What's a strong bolt?

[fatjames]

I need to replace some m8 allen head bolts. The norm seems to be A2 Stainless. What are my other options? Ideally, somewhere I can buy from tonight or tomorrow. (bristol)
thanks

[Robby]

What are they being used for, and what happened to the originals?

I generally avoid stainless - it makes alloy corrode, and if it snaps, you're fucked. If you want something shiny, go for zinc-plated mild steel.

Can't help you on having them in Bristol by tomorrow, but you'll find them on ebay, probably with an option for fast delivery.

[fatjames]

They're for holding the top box rack onto my bike. I've had 2 snap on me. Originally, I was sent a new rack as the whole thing was rusted within a month. This time Rentek don't wanna know saying I over tightened them. (it damaged my fairing in the process, but that's another story!)

I've managed to get the snapped bolt out but obviously need to replace it.

I'll only just tighten them this time and use thread lock.

Anyone used something like this? I might be able to find some in b&q or screw fix (I only need 2)
https://www.amazon.co.uk/Tensile-Socket-Button-Screws-Delivery/dp/B00CVU8DGA/ref=sr_1_8?ie=UTF8&qid=1403261848&sr=8-8&keywords=m8+high+tensile+bolt

[smegballs]

Got any pics of how it mounts to the bike? Any chance of getting a nylock on the bolt? I'd say 10.9 class should be fine.

What do you think caused the previous one to snap? Any excessive loads on the rack? Maybe the stock ones were just cheese bolts?

[fatjames]

The first time, one of the brackets snapped (front left) so the bolt snapped (rear right)
It uses the stock mounting points for the rear grab rails. The front take a bracket and the rear just bolt straight in.
The first one got rusty quickly, rentec said there might have been an issue with the welding.
This time, no clue. I generally just use the box for lunch or lid sometimes a bit of shopping. I could well have overloaded it, but when it snapped the box was empty. I was very glad nothing got stuck in my chain!
Also, it's the same one that snapped, rear right.
If I need to order from ebay, I'll post some pics. Otherwise, I'll just fix it.

[bladerunner]

Those are a high tensile bolt...but will rust up first time it rains...better off with nickel plated as said....and me I hate domed bolts like that because of the cosmetic head means they are prone to rounding when you do try to remove them. Engineering,it's always a compromise!
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Current bikes...cbr929, KDX200's, Rd125lc mk2, RGV250's

[DrDonnyBrago]

Stainless isn't great for everything. Buy high tensile steel bolts (grade 8.8 is strong, grade 10.9 is very strong).

Bolts like that normally have 8.8 or 10.9 stamped on the head and you can usually find them more easily than st/st. Might struggle in a chain DIY shop but a bolt supplier will have them, screwfix etc might.

Nuts and bolts in WSM will have them.

[fatjames]

Thanks both. btw, doesn't have to be allen head, that's just the what was supplied.
If I can't any tonight, my options open up a bit and can have a ride to WSM or somewhere else in the morning.
I'm trying to think how long they are and they might be 90mm. When buying stuff like this, I tend to buy long and get a few anyway.

[Pete.]

A2 stainless is near the bottom of the pile for tensile strength. Why they would waste their time supplying A2 bolts for something that rusts in a month is anyone's guess.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[stinkwheel]

You really shouldn't need anything stronger than grade 8,8 to hold a rack on with.

If you are worried about overtightening, use a torque wrench and,
assuming you are using M8, standfard pitch bolts:

Tighten a grade 8,8 steel bolt up to 17ft/lb.

If you really want stainless, buy A4/80 stainless and tighten them to 8 ft/lb.

Buy them from screwfix, not B&Q.

Notice the stainless needs to be tightened to HALF the tension of a simple, high tensile steel bolt and A4 stainless is twice as strong as the normal A2 stainless fasteners. That ought to tell you everything you need to know about the relative strength of the materials involved.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Raffles]

[fatjames]

Hey, just to clarify, I wasn't going out of my way to buy stainless steel, but it's what I was seeing. Bolts I was using prior to this came with the rack and I don't know what they were.

I found some zink plated hex head bolts on my lunch, so if nothing else I'll use those with threadlock and use my torque rench.

[Raffles]

[fatjames]

I don't think I need to eat multi vitamins for a while..