Z25 Bolt Sheared - Post Mod!

B1gneil · Aug 28, 2016

Just in the middle of 7k mile service of the Nelli. Valve shims all out, reversed then measured again as they were all dimpled. Following Engenia's xl service plan I decided to whip off the alternator and check the z25 bolt was tight even though I had no starting issues.

Was shocked to find the head of the Z25 bolt and Belville washer fall out onto the crankcase and we all know that is not good!!!! (Polite version)

Took the clutch cover off to inspect likely damage in the clutch area. The z25 shaft was protruding from the post mod z25 gear causing a slight bur on the inner edge of the large clutch basket gear. That would explain the slightly hairy magnet sump plug when I drained the oil.

Lesson learned so far - Pull the alternator off every service and check the z25 retaining bolt and alternator shaft nut.

So I really need some advice from the experts out there

Has anyone ever heard of a post mod bolt shearing before?
What could have caused it to shear - over torqued, fault in bolt metal????
Can I knock the shaft out from the alternator side with a 13mm bar, leaving the z25 gear in place?
Should I replace the z25 shaft with new if I can get the remain of the bolt out?
Finally the worrying question -should the main clutch basket gear have any play - it rocks slightly like when you find wheel bearings goosed? If it shouldn't rock - what is the likely cause?
Will definitely need the alternator cush drive gear seen in the picture - wonder if they are readily available?

I put a lot of effort into ensuring that all the relevant mods had been done which was confirmed at the 600 mile service - hence worried to find this.

Engenia · Aug 28, 2016

B1gneil said:
So I really need some advice from the experts out there.
Has anyone ever heard of a post mod bolt shearing before?

Yes you are not the first

What could have caused it to shear - over torqued, fault in bolt metal????

Benelli supply that bolt with a shortened head which will weaken it, although yours broke further up the shaft, so it was probably a re-used bolt or it had a fault.
Replace it with a standard R8.8 M10 x 1.25 x 25 set screw from your local bolt shop and use the OEM Belville washer. Don't be tempted to add a flat washer. That defeats the Belville washer and will cause it to loosen.

Can I knock the shaft out from the alternator side with a 13mm bar, leaving the z25 gear in place?

Yes, that's the only way to get it out.

Should I replace the z25 shaft with new if I can get the remain of the bolt out?

If it isn't damaged you should re-use it

Finally the worrying question -should the main clutch basket gear have any play - it rocks slightly like when you find wheel bearings goosed? If it shouldn't rock - what is the likely cause?

The gear can have some play in the rotating direction if the cush springs aren't tight, but if you are talking about movement perpendicular to rotation that should be barely perceptible. I have a perfectly good '03 basket that I just checked and all I can see is an ever so slight movement of oil between the basket and primary gear. If yours has more than that the rivets (three) may be loose. If that is the case, I don't know how to tighten them.
Colin has had one apart, so he may be able to advise.

Will definitely need the alternator cush drive gear seen in the picture - wonder if they are readily available?

I believe so. Talk to Maniac or Bennets.

B1gneil · Aug 29, 2016

Thanks for the quick response and advice Engenia.

Waiting for a clutch holding tool to arrive that I ordered of fleebay. Then plan to get the clutch off and pull the z25 shaft out completely to examine and remove the broken bolt. Hopefully will be able to recover it and just fit a replacement bolt as you advised.

Will keep the thread running with any info I think will help.

Engenia · Aug 29, 2016

You don't need the tool if you select top gear and jam the rear brake on.

B1gneil · Sep 1, 2016

So managed to strip the clutch down tonight. Basket is post mod and all secure with no damage. Found that the axial play in the clutch is caused by the play between the needle roller bearings and the 'spacer' sleeve that the bearings run on. No signs of damage or wear?

Found the following thread which seams to indicate axial play as normal? No difference in axial play After replacing the bearing's with new.

http://www.benelliforum.com/forum/tornado-tre/10361-revised-clutch-basket-bearings-uh-oh.html

Can anyone else confirm if this axial play is normal.

