Miraz Posted January 22, 2003 Share Posted January 22, 2003 Picture here I've just pulled the diff out of my car to discover that one of the bottom diff mounting bolts has sheared off in the diff at some point in the couple of months since I last checked it. Has anyone had this problem before? Any idea what I can do stop it happening again, or what might have caused the failure? Link to comment Share on other sites More sharing options...
EFA Posted January 22, 2003 Share Posted January 22, 2003 I've had it. No real cure. The bolts are a std bolt machined and re-tapped by Caterham, hence not available in higher tensile elsewhere. Arnie Webb The Fat Bloke back @ 512k Link to comment Share on other sites More sharing options...
Miraz Posted January 22, 2003 Author Share Posted January 22, 2003 Is this a regular problem? Link to comment Share on other sites More sharing options...
AMMO Posted January 22, 2003 Share Posted January 22, 2003 Sorry, I don't understand. Is it one through bolt with a nut on one end or two separate bolts? If one long through-bolt maybe you can't get it in high tensile. If relatively short bolts they should be available in 12.9 cap heads. If an oddball thread and pitch and if they are bolted into a threaded portion of the diff then you can always change the thread and pitch to something more common and use a 12.9. You can do this with a helicoil, timesert or custom made insert. Thanks for bringing this to our attention guys. I will check mine as part of the winter jobs. AMMO Link to comment Share on other sites More sharing options...
Miraz Posted January 22, 2003 Author Share Posted January 22, 2003 It's not the long through-bolt at the top-rear of the diff, it's one of the two shorter bolts at bottom-front that are attached to the threaded portion of the diff. A high-tensile replacement sounds like a sensible way forward.... Link to comment Share on other sites More sharing options...
AMMO Posted January 22, 2003 Share Posted January 22, 2003 I'll take mine out tomorrow and check the threads. I would have thought it would be metric. I'll check with my fastener supplier and post any findings. AMMO Link to comment Share on other sites More sharing options...
scooby dooby doo Posted January 22, 2003 Share Posted January 22, 2003 HOW did you break it 🤔 You've only been running.... 180 bhp 🤔 Is it the extra loadings / leverage of the wider rear track do you think 🤔 Or have you hit a kerb 🤔 *confused* HOOPY 500 kg R706KGU Link to comment Share on other sites More sharing options...
F355GTS Posted January 22, 2003 Share Posted January 22, 2003 Geoff I found one snapped on mine when taking the Diff out a few weeks ago It's an M12 thread I believe but the untapped part is a slightly bigger diameter making the bolt slightly stepped, I've got a new one here if you need it urgently Mark Link to comment Share on other sites More sharing options...
Miraz Posted January 22, 2003 Author Share Posted January 22, 2003 Mark - no urgency for a replacement, I'm taking the chassis up to Arch tomorrow to get a few little jobs done and I don't expect to get it back for a week or so. Hoopy - The bigger chassis shouldn't make any difference to the load on the diff mountings...unless something is very wrong. Ammo - interested to see what you come up with... I've not been putting a lot of power through this, although I have done a few trackdays recently....anyone else hit this problem? Link to comment Share on other sites More sharing options...
EFA Posted January 22, 2003 Share Posted January 22, 2003 Like I said, Its a cheap M12 bolt machined down. Guess little attention is paid to the relief of stress at the taper point hence the shear. I don't think its load related - I have run these bolts for years before. The one I had (which broke exactly as yours has Geoff) was actually quit new! Arnie Webb The Fat Bloke back @ 512k Link to comment Share on other sites More sharing options...
Mr Locust Posted January 23, 2003 Share Posted January 23, 2003 Plated items can be prone to brittle failures if not correctly de-embrittled as part of the plating process. Ref the recent thread which led to the 'r'-clip safety replacement recently. Most of the fasteners I have fitted so far have been plated. Link to comment Share on other sites More sharing options...
