24 replies to this topic

[mini13]

Yep,Not an application for a stainless bolt.

 

actuallty Ive never liked that little bolt in this application anyway, when using aftermarket tie rods where theres more meat at this joint, I drill oversize and use a cam6126 998 bigend bolt and A+ head nut.

Edited by mini13, 30 September 2016 - 12:59 PM.

[tiger99]

Depending on the grade of bolt, that may not be good. Engine bolts work in tension and higher grades like 10.9 or 12.9 are ok. But the tie bar bolt operates in shear. A 10.9 may be ok but a 12.9, although having more tensile strength, is far too brittle.

The head nuts are usually grade 8 and will not be appropriate for a higher grade bolt. You can get the situation when tightening, when the nut is dangerously close to stripping. With a correct combination of nut and bolt, the bolt always fails first, in tension, so you get very clear warning of the problem.

Also, is there really enough metal on the lower arm to drill safely? Maybe there is on an uprated arm, but there is not always very much on the standard part.

[mini13]

good point, I always thought the bolts we 8.8 as an upgrade was to use capheads, but that might have been pre A+,  they are marked WWE and T I assume T is the grade as I know R and S are grades but cant find a refference to T...

 

I do remember there was tonnes of meat around the hole on my arms, think i might swap the bolts out for somthing 8.8 that I know what the grade is.

[Cooperman]

If you do a calculation of the ultimate shear strength you will find that it is in 'double-shear' and to snap that bolt would probably mean a huge amount of damage to the sub-frame. It would almost certainly snap the 'ear' off through which the front of the tie-bar is mounted.

I have seen many of the 'ears' broken off, sub-frames bent, but I have never, ever, seen one of those bolts sheared off, even in big rally accidents.

I'm not saying don't bother to fit the correct bolt, of course you should, but let's not go overboard with this.

[tiger99]

Yes, a standard 8.8 bolt, of the correctly fitting dimensions, is more than up to the job. 8.8 bolts are tough, rather than brittle, and are the correct thing almost everywhere on the Mini, certain engine parts excluded.

 

We have in the past seen people talking about fitting full sets of stainless bolts. I wonder how many of them are no longer alive, or are too embarrassed to come back and admit their mistake, as a result of a critical bolt failing somewhere? A2 stainless is too weak in tension and A4 is too brittle. Neither have adequate fatigue ratings.

 

Seat belt bolts are required by law to be 7/16" UNF grade 8.8 or better, which completely rules out stainless there. Metric are not allowed, the coarser thread means greater root depth and therefore less strength. The IVA Testers Manual allows 12.9 bolts, which I believe to be a serious error as the primary load is often in bending/shear and a ductile bolt will be safer. I would use 10.9 if I wanted a caphead.

 

I often wonder about the bolt kits sold on Ebay. Personally, I would go to the likes of Namrick, where you know what you are getting.

[mini13]

yeah those ears are a bit weak, Ive done the usual HTMYM triangulation gusset on there,

[tiger99]

Gusset? Any chance of a picture? I have never heard of this but may want to do it one day.

This is one area of the Mini that I don't like. I have other ideas but they will keep till I get my workshop and can make things again.

[tiger99]

Oh, maybe I misread. You meant a gusset on the tie rod front mounting ear? I was thinking about the lug on the lower arm.

I want to improve both in the long term...

[Spider]

Getting well OT now, but sorry guys the Tensile Numbers you are quoting all seem to be metric numbers and don't readily translate to imperial tensile numbers.

[tiger99]

That is because in the UK new UNF bolts are marked 8.8 when in fact they are grade S etc. I know it is probably contrary to the ISO standards but that is how it is, and the actual material properties are the same. In earlier Minis the bolts were marked S, later the same bolts, in UK production at least, were marked 8.8.

I would be extremely interested to know the story behind why this happened.