Redbaron57 · Feb 22, 2017

Alternator drive failure on 2013 TRE K

I bought a 16,500km 2013 TRE K a month ago. A horrible clatter at low revs got the better of me so I proceeded to investigate. I found both alternator drive coupling halves were loose on their shafts, despite the alternator nut and Z25 shaft bolt being held in place by the Loctite. With help from this forum I ordered new bits from Maniac Motors and proceeded to consider the root cause of the alternator drive failure. After reading everything on this forum and finding via the Triumph forum that those triples have the same problem, I have resolved to do a couple of things differently on reassembly. As I see it there are two issues: 1/. Spline clearance: Inertia effects subject the alternator drive to torque reversals on engine compression and firing cycles. If the centre bolt generated enough friction to avoid slipping under these reversals there would be no need for a spline, but it doesn't, so there is! It stands to reason therefore that the splines should have an interference fit, or at least zero clearance when clamped in position, to prohibit slipping when the torque reverses. If the splines on my new bits fit with even the slightest clearance I will Loctite them (and worry about disassembly only if and when the need arises). 2/. No Free Bolt Shank: A bolt is in effect a spring that clamps faces together. It works best if it has some free length that can be stretched as the bolt is tensioned. That provides tolerance in bolt tension and ensures tension is maintained under dimension changes due to temperature variation. On the Benelli almost the entire bolt is bound by the thread inside the Z25 shaft. There is no unrestrained bolt shank free to elongate under tension, so there is hardly any margin between under-tension and over-tension. Benelli introduced the Belville washer in an attempt to overcome this issue. That has proven inadequate by the failure on my bike and the bolt failure that is the original subject of this thread. In my opinion it would be better if Benelli counter-bored the Z25 shaft for some distance at the outer end, before the thread starts, to provide some unrestrained bolt length. Rather than risk messing-up a new shaft I propose to achieve the same result by turning the thread off a new bolt for some distance under the head. This will allow unrestrained elongation. I think a 30 mm bolt length will work, so I propose 15 mm unthreaded shank, leaving 15 mm thread on the end of a M10 x 1.25 mm pitch Class 10.9 bolt (with the head machined down to 5 mm high). I proposed to use 60 Nm torque, which should be about 1/3 turn from snug, as recommended for this size and class of bolt. I will run a trial on the old shaft first to confirm the thread can take it. I proceed in anticipation that my alternator drive will finally run quietly and be found tight on subsequent checks. Anyone who reckons I am barking up the wrong tree is welcome to explain the flaws in my reasoning.

Kevin S · Feb 22, 2017

If you have trouble getting the alternator cush-drive rubbers you can try buying Kawasaki ZX750N rubbers (Kawa part number 92075-1790). They fit and work just fine.

Redbaron57 · Feb 22, 2017

Kevin S said:
If you have trouble getting the alternator cush-drive rubbers you can try buying Kawasaki ZX750N rubbers (Kawa part number 92075-1790). They fit and work just fine.

Yep. Thanks for the tip. I have already been down that road but I decided to get Triumph rubbers from the UK - the price was the same and for little extra the o-ring was included. I do like the look of the Kawasaki rubbers though. They appear to have extra ribs to ensure a tight fit. That is important to avoid the slack that results in impact and sees them hammered to bits. Maybe I will use them at the next service. I now wish there was an alternative to Benelli gaskets. The price of those is outrageous (valve cover and clutch cover). I did try to find gaskets from a Chinese supplier but they only seem to offer bits for the smaller bikes.

