Bolt grading question
 

I think this is a stupid question, but that’s not stopping me.


I’m removing and replacing my seats and because some of the bolts are stripped or near stripped, I’m planning to replace them all. I notice that the current bolts are 8.8 grade. Is there any reason that I couldn't/shouldn't substitute a 12.9 grade bolt? 12.9 seem easier to find.

no reason at all.
12.9 is stronger than 8.8 and you do not want the bolts that secure the seats to be a mode of failure. So the stronger, the better.

Be sure to put some anti-seize on the threads when you reassemble. Another nice thing about the harder bolts, the allen head is way less likely to strip.

Not necessarily true. The higher grades have a greater tensile strength but are less elastic (trick question: which is more elastic? A rubber band or a piece of steel). This means that when a bolt yields and subsequently fails the transition of a lower grade is longer (takes longer to deform) than s higher grade.

Perhaps. But if you look at the yield strength the higher grade bolt wins. 12.9s are pretty tough. Take one in a vice and see if you can break it with a hammer. Then compare that to an 8.8.

The practical benefit of higher grade seat bolts is the Allen socket will be much less likely to strip out. The challenge is finding some at 12.9 with the low profile head. These guys have some at grade 10.9...

http://www.classic9leathershop.com/screws-nuts-bolts-brackets/seat-bolt-kit-for-porsche-911-993-964-944-968-928-oem-type-low-head-yellow-zinc-1985-1998/

IMHO the bolt and nut need to be matched properly. When matched properly the bolt should always break during torque to failure test . I do this so when the assembly plant over torques the joint it will be obvious to everyone there is a problem to be fixed. Also it is typically easier to replace a broken bolt then to helical an engine block. As the bolt gets stronger so must the nut.

Hopefully this is helpful

David

Instead of internal hex drive please consider internal torx drive. They are way more robust

I love this suggestion. Do you know a source?

PM me your address and I will see what I can do

David

What is the diameter of the current m6-1.0 bolt?

On my....so what??

Are Grade 8 bolts more brittle than Grade 5? (8.8 vs 10.9 Metric) | Helping you work smarter not harder

12.9 hardware is always stronger and better than 8.8 or 10.9 hardware. The question is do you need it to be better?

better check into what Porsche's engineers spec'd

but No, 12.9 is not always "better"

PM sent

This is funny same discussion going on the jeep forum right now
Grade 5 Or Grade 8 For Winch? | ECJ5

Excluding cost considerations, when are 12.9 fasteners not better than an 8.8 or 10.9 fasteners?

Oh, read this too:

https://www.rockcrawler.com/2003/04/fasteners-making-the-grade-a-technical-discussion/

Bolt stretch and clamp load is what keeps the fastener from loosening up due to the external influences acting upon the bolted joint. If you have a 12.9 bolt and a grade 5 nut you will never be able to apply enough torque to keep the joint together due to the nut. But then again maybe you are safety wiring them.

Of course. You can use higher grade nuts than bolts but the bolts should not be higher grade than the nuts as you need to tighten the bolt or nut to the torque of the bolt to get the proper clamp load. The bolt is what stretches to supply the clamp load...not the nut.

The "nut" on the seat rails for my 85 Carrera are actually little blocks of steel, with a tapped hole. They are prevented from spinning by the seat rail themselves. I have no idea what rating they have, but the are not traditional nuts.

In the absence of more detailed information on those threaded blocks of steel, I would assume that they are the same grade as the factory specified bolts.