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Broken cam chain tensioner


prr

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So I had my first issue with the bike. At around 40k kms on the clock the engine become noisier at around 3500rpm. Slowly the noise moved down to 2500rpm but become heavier. As I was not sure if the dealer had checked the valve clearance I decided to check them. It was a bit of surprise to find some "spare parts" in the engine head

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It turned out to be a broken cam chain tensioner

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(Seen in the upper left corner)

 

I ordered a new one and replaced it. As per the service manual to do so the cam head cover should be removed as well as the thermostat. However I managed to remove the tensioner by just unscrewing the two bolts on the block itself. It was a bit tricky as the one under the thermostat cannot be removed but the space was just enough to pull it up and remove the old and to install the new tensioner. Unfortunately one piece of the old tensioner is missing and I suspect it has fallen in the oil pan. 

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The procedure takes about an one and a half hours but it is worth checking the chain tensioner condition once every 15-20k kms ...

 

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That’s the second one of those to go (I seem to think). I would make every effort to get the missing piece out of the oil pan, though.

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Slowboy

Good advice there. Well worth a quick check.

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kayz1

.

Edited by kayz1
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embee

That's the first pics I've seen of a broken one, useful. Need to look at the area which has broken for any signs of cracks in future.

I had mine out to look at it when I replaced the DCT shift pin, didn't see any signs of problems but having not seen a broken one before I may have missed the critical area.

I agree about the idiot who designed the tensioner bolt directly under the thermostat cover …………………..

 

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motorbykcourier

Crikey....

And I've done 200K miles!!

Will check next time I do valves,,,,,,

 

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embee

It's a pretty rare occurrence, of all the NC engines out there racking up the miles there have been only a handful of reports of broken tensioners. It's probably another of those odd production/batch issues like the DCT shift pin, it certainly isn't an epidemic. At least so far the breakages haven't proven to be catastrophic ………………………… (stand back, wait for the bang...……….).

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Guest machinman
15 minutes ago, motorbykcourier said:

Crikey....

And I've done 200K miles!!

Will check next time I do valves,,,,,,

 

Nice to hear from you Mr. Awesome milage. Any secrets??

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Graham NZ

Ouch!  If Steve Lee has been 200k miles without a problem it can't be a common problem.  Maybe bikes ridden a lot at high revs are more likely to have a problem?

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Grumpy old man
8 hours ago, Graham NZ said:

Ouch!  If Steve Lee has been 200k miles without a problem it can't be a common problem.  Maybe bikes ridden a lot at high revs are more likely to have a problem?

NC?- high Revs?😊 ( but I do understand )

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Graham NZ

High revs is a relative term.  In the case of NC engines anything over about 4,500rpm could be considered high revs when 3,000rpm is about 100km/h.  About the same as my Buell.

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Slowboy
On 19/07/2018 at 21:41, Graham NZ said:

Ouch!  If Steve Lee has been 200k miles without a problem it can't be a common problem.  Maybe bikes ridden a lot at high revs are more likely to have a problem?

It's not a common problem, it's an unusual occurrence, otherwise this forum would be full of it. Nevertheless it's a valuable point and thanks to Peter we've been made aware of it. It would make sense to have a look when you've got the cap off to check the valves. If I still had my NC, I don't think I'd be that concerned but I'd definitely check it. There are a significant number of high mileage NCs (well over 50k) that haven't had a problem.

Doesnt make it less annoying if it happened to ones own bike though, granted.

Edited by slowboy
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Graham NZ

I'm about to do the tappets and change the coolant.  What do I look for regarding the cam-chain adjuster?

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Grumpy old man
4 hours ago, Graham NZ said:

I'm about to do the tappets and change the coolant.  What do I look for regarding the cam-chain adjuster?

Saved me asking 

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Slowboy
6 hours ago, Graham NZ said:

I'm about to do the tappets and change the coolant.  What do I look for regarding the cam-chain adjuster?

 

1 hour ago, Grumpy old man said:

Saved me asking 

 

I'd suggest, for starters, removing the tensioner as per the manual and checking the areas, which you can see in the photos on the original post, where the component failed to see whether there is any evidence of cracking starting and replace if there is.

It looks to me like there are two critical points.

Peter, Embee, any other areas on the tensioner worth a look?

Edited by slowboy
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embee

As I said earlier, this is the first time I've seen pics of a failed item so don't know if anything else could fail. It's a bit of a pain to get it out, the bolt directly under the thermostat housing is a real PITA, but if you have the rad off for valve check anyway it's not a big deal to remove the cover. I suspect as the OP said you can just undo the bolt and leave it in the hole if you prefer.

I doubt you'd be able to see much in situ down the tunnel,  which would mean taking it out for inspection. Possibly worth the effort having got that far. The lower end just drops into a slot in the crankcase casting.

The pressed fame of the tensioner looks like it has somewhat undesirable bend/corner features which are stress raisers and in that type of material can lead to cracking, things like that often use steel in a semi-hardened state.

It looks like the top end is cantilevered out above the top bolt fixing pad, maybe a support would be desirable so it effectively rests against the head casting, but that's modification territory.

 

 

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All the pictures I've seen of a broken tensioner the issue is either the pin or on the top part (in my case). So I would not spend time removing the tensioner if visually there are no issues there.

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  • 1 year later...
Black15

I just took my 2019 NC750 in for its 8,000 mile service. It has had a rattle just off tick over since I bought the bike new and was told this was normal tappet noise. Guess what? It is a broken cam chain tensioner.

The mechanic tells me that this is the first one that he has seen.....

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ChrisCB

Chances are he only became a mechanic yesterday!

 

However I've known of three failures, one on my own bike but not on such a late low mileage model.

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Black15

That’s interesting Chris. Was valve timing affected as a result? I am concerned that the low power/higher than normal fuel consumption that my bike has been experiencing could be connected to the tensioner problem?

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ChrisCB
29 minutes ago, Black15 said:

That’s interesting Chris. Was valve timing affected as a result? I am concerned that the low power/higher than normal fuel consumption that my bike has been experiencing could be connected to the tensioner problem?

No valve timing issues, I think it helps that the engines are relatively low revving, I noticed a loud tapping on my own bike.

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fj_stuart

Not really relevant to the NC but my CBF250 had a failed cam chain tensioner a couple of years ago. Unlike other bikes the CBF’s tensioner uses a spiral spring like a clock. This is pretty thin and it snapped. This caused the cam chain to jump on the sprockets and the piston to hit the inlet valves. The bike had done 65,000 miles so I had to think whether it was worth fixing. It was going ok otherwise so I did.

 

The CBF is quite nice and easy to work on. The piston was undamaged and I was surprised to find that the bore did not have any detectable lip on it. Amazing for such miles on a high-revving bike.

 

I wasn’t keen on giving Honda cash for a new tensioner unit. I found a manual tensioner (just an aluminium plate with a bolt) This has worked fine for the past 6,000 miles.

 

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