Flex plate to TC bolt hardness specs? - Third Generation F-Body Message Boards

my3rdgen · 03-11-2004, 12:22 PM

I just installed my engine and tranny combo into my 87 T/A. When replacing my flexplate to TC bolts (old ones went with old engine) I got a set of 3 hex head "M10 1.50 X 15" with a rating of 10.9 stamped on the head.

The old bolts were rated with a 9.8 on the head and they were the stock pieces. I understand how the american hardness rating works. But the metric stuff is a little different?

I checked it out and did a search on this site and the web and have concluded that a metric bolt with a hardness rating of 10.9 is somewere between a standard grade 5 and grade 8 bolt.

Is this correct?

Also, if this is correct that puts the stock bolt that I pulled out with the 9.8 hardness rating, closer to a grade 5 bolt. Isn't a grade 5 bolt a little too weak to be using in this application, and will a metric 10.9 hard bolt be strong enough?

I am by no means a metallurgist so please give me some insight.

ede · 03-11-2004, 01:18 PM

you should use bolts made for the flex palte to TC connection and not a standard bolt. as for your question as i understand it the bolt strength goes up in relationship to the diameter and is equall to or exceeds our grade 8. it's not hardness they rate it's tensil strength.

383 rocket bird · 03-11-2004, 06:10 PM

What I did when I replaced mine was to use ARP bolts. They are purely a fastener company and make high quality bolts better than anything from a hardware store. They are like 7 bucks from summit or jegs. -John

my3rdgen · 03-12-2004, 09:39 AM

Thanx for the help. My goal was to make sure that I got a good fastener, and I didn't quite understand the numbers on the top of the head of the bolts. I didn't want to have to crawl under the car on the highway and dig the broken bolts out of the TC on my way to the CC Summer Nats. That would suck.

dankhound · 03-12-2004, 11:23 AM

The higher the number on top the stronger the bolt is. So replacing a bolt with a higher grade bolt is ok.