Grade 8 vs Grade 5 bolts in under car suspension service

[Tmoose]

I'm silently observing A discussion on a car bulletin board dedicated to a particular foreign make from the 60s and 70s.
A recent thread runs something like this - There is danger of using Grade 5 instead of Grade 8 bolts on the upper A-arm rods............ Both alignment shops that worked on the poster's car insisted on installing Grade 8 bolts in place of the perfectly good Grade 5 bolts that were installed. One of the new bolts broke as he was going over a speed bump at less than walking speed. Now, the front end is making groaning and popping noises. He feels Nothing’s broken yet, but plans to replace the bolts today.

My guess is they may be discussing item 29 here -

http://www.v-performance.com/content_files/early type front susp_Section_6_page_7(2).gif

Not all OEM chassis fasteners I've seen are Grade 8, but it Seems like there are plenty of 10 to 15 year old Grade 8 / 10.9 bolts surviving just fine in the under car environment.
There are MIL specs and others warning that fasteners close to or slightly above a tensile strength close to Grade 8 and Socket head cap screws should NOT be used in nasty environments, which I consider the road salted winter New England road to be.
A buddy even had a couple of small Gr8 fasteners that came with his replacement ball joints spontaneously snap within a bout a year.
I attribute that failure to general crappiness, not being Grade 8.
Ditto for the alleged bolt failures on the bulletin board.
I can also see how a crappy lower grade bolt might have better ductility than a counterfit Grade 8, which might make short term failure less likely.

 

[Tunalover]

Tmoose-
What industry or government entity is in charge of assuring that manufactures aren't producing inferior quality?


Tunalover

 

[desertfox]

Hi

Take a look at this:-

http://www.youtube.com/watch?v=FVndKEtazNg

The interesting point in the OP above is that the grade 8.8 bolt failed and I assume doing no damage to the female counterpart as it is not mentioned, also a well designed joint would ensure that the bolt or screw failed first.
The biggest danger I see in replacing a grade 5 fastener with a grade 8 fastener is that the fastener may no longer be the weak link.

The tensile strength curve shown on page 2 of this link clearly shows the grade 5 bolt to be more ductile than the grade 8 bolt however if we consider failure of the bolt/joint to be the yield point on the curve then I would prefer the grade 8 bolt provided it would still fail prior to anything else.

https://www.fastenal.com/content/feds/pdf/Article - Bolted Joint Design.pdf

 

[CoryPad]

There is danger with hydrogen assisted cracking when using fasteners with Grade 8/Property Class 10.9 or higher strength. Use of coatings like zinc-rich types (Magni 565, Geomet, etc.) has mitigated most of those concerns, which is why Property Class 10.9 has been used for highly stressed vehicle underbody locations for decades without problems.

 

[Duwe6]

Standard Gr.8's [not 8.8] can be somewhat brittle, so I only use them when a Gr. 5 is too weak. Sometimes 'extra' is detrimental.

 

[EdStainless]

And of course if they weren't torqued to the correct preload then it doesn't matter what grade they are.
A clean joint and little oil are needed to assure getting the correct results.
This gets overlooked often.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[Tmoose]

I pasted the info incorrectly initially. Should have read -
There is danger of using Grade 8 instead of Grade 5 bolts on the upper A-arm rods.

Also I would really like to see what came installed from the OEM, in regards Grade and coatings. I may be able to find the size and torque spec in a manual.

 

[tbuelna]

The bolts used to attach the upper A-arm shaft to the frame are not likely highly stressed. So a Gr.5 bolt should work fine if that is what it was originally designed to use. However, I can't see where using a higher TS Gr.8 bolt would create a problem, unless this bolt is installed using a torque-to-yield technique.