Best Bolt Material?

[scz4]

Looking to purchase some bolts, nuts and washer to fit a strut brace. The rear bolt and nut will be exposed in the front inner arch, so corrosion resistance is important as is strength of course.

Which of these from the drop down list is best for this use case?

[E-bmw]

Do not fit stainless bolts where they can't be "isolated" from dissimilar metals or the surrounding metal will have accelerated corrosion.

On top of that HT stainless is many times less strong than HT steel.

Get some good quality 12.9 HT steel bolts and apply a light film of suitable grease to offer additional protection.

Edited by E-bmw on Monday 5th February 11:05

[mmm-five]

It's got a titanium option, so go for that...even if it's purely for the kudos and the 'weight saving'

[Pica-Pica]

Check what strength is already there and match it. It should be on the top of the bolt head
8.8
10.9
or
12.9

[scz4]

mmm-five said:

It's got a titanium option, so go for that...even if it's purely for the kudos and the 'weight saving'

Titanium is out of stock

[scz4]

E-bmw said:

Do not fit stainless bolts where they can't be "isolated" from dissimilar metals or the surrounding metal will have accelerated corrosion.

On top of that HT stainless is many times less strong than HT steel.

Get some good quality 12.9 HT steel bolts and apply a light film of suitable grease to offer additional protection.

Thanks, doesn't look like they offer normal steel bolts.

Surely marine grade stainless would offer good corrosion resistance.

[Tye Green]

every bolt in the engine bay will just be 'normal' steel with a thin plating to offer a couple of years corrosion resistance

[shalmaneser]

scz4 said:

E-bmw said:

Do not fit stainless bolts where they can't be "isolated" from dissimilar metals or the surrounding metal will have accelerated corrosion.

On top of that HT stainless is many times less strong than HT steel.

Get some good quality 12.9 HT steel bolts and apply a light film of suitable grease to offer additional protection.

Thanks, doesn't look like they offer normal steel bolts.

Surely marine grade stainless would offer good corrosion resistance.

Galvanic corrosion works differently do the corrosion you see on normal steel fasteners. Stainless fasteners also have different failure modes to quality normal steel fasteners and shouldn't be using in a structural application (like the one above) unless designed for the job.

Get some OE bits and paint them if you must.

[GreenV8S]

Don't use stainless stell. It's relatively weak, brittle, and can still corrode badly when in contact with other metals.

If you can find it, your best bet would be a suitable grade of high tensile steel, passivated. By suitable, I mean rated to carry the loads it will be carrying - you don't necessarily need the highest HT rating. If it's only attaching something to the shell the loads may not actually be very high.

[AW111]

You can get zinc plated cap screws in grades 8.8 and 12.9 (and probably 10.9).

8.8 grade 8mm should be pleinty for a strut brace.

[scz4]

Thanks for all the advice.

This looks like the right thing then?

[richhead]

scz4 said:

Thanks for all the advice.

This looks like the right thing then?

The hardness is exactly that, so the harder the material , the more britle it is, ie less flex, a strut brace fixing is likely to require some flex due to the sidways loading, so use a bolt 10.9 or lower, 12.9 isnt widly used for this reason,and coat the threat with a suitable grease and tourque correctly.
However in reality i think you are probably over thinking this, and most bolts would be fine.
Stainless bolts are the work of the devil. as are titanium. when not used in the right places.
Rule of thumb a 8.8 steel bolt is fine for most aplications, if more strength required use 10.9.

[AW111]

richhead said:

scz4 said:

Thanks for all the advice.

This looks like the right thing then?

The hardness is exactly that, so the harder the material , the more britle it is, ie less flex, a strut brace fixing is likely to require some flex due to the sidways loading, so use a bolt 10.9 or lower, 12.9 isnt widly used for this reason,and coat the threat with a suitable grease and tourque correctly.
However in reality i think you are probably over thinking this, and most bolts would be fine.
Stainless bolts are the work of the devil. as are titanium. when not used in the right places.
Rule of thumb a 8.8 steel bolt is fine for most aplications, if more strength required use 10.9.

8.8 will be fine for that strut brace. It's more likely to deform or tear the sheet metal it's bolted to than break the bolt.

As richhead says, 12.9 will be way brittle.

[scz4]

Thanks all.

Finding 8.8 grade non-stainless isn't very easy. 12.9 yes.

Will keep looking.

Edited by scz4 on Monday 5th February 13:48

[donkmeister]

richhead said:

Stainless bolts are the work of the devil. as are titanium. when not used in the right places.

Galling... It's the reason I bought my first impact wrench.

Didn't stop me fitting Ti screws in a couple of places simply for the lolz, but it's nothing importantly structural or subjected to harsh loads and environments.

[JoeMk1]

Check the thread pitch of the mounting holes too, they're likely to be fine pitch which are not so easy to find.

I've used this seller on ebay before, have a good selection of fine pitch, high tensile fixings. Also one of the rare places that offer them in the modern Zinc flake (Geomet) coatings that are almost industry standard these days.

https://www.ebay.co.uk/str/highgradefastenersandfi...

(The galvanic corrosion points people have made is particularly important if this is for a Supra as the picture suggests, as the strut tower is aluminium, which would exaggerate the issue of using stainless.)

Edited by JoeMk1 on Monday 5th February 15:02

[scz4]

JoeMk1 said:

Check the thread pitch of the mounting holes too, they're likely to be fine pitch which are not so easy to find.

I've used this seller on ebay before, have a good selection of fine pitch, high tensile fixings. Also one of the rare places that offer them in the modern Zinc flake (Geomet) coatings that are almost industry standard these days.

https://www.ebay.co.uk/str/highgradefastenersandfi...

(The galvanic corrosion points people have made is particularly important if this is for a Supra as the picture suggests, as the strut tower is aluminium, which would exaggerate the issue of using stainless.)

Yeah need different pitches, although they seem quite common sizes. It is indeed a Supra. The earlier cars don't come with threaded turret, so have to pass a bolt through the existing hole.

2x Short flange bolt - M10x 1.5 mm thread, 35mm long
2x Long flange bolt - M8 x 1.25mm thread x 50mm long

Thanks for the link, will take a look.

Definitely avoiding stainless! Will need to find non stainless nuts and washers too.

[gazza285]

What do you mean by long and short flange?

[scz4]

gazza285 said:

What do you mean by long and short flange?

I assume they're referring to length. Was copied from a US Supra forum.

[TwinKam]

They are both Metric Coarse ('standard') threads, the most common for their respecive diameters, so much so that they are usually simply referred to as 'M8' and 'M10'.