R2D2 Posted January 24, 2003 Share Posted January 24, 2003 It is only generally chrome plating that tends to give rise to this problem. The "hydrogen potential" used during the chroming part of the process can cause "nascent" or atomic hydrogen to form on the surface of a component. If you are plating a reasonably high strength steel the individual hydrogen atoms can diffuse into the material. When hydrogen molecules form the internal pressure that can results has a tendancy to form small cracks which then result in good sites for fatigue crack initiation. You don't tend to get this problem with mild stell beacause the crystal structure is different and there tends to more "space" available. I wouldn't be too concerned about this problem with zinc electroplate and with bolts that are not very high strength. The failure of this is more likely top be due to fatigue and changing the bolt for a better grade seems a reasonable idea. Link to comment Share on other sites More sharing options...
AMMO Posted January 24, 2003 Share Posted January 24, 2003 Miraz I've just taken the bolt out of my diff. It's a plated hex head bolt with a 1/2" shank and an M12x1.75 metric thread. This is probably because the metalastic bush in the chassis is imperial and the diff is metric. The bolt is either a one-off or a 1/2" bolt that has been screw cut to metric. The easy fix to put a stronger bolt in is to helicoil the diff to 1/2" and use a 12.9 cap head bolt. This way there would be no step in the bolt, which from your photos is where the bolt failed. Regarding plating I was told not to plate 12.9 high tensile bolts as it weakens them, so I have never done it. If they start to look a bit grotty I just replace them. Hope this helps. AMMO Link to comment Share on other sites More sharing options...
rgrigsby Posted January 24, 2003 Share Posted January 24, 2003 Geoff, It's a bit wierd as I've done 50+ track days in the last 3 years and loads of other mileage yet the bolts in my diff were fine when we removed the diff a few weeks ago, I've only got 155bhp but even so the car has been driven pretty hard on track. I wonder if there is some other reason for their failure? Cheers Rob G www.SpeedySeven.com Link to comment Share on other sites More sharing options...
EFA Posted January 24, 2003 Share Posted January 24, 2003 Commonality of failure here: They only break if you're a fat bastard. Clearly plating has nothing to do with it. Arnie Webb The Fat Bloke back @ 512k Link to comment Share on other sites More sharing options...
R2D2 Posted January 24, 2003 Share Posted January 24, 2003 The plating debate is quite interesting. Most high spec. aircraft fastners are still cadmium plated and there are two main benefits associated with this approach. Cadmium and aluminium don't have a strong potential to form a battery, if plain steel bolts were used the aluminium near the bolt would corrode quite badly, salt water would make it even worse. Cadmium is supposed to "lubricate" the head of the bolt during tightening and give more consistent results when checking the ratio of torque to bolt stretch. Plain steel bolts are quite poor in this respect particularly if the head is put into contact directly with aluminium. Most aircraft bolts are electroplated and they are then heated to aorund 175 deg C for 2-3 hours to eliminate the "hydrogen" embrittlement that can occur. In reality the cell that is used for the plating has to be poorly regulated for this to be a real problem Very high strength bolts are not e cadmium plated they are coated using vapour deposition and are hideously expensive. I think that only aircraft component manufactureres can get a licence to cadmium plate these days. Zinc is chemically quite similar to cadmium and shouldn't cause too much of a problem. I would imagine that failures must be fatigue related and I wouldn't mind having a good look at the facture surface of one of the broken bolts. The photos already posted aren't quite detailed enough and i don't know how to enlarge them. Link to comment Share on other sites More sharing options...
F355GTS Posted January 24, 2003 Share Posted January 24, 2003 Chris If I can find my failed one I'll send it up with the wiper motor Mark Link to comment Share on other sites More sharing options...
Miraz Posted January 24, 2003 Author Share Posted January 24, 2003 Chris - if you look at the page containing the photos, there is a "full size" link in the top right hand corner of the screen which will bring up a much larger version of the photo. Link to comment Share on other sites More sharing options...
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