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Stainless bolt tensile strength?

Gary Stockton

Super Moderator
Sep 10, 2012
Country Flag
Hi all,

On a recent trip to Wales I managed to whack the head off one of the bolts holding my bash plates on, resulting in a lot of fun and games on Saturday to extract the remaining bit of bolt from the chassis (mostly successful ;) )

So I now want to replace the mild steel bolts that came with the plates with stainless ones that are high-tensile. If I were to look at non-stainless, I'd go for something rated 8.8 IIRC, but waht is the stainless tensile equivalent - I can get something labelled 'A2' and STAINLESS STEEL DIN933

How do these compare, or should I just go with the high-tensile zinc-coated?

Come in, Dr Wildsmith ... :D
Shouldn't 10.9 rated bolts be used for cars. AFAIK 8.8 is pretty much what you can buy at B&Q and It's not all that strong. Ok for domestic/ household use but i wouldn't use them on a vehicle.

I'm not clued up on the stainless side of things so cant help there.

Apart from the benefit of not being as prone to rust, does stainless offer any strength advantage over mild steel?
You might consider having domed head bolts Gary with something like recessed allen or torx type fixings. Difficult to knock a domed bolt off.

You might think that the head might get peened over. It doesn't tend to in my experience. Thing with MS is that it's easier to weld onto if you have an issue. OK stainless is easy to weld too, but not everyone has the kit to do that. Ask Gav about stainless bolts. They are a right pain to do up unless they are lubricated. Dunno what they'd be like to undo after going off road. Especially at the side of the road on an overlanding trip!

I would think that you'd need to be careful not to over tighten bash plate bolts. If they are highly stressed then bashing them might perhaps cause them to shear. Especially if they have been used several times. I'd rather use an anti vibration washer and not do them too tightly. And make sure they are greased when they go in. Not with copper grease though, just normal grease.

Gary Stockton said:
should I just go with the high-tensile zinc-coated?
I think this is a good idea ;) Depending on the grade of stainless, you run the risk of galvanic corrosion - try getting a nice stainless bolt out when it's effectively welded itself into the chassis or cross-member :o
A2 stainless spec

Grade comparisons

'Fraid I haven't read the above links Gary but they seem to give penty of figures...

On a purely practical note, if you fit stainless bolts, they will be harder to drill out if you snap them in the future...

Can you countersink your protection plates & use a countersunk bolt that would be flush?

If you do go for stainless, be wary of the threads binding up when you tighten them up :shock:

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I was thinking of counter-sinking the holes, but they're all slotted (I was going to go with counter-sunk allen-key bolts ...)

I hear you on the over-tightening - and as I've had the plates on and off a few times I wonder if that's not what was a partial cause - the corresponding bolt on the other side is ruined - totally warped and stretched ...

I was keen on the stainless aspect purely from a rust POV - a Scottish winter looms, and if they salt the roads like mad this year it could be fun. Also not sure about the combo on ally plate, stainless bolt and mild steel chassis, and electrolysis between them all :roll: Doubt it's an issue, but the plates certainly seem to show some signs of it. Maybe it's just from the salt ...
ha - we're all over this one then - multiple concurrent postings - love it :clap:
Who's going to beat me to this post I wonder...

Once the plates are on with hex head bolts, you could go round countersinking with the plates in situ if you can get under el camino with the drill...

Fit & forget :D

I'm note sure how long it takes for these galvanic (??) reactions to start TBH...
From Gav's links above, it seems 8.8 bolts at =<16mm are rates at 800nm and 640nm stress at permanent set. There are 3 corresponding scales for stainless, depending on the stainless composition (bit like Animal House, this ...) and essentially A2 on the Austenitic scale is equivalent to 8.8 stuff.

You see how much useless information you can accumulate here - isn't it great :lol:
When we use stainless bolts for work, its generally A2 that is used... Not from specification by use like, maybe its just the easiest to get hold of... :roll:

Let me know what you need bolt wise Gaz - I can always post you some out... We've got hod loads kicking about downstairs :whistle:
If I could weld ally, I could weld you some little protection bumps in front of the bolts. Now, I wonder if you could get what amounts to a cup washer Gary, that has raised edges that the bolt head then sits in. You could machine some up but I wonder if they already exist. They could be swapped every time you take the bolts out if they have had a battering. If you had them made up, they could be as thick walled as you like.

What grade are the bolts that are in there now Gary? I wouldn't expect 8.8's to be shearing just from scraping a bit of rock unless they're standing very proud :) Stainless has it's own problems so I'd stick with coated HT bolts.
Mild steel IIRC. There is also a dent in the plate in front of that bolt and a massive scrape - I remember hitting hard and thinking 'ooops' !!
Gary Stockton said:
I remember hitting hard and thinking 'ooops' !!
Where was that then? I remember you scraping a bit after the axle twister on Sunday but that shouldn't have bust anything. At least they're earning their keep :mrgreen:
Saturday in the dark on the way out the bomb-hole ;)

Yes they're doing ok but starting to look battered now. The front suspension is in, hooray, so that should help a bit now. Picked up an extra 40mm clearance :p :clap:
Why did we even have to ask??? C'mon Gaz, don't be shy... :p