mikerj · Feb 23, 2017

Redbaron57 said:
2/. No Free Bolt Shank: A bolt is in effect a spring that clamps faces together. It works best if it has some free length that can be stretched as the bolt is tensioned. That provides tolerance in bolt tension and ensures tension is maintained under dimension changes due to temperature variation. On the Benelli almost the entire bolt is bound by the thread inside the Z25 shaft. There is no unrestrained bolt shank free to elongate under tension, so there is hardly any margin between under-tension and over-tension. Benelli introduced the Belville washer in an attempt to overcome this issue. That has proven inadequate by the failure on my bike and the bolt failure that is the original subject of this thread. In my opinion it would be better if Benelli counter-bored the Z25 shaft for some distance at the outer end, before the thread starts, to provide some unrestrained bolt length. Rather than risk messing-up a new shaft I propose to achieve the same result by turning the thread off a new bolt for some distance under the head. This will allow unrestrained elongation. I think a 30 mm bolt length will work, so I propose 15 mm unthreaded shank, leaving 15 mm thread on the end of a M10 x 1.25 mm pitch Class 10.9 bolt (with the head machined down to 5 mm high). I proposed to use 60 Nm torque, which should be about 1/3 turn from snug, as recommended for this size and class of bolt. I will run a trial on the old shaft first to confirm the thread can take it. I proceed in anticipation that my alternator drive will finally run quietly and be found tight on subsequent checks. Anyone who reckons I am barking up the wrong tree is welcome to explain the flaws in my reasoning.

I thought long and hard about exactly this when I put my RS back together. The primary problem IMO is torsional vibration in the Z25 shaft, which can be felt at 4-5000 RPM with the engine loaded even with all the alternator drive parts in good condition. I try to avoid loading the engine in this RPM range for this reason.

The further the bolt thread is engaged into the shaft past the alternator coupling, the greater the torsional vibration the bolt will have to cope with; effectively the shaft will be twisting more towards it's middle and will be pretty much static at end where the bolt head sits. This suggests to me that a longer (fully threaded) bolt would likely make things worse, though with an unthreaded, slightly reduced shank the bolt would be more free to twist with the shaft rather than trying to resist it, as well as being better able to maintain clamping force.

It's a shame there isn't space to fit an overrun clutch as this would help significantly to cut down on torsional vibration.

Redbaron57 · Feb 24, 2017

You raise an interesting point Mike, thanks for troubling to respond. My underlying assumption was that the torsional strain in the Z25 shaft is insignificant. You are correct though. However minor, the phenomenon does exist. I hope that friction and Loctite have sufficient strength to absorb the torque developed in the unrestrained bolt length. If my modification fails I will declare your concern vindicated.

Yes, an overrun clutch would avoid the torque reversals, as the car industry discovered. Perhaps with some determination one could be incorporated using a housing spacer and longer bolts but that is a bigger project than I am contemplating. The rubber element coupling would have to be retained to absorb the torsional spikes. The type of spring damper used in the later automotive alternator decoupler pulleys might be a more compact and lasting solution but selection of the correct characteristics would require dynamic analysis my grey, threadbare head is unwilling to consider.

Part of the Benelli problem is that the Denso rubber element coupling design is not ideal. In industry it is understood that the rubber elements must be in compression at all times so that they never lose contact with the coupling blades, as this causes wear and impact. The elements are therefore made larger than the coupling cavity. They must be assembled by inserting half the elements then applying more than the rated drive torque to open-up the remaining cavities and enable insertion of the remaining elements. In the motorcycle situation that would make assembly impossible, thus we have an imperfect compromise prone to failure. However I do not fully understand why apparently the Denso coupling on Benellis have a chronic problem, on Triumph triples an occasional problem and on Kawasakis they last the life of the bike. While four cylinder engines have smaller torque variations at higher frequency that result in lower dynamic loads, I suspect there are subtle differences in the detail designs that could make an interesting investigation.

