Tensioner Body Torque setting

Hi Charles, looking at your profile and your handle it looks like you have a GT4. I'm not sure if it's the same on your car, but on my 328 the cam belt tensioner assembly is secured to the head via three M8 x 1.0mm studs/nylock nuts. The 328 WSM doesn't provide a specific torque figure for these fasteners but they do provide torque specifications for other fasteners with the same diameter/thread pitch. Based on that I torqued the nuts to 9 ft/lb or 108 inch/lb. The other thing that I sometimes use when I can't find a specific torque spec in the WSM is a fastener torque calculator. Futek provides one that is pretty good: http://www.futek.com/boltcalc.aspx?mode=metric. As an example, using this particular fastener the Futek calculator provides a recommended torque of 8.7 ft/lb with a maximum torque of 11.5 ft/lb which is right in range with what is recommended for similar fasteners in the 328 WSM. Hope this helps

 

M8 is pretty decent size and 9 ftlbs is not enough. I would go to 25Nm. That is pretty standard for that size fasteners on Ferrari and Porsche.

 

Charles, I'd have no hesitation taking Mitchel's advice. I'm pretty sure he has more real world experience wrenching on these cars than I do.

Mitchel, I looked at several WSMs I have on file including the 328, 355 and testarossa and while there are many M8 x 1.25 fasteners with recommended torque values of around 20-25 Nm (15-18 ft/lb), the few M8 x 1.0 fasteners that are listed show lower torque settings than that. The one exception is the dome nut which secures the water pump impeller to the water pump shaft which is an M8 x 1.0 and calls for 18 ft/lbs. The water pump shaft is hardened steel which I suspect is the reason for the higher torque figure in this M8 x 1.0 application.

For what it's worth, I've performed two major services on my 328 over the 9 years that I've owned the car and I have used 9 ft/lbs for these fasteners both times. During removal of the cam belt tensioner assemblies during the second major service there was no evidence of fastener loosening.

 

Why is this a mystery? It's given directly on page M5 of the 308GT4 WSM:

"...belt stretcher support"
M8 x 1.25
2.5 Kg-m

 

and multiplied by 9.8 gets you pretty close to 25 Nm.

 

I did this yesterday.

Are these 3 nuts supposed to have washers? The studs are very short and if washers are used the stud does not engage with the nylon of the Nyloc nuts. Or maybe the nuts are supposed to be half-nuts. Something not looking quite right here.

 

Since you don't have a Parts Manual/Catalog, go to one of our parts vendor's sites and use their on-line Parts Cataloge.

Sorry, on my phone or I'd post a link or two.

 

If this information were in the parts lists I would not be asking the question.

 

It always helps me to go back to basics - torque specs are based not only on the size of the specific fastener, but also the materials and details of the specific application. Thread diameter and pitch are two important factors, but the materials involved in the joint and the bracket matter just as much, as do the physical conditions, such as temperature, vibration, # threads engaged, # of fastener/joints in the system, etc. Finally, the loading on the joint is a factor - whether an axial tension or compression load is applied, or maybe the fastener is just trying to clamp two materials together so they don't slip, in this case, losing adjustment.

So, importantly, in this application there are three different thread engagements in the system - the stud-to-the-alloy-belt-cover, the stud-to-alloy-engine-block (as I recall, at least one of these studs is anchored in the block and passes through the cover and tensioner bracket), and the nut-to-the-stud.

The torque specified for a given fastener in a lightweight alloy should be different than for the same fastener in steel, or hardened steel, simply because of the physical properties of the materials. So, where the nut-to-stud joint would withstand a greater torque, the axial tension in the stud resulting from that much torque could pull the stud out of the alloy cover, or deform the soft alloy bracket. I use a rule of thumb: always use a flat washer, and never tighten the fastener so much that the washer leaves an impression in the face of the material it's clamping against. If it does leave an impression, it could also be deforming any threads in similar materials, anywhere else in the system.

Also, the number of fasteners in the system is a factor, and in this case, it may be that three were used instead of two, (6 threaded engagements) to keep the torque low enough for each, to avoid failure of the alloy threaded joint, by distributing the required holding force over more fasteners.

Finally, the use of a locking nut is a factor, especially in systems where temperature and vibration are factors. Generally, locking nuts are advisable wherever the torque specified is relatively low compared to the maximum for that particular joint, so that the nut doesn't loosen. (a mild steel nut to a mild steel stud can sustain a relatively great torque, but if it is only tightened to a torque well below its maximum, then the threads are more likely to slip)

This system seems to have been designed with relatively low torques in mind, so it would be advisable to use a locking nut.

Personally, I always have to be careful not to just mindlessly use the general torque specified for a given thread size and fastener material... because that assumes all elements in the system are made of that same material. If the joint I'm torquing is part of a fastening system that includes another threaded joint, I also need to consider that joint in my decision about what torque I should apply.

 

Pretty sure that they should have washers (as rubbing the face of a steel nut against an aluminum casting isn't the best practice). Have a look at the "crankcase" page 2 of the SPC (of all the 308 types, not just 308GT4) -- I think that might be where the nut/washers reside for those. Also, there must be some pictures previously posted of assembled tensioner assemblies in the various cambelt threads. Many washer F uses are only about 0.5mm thick -- even for fairly big diameters.

 

Makes sense. I am going to need to get some thinner washers and "Type T" nuts which are less tall than the ones on there, and which I have new (using new nuts of course).
I think the stud will just about be long enough to go into the nylon.

 

Mine had three flat washers associated with each of the fasteners.
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Is your hardware situation far different from the good photo that Ettore posted? I wouldn't go to a "half-high" nut -- are the "new" Nylock nuts that you have significantly taller that in the photo?

 

They hid it. They are shown on the head diagram instead of the timing belt diagram as you'd expect.
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Sorry, I meant to write that there were three washers, one with each fastener.

Take a look at my photos as well as the prior post with the parts diagram as they clearly show what is supposed to be there.

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Hello together,

a common torque for 8.8 M8 regular thread (x1.25) bolt or stud is 25Nm, if there are not more than two parts clamped together and no longitudinal operation forces adding stress to the bolt.
But pay attention to the fact, that the material of the mating thread is taken into account by the thread depth or better said, screw-in depth. And these studs, Ferrari used for the tensioner body are too short for aluminium.
As a rule of thumb for the thread depth: Thread diameter * x
x steel: 1
x cast iron: 1.2
x aluminium: 1.4

I had one thread damaged and repaired with helicoil. Then I replaced all six with studs from GM. They had the correct ones for their aluminium cylinder heads. Go figure! The thread depth in the Ferrari-head is enough for using longer studs.

Washers are counterproductiive. They just save the aluminium surface around the holes, regarding wear caused by torquing down or undoing the nuts.

Best Regards
Martin

 

The photos answer the question, thanks to Ettore. I need thinner washers and shorter nuts. The ones on there were Type P which are too tall, I need Type T.

It can be seen from the photos there is no "spare" stud length.

 

That's one of their least-confusing mangled Italiano-to-English translations

Seriously, based on some of the bizarre English nouns they've used, it's like the translator spoke perfect conversational English, but had very little background in technical English jargon.

 

By the way Chaps the washers on there are infact spring washers because obviously you don't want to 'drop a nut' now do you.....;-) I actually used slightly shallow nylocks for good measure also...
Regards Bell Bloke