and I return the kindness by taking care of its needs at the right time. Actually, it's quite common if you consider any biker who loves riding. There, I said it :D

Last Year:

Now before the eyeballs start rolling, let's get back to the topic; Sprag clutch. Well, I got to know about it a year back when I was trying to switch start the bike for going to work. There was this loud (relative, to other noises I usually hear from the bike) 'clang' from the engine (wasn't sure from where) when the switch didn't really start the bike. Though knowing bullets are an ancient tech, a noise once in a while is nothing new. But a metal-hitting noise was a bit too much for me to ignore. So I called up my mech and got to know it could be the starter gear and its nothing to do with the engine. That was a relief. So I decided to continue with my day and get that noise checked once I get some free time.

During the next servicing, I got the following update from Sami (my previous mech):

The starter gear (that's what my then mech referred to it as) slipping mechanism is faulty and the noise comes when the engine backfires. So every time I switch to start (or even kick) and if it causes a 'kick back', the crank is trying to rotate the starter gear backward (which should ideally rotate in only one direction and slip in the other). As the slipping mechanism is not working, the starter gear is rotating during kickback. It has to be replaced, however, I can continue using it and in case of a breakdown, kick start can be used.

Upon searching online for more details on the sprag clutch I came across the following post which made some sense to me:

Royal Enfield recommends 420ml of oil in the primary case(clutch case). While this is the right amount of oil for a CI/AVL Bullet without an electric start, it is insufficient for Bullets with electric starters. The reason is the fact that bullets with electric starters need more oil to lubricate the sprag clutch bearings and gear assembly for the electric start. 

Bulleteers in the UK and the USA have been instructed by their dealers to use 1 liter of oil in the primary case since a large number of sprag bearing failures have been attributed to lack/insufficient lubrication. So, the Bulleteers abroad have been following this advisory with good results. 

As far as my little knowledge goes, Royal Enfield India for its part has not circulated this advisory in India either through the dealers or through any means of communication, amongst the large number of owners who own electric start-equipped CI/AVL Bullets. 

I had a word with a well-known and highly respected Royal Enfield dealer and mechanic from the US of A, who strongly advised the usage of 1 liter of oil in the primary case. Hence, I thought I'd let fellow bulleteers know about this as this could save many sprag clutch/electric start failures which otherwise could have resulted due to the lack of sufficient lubrication.

Note: This advisory does not apply to the Unit Construction Engined(UCE) bullets.

I brought this topic up with Nandan (my mentor in bullet maintenance, whom I approach occasionally when I need to know the engineering-centric explanation for certain bullet behavior, which is usually beyond a mechanic's cup of tea), and here is what he had to say:

What is good for preventing jamming, will be bad for gripping.
The sprag itself has to grip in one direction &  slip in another.
The clutch has to grip well, or else you may as well buy a 100cc bike
with mileage-problems!
And even with small amounts of oil in the bike, it leaks.
In my opinion, extra oil is too much headache for a very small problem!
The best way is to avoid the starter and learn to kick-start the bike.
You can buy an ignition unit...the OLD model TCI, which did not skip a
spark like the new models.
It will make starting even easier. This unit can be kept as a spare.

After the above discussion, I continued with my rides on my bike, and other than occasional 'clangs' nothing really happened. 

Last Month:

I was looking for a new mech (as the old one is no good for my work anymore), and I came across Nizam from BSK. Very humble guy and currently the chosen one to put their hands on my bike. Even he told the same fear of a defective sprag clutch. Better to remove it for my Ladakh ride or replace it completely. However, he would still have a look at the clutch during servicing and let me know of any hope of repairing it.


As part of some fabrication work getting done for my fuel tank, I was speaking with Imran (a well-known bullet mechanic who also travels with groups as a support mech). Even he suggested removing the 'sprag clutch' for the trip and using a kicker. That's the safest thing to do and commonly done by other CI bullets as well. Apart from that he was quite impressed with my bike's looks and the condition of the engine ;)

Reference Links:
The best and simple description of detecting known behavior of a bad sprag:


Parts Diagram of Sprag Clutch Assembly:

Picture of the spare parts:


So I have decided to remove my sprag clutch assembly for my upcoming ride to the mountains. I will try and record the removal process for my own reference.




Govind said...

Can you please share the parts numbers or name of the parts in the above diargram my motorcycle missing those parts... please

Unknown said...

Can I get this type oneway clouch