P.S. It has just occurred to me that one of the aforementioned subtle design differences I think I see, from coupling offers on eBay, is a shorter bolt on some Kawasakis, maybe only half as long as on my TRE K. It could well be that you are correct and I am barking up the wrong tree. A shorter bolt would only engage at the end of the shaft within the coupling spline where the stiffness of that component would prevent significant torsional strain. My bolt is 25mm long and goes into the Z25 shaft 10mm beyond the coupling. The looseness of my coupling and the broken bolt that started this thread could both be explained by rotation in service resulting from the Z25 shaft twisting back and forth. However further examination of the photo at the start of the thread shows the Belville washer had been bearing only on the outer edge, indicating there was not much bolt tension. The failure would therefore likely have been by fatigue due to stress cycles imposed as the coupling wobbled about. A bolt is supposed to be tensioned more than the applied load so it never sees load cycles hence will not fatigue. Interesting question though - perhaps I should buy another TRE K and fit a very short bolt for comparison with my long bolt proposal!

mikerj · Feb 24, 2017

Redbaron57 said:
Y However I do not fully understand why apparently the Denso coupling on Benellis have a chronic problem, on Triumph triples an occasional problem and on Kawasakis they last the life of the bike. While four cylinder engines have smaller torque variations at higher frequency that result in lower dynamic loads, I suspect there are subtle differences in the detail designs that could make an interesting investigation.

Again I suspect this is due to excessive levels of torsional vibration when the engine is under load. After all the Z25 shafts are known to break, particularly the older ones with the oil hole and I suspect this down to very high cyclic torque reversals when the alternator drive train hits it's resonance point.

The primary difference with the Kawaskai engine is that it has four cylinders which give torque reversals from the crank only twice per revolution since all pistons reach TDC/BDC at the same time. A three cylinder engine with 120degree crank creates much higher frequency torsional vibration components, even if the amplitudes are a little lower. The difference between the Benelli and the Triumph is clearly more subtle since the designs are fairly similar. It may be as simple as the resonant frequency of the alternator drivetrain is shifted to less used engine RPM range.

Redbaron57 · Feb 26, 2017

Kevin S said:
If you have trouble getting the alternator cush-drive rubbers you can try buying Kawasaki ZX750N rubbers (Kawa part number 92075-1790). They fit and work just fine.

Kevin, have you found that the Kawasaki coupling rubbers perform or last any better than the originals? The used originals from my bike are very hard and feel more like plastic than rubber. I wonder if part of the coupling reliability problem lies in the shock absorption capability of the rubbers. Thanks, Rolf

B1gneil · Feb 26, 2017

Redbaron57 said:
Kevin, have you found that the Kawasaki coupling rubbers perform or last any better than the originals? The used originals from my bike are very hard and feel more like plastic than rubber. I wonder if part of the coupling reliability problem lies in the shock absorption capability of the rubbers. Thanks, Rolf

I bought the Kawasaki rubbers and even though certainly offered a usable alternative I decided that there was too much movement even by hand let alone with the engine forces that they will endure. So stuck with OEM rubbers. Is this the right decision - my decision was based on the fact that I imagine that there are a majority Nellie's successfu<script id="gpt-impl-0.10316817472314749" src="https://securepubads.g.doubleclick.net/gpt/pubads_impl_110.js"></script>lly running on the OEM units without any problems.

Agree that it would be interesting to know if the OEM rubbers are a regular unrecorded replacement item and those who installed Kawasaki or Triumph items have better longevity?

However I'm sure we all agree it would be fantastic if there was a fix that would eradicate this Z25 issue ever happening or being discovered at any interval afterwards.:clap:

Redbaron57 · Feb 27, 2017

Search for Root Cause

B1gneil said:
I bought the Kawasaki rubbers and even though certainly offered a usable alternative I decided that there was too much movement even by hand let alone with the engine forces that they will endure. So stuck with OEM rubbers. Is this the right decision - my decision was based on the fact that I imagine that there are a majority Nellie's successfully running on the OEM units without any problems.

Agree that it would be interesting to know if the OEM rubbers are a regular unrecorded replacement item and those who installed Kawasaki or Triumph items have better longevity?

