86 Cutlass

Does anyone know what a 10.9 metric equal to in std hardness? Grade 5 or 8? We are rebuilding a 86 Cutlass Supreme and started to use grade 8 for all of the upper and lower control arms, and A arms, but someone told me that the grade 8 were brittle and would break from the vibration? We had to cut a lot of the original bolts for removal. Any help would be great.

Thanks

henryk8398

This may help...

http://www.dansmc.com/torque_chart.htm

Lady72nRob71

Looks like grade 8 is similar to 10.9. If I was you, i would just use all grade 8. Tractor Supply sells it by the pound!

The harder a bolt is, the more brittle it would be, but it would take a heck of a lot to break them.
My 72 uses grade 8 for the suspension and the 86 chassis design is not a while lot different.

joe_padavano

Yeah, so long as all you need is coarse thread. I needed 7/16 fine thread grade 8 bolts for the seatbelts in my 62. I had to get them from McMaster Carr.

Lady72nRob71

My 72 used coarse threads, so I lucked out and the threads were the same, new and old.

joe_padavano

Brittleness goes with hardness, not necessarily strength. In addition, vibration causes fatigue failure, which is completely different from a brittleness failure. Bottom line is that the factory bolts are Grade 8 (or equivalent metric). Anyone who says not to use Grade 8 in any application due to brittleness does not know what they are talking about.