However I'm sure we all agree it would be fantastic if there was a fix that would eradicate this Z25 issue ever happening or being discovered at any interval afterwards.:clap:

Thanks for that feedback. I appreciate every perspective. Certainly if that movement was free play it could not be tolerated but elasticity might be helpful. I ordered Kawasaki rubbers for comparison.

It could be that the failure of my bike was simply an installation error but I get the impression from the maintenance schedule and experiences posted on the forum that Benellis have a weakness here. Old habits die hard. I am new to Benellis but in my earlier life, responsible for big machinery where downtime lost megabucks, I learn't that doing the same thing gets the same result. I enjoyed the challenge of finding the root cause and improvements for equipment reliability. That mean't discussing observations with operators and maintenance people, examining the failed parts, engineering analysis and sometimes attaching sensors to measure vibration and actual loads to eliminate assumptions & guesswork. In retirement I have little access to software & equipment to perform proper analysis but at least I can examine the failed parts, find-out about other people's experience, quiz some old contacts then pester the new generation of highly qualified engineers in my family with my hypotheses. Finally I can test a proposal and rejoice in success or cringe in embarrassment, depending on the outcome. Analysis of very failure increases understanding.:doh:

Kevin S · Feb 27, 2017

I haven't put the Kawasaki rubbers to a long term test yet, but here is my experience so far: I checked the standard rubbers after 3,000 miles when I was doing the Z25 upgrade and they were showing signs of breaking down. I couldn't get hold of Benelli rubbers at that time and, as I wanted to get the bike back on the road I refitted the originals. At the 6,000 mile service they were completely shot. I thought it an unacceptable risk to continue fitting standard rubbers if they were likely to to fail so quickly and looked for an alternative. Somewhere on this forum there was a thread mentioning the Kawasaki rubbers and I bought a set. I rode with them for 3,000 miles, inspected them while I was checking the Z25 bolt torque and they looked perfect. Pretty soon I'll be checking them again at 6,000. If they are OK again then I won't bother checking them for a while. As I said, no long term test results, but to date and bearing in mind that this is a test sample of one, I think they are better than the ones which were fitted at the factory.

Redbaron57 · Feb 27, 2017

Thanks Kevin, that is great news!
It supports an inkling that the Benelli coupling elements might be plastic and the Kawasaki elements rubber.
Plastic is very different to rubber. It is cheap to mould but has two characteristics that make it unsuitable for shock absorption: 1/. Its spring constant increases with the rate of strain. 2/. It has little or no natural damping. It therefore does not soften the torque impulses that can overstress the drive when they try to instantly accelerate a large inertia like the alternator rotor. Plastic transmits shock like a rigid element. I visualise it acting almost like glass, hammering the drive and breaking itself into bits.
On the other hand rubber has a pretty constant spring rate regardless of the strain rate and a natural hysteresis that slows down (dampens) the rebound. This makes rubber very suitable as the compression element in a coupling (when sized correctly). It softens the torque impulses on compression and firing cycles, accelerating the alternator rotor more gently. That avoids the torque peaks that hammer the drive components, loosen splines and cause metal fatigue.

Did you notice a reduction in handlebar vibration with the Kwakka rubbers?

Kevin S · Feb 27, 2017

A couple of other things, unlike B1gneil I found that there was no slack in the Kawasaki rubbers, in fact they were really quite snug, which was reassuring. Earlier in the thread you were bemoaning the cost of the gaskets for the primary drive and camber. I agree, fifty odd quid is a lot to pay for a primary drive gasket, however the aluminium composite ones are definitely reusable. The gaskets seem to come in two flavours, one made from a fibrous material which tends to break very easily and is essentially a use once gasket, but Bennetts sold me one which has an aluminium core coated another side with a softer material. I've used it three times now with a very light swear of Threebond 1215 and it is perfectly leak-tight. Similarly the cambox gasket is rubber (nitrile?) and is reusable. I use a smear of the same sealant on the semicircular parts on the cam-chain side and have had good results. I assume that if the valve seat are in good enough nick the chamber is vented well enough and there is very little pressure in that volume

Redbaron57 · Feb 27, 2017

Thanks again Kevin, what you have told me is what I hoped. You have made me a happy man with confidence that my money was well spent and this will be reliable bike when the new alternator drive is installed using Kawasaki coupling elements. I also hope that, as I read somewhere in this forum, elimination of the harshness in the alternator drive will stop the nasty vibration of the right handlebar that numbs my hand. I considered leaving the gaskets off my parts order and reusing the originals but given the high shipping cost I decided to be safe and include them. Your advice gives me hope that this will be an infrequent cost while Nellie and I grow old together. I love Nellie's note, torque and handling so I see no reason to part as long as I can be confident I will not have to walk home. I invested in a top box to carry luggage securely and plan to have the ECU remapped on a dyno to eliminate Nellie's crankiness at low revs. For a total cost of about $10k I should have an excellent bike for my purposes.:clap:

Redbaron57 · Feb 28, 2017

Difference between urethane and buna-n alternator coupling elements

If I am right about the materials used for Benelli couplings vs Kawasaki as mentioned in Post # 16, the following information pinched from the Lovejoy site is relevant. Neither material is perfect but Buna-N is the only choice for shock absorption, although it must be replaced when it has taken a set leaving clearance in the coupling. Note the bits in bold:

1. NBR (Nitrile Butadiene Rubber) a.k.a. Buna-N -- is the standard and most economical material for jaw coupling spiders. It offers the best combination of temperature and chemical resistance, misalignment, and damping ability. Rubber has the best resiliency in bouncing back from deformations that occur in cyclic or heavy shock loads. This is the only material suitable for reciprocating engine applications.

Most sizes of NBR spiders are 80A-shore hardness and are black in color. Temperature range is -40°F (-40°C) to 212°F (100°C). Also referred to as "SOX" by some manufacturers. This material will allow the Jaw coupling to experience a torsional wind-up at full torque load of 4°-10°, depending on the coupling size.

Another attribute of natural rubber products used in compression is that they take a permanent "set" or loss of volume after just a short time in operation. This does not become a performance problem until the spider thickness is anything less than 75% of its original size, at which point it should be replaced. This limits their selection in motion control/precision applications since increased free-play in the coupling results from the "set". Shelf life of natural rubber elastomers is 5 years.

2. URETHANE has a 1.5 times greater torque capacity than NBR due to its greater compressive strength (either 40D or 55D shore hard ness is used) as well as better abrasion/wear characteristics. It holds up better to environmental conditions such as ozone, ultraviolet, and some oils-chemicals versus the NBR. It is limited to -30°F (-34°C) to 160°F (71°C) temperatures however, and should not be used in heavy cyclic or start/stop applications since the damping ability is limited. ..... Urethane spiders ... offer a shelf life of 5 years.

Redbaron57 · Mar 3, 2017

Triumph rubbers too small

Triumph rubbers arrived from UK. They smell like rubber so I presume they are Buna-N, which is good. They are unsuitable for Benelli unfortunately, as they are narrower and longer than required, to suit a smaller diameter, deeper coupling. That results in radial play and probable axial thrust in the Benelli coupling. I await Kawasaki rubbers from Partzilla, USA, the only supplier I found with new stock and reasonable shipping charges.

Z25 Bolt Sheared - Post Mod!

B1gneil

Attachments

Engenia

B1gneil

Engenia

B1gneil

Redbaron57

Kevin S

Redbaron57

mikerj

Redbaron57

mikerj

Redbaron57

B1gneil

Redbaron57

Kevin S

Redbaron57

Kevin S

Redbaron57

Redbaron57

Redbaron57

Top Contributors this Month

Recommended Communities