New problem tighten bolts

[ch1ch2]

JD 302 old construction grade tractor work, horse for a long time.
Two stage clutch. PTO quit clutching. Split the tractor found 1 or the 3 arms broken. JD does not sell a clutch pack assembly, you have to buy individual parts at ridiculous prices.
Bought a new arm and started reassembly. Six bolts attach clutch assembly to the flywheel. Manual says to replace the bolts from grade D to grade F.

New to me. Never heard of grade D or F bolts. Manual says to torque to 120 ft/lbs. Figured out I already have grade F bolts. As I tighten every thing seems normal until @50 ft/lbs and the 2 bolts seem to not get any tighter but like something is stripping. One bolt breaks. Bought 6 grade 8 bolts. Bolts tighten to around 50 ft/lbs and that odd feeling returns. Does not seem to tighten. Pulled it apart again and nothing seems to be stripping. Scared to go on. What should I do??????
HELP!

 

[AWSubie]

Grade F is equivalent to Grade 5. What is the diameter of the bolt shank?

 

[ch1ch2]

Grade F is equivalent to Grade 5. What is the diameter of the bolt shank?

Not sure. I used a 5/8 socket if I remember right, if not it was a 9/16.

 

[ch1ch2]

Grade F is equivalent to Grade 5. What is the diameter of the bolt shank?

From what I found on the internet an F had a lot lower shear strength.

 

[ch1ch2]

The tractor is at my sons house about1 mile away.

 

[ch1ch2]

I see you are in FTW. I called Arrow Bolt in Euless, and they never heard of a grade D or F.

 

[AWSubie]

Once you know the shank size you can figure the torque you need from a chart like the one found here
Bolt Depot - US Recommended Bolt Torque Table

Do not lubricate the threads.

 

[ch1ch2]

Once you know the shank size you can figure the torque you need from a chart like the one found here
Bolt Depot - US Recommended Bolt Torque Table

Do not lubricate the threads.

That is a very good thought!
Maybe the manual is mis-printed. The 120 does seem way out of line for the weak bolts and the chart listings.

 

[ch1ch2]

I went to another bolt companies web site for a torque chart and it shows torque for 1/4 and 5/16 bolts in in/lbs. Maybe the manual is mis-printed in ft/lbs instead of in/lbs.

 

[Creamer]

A 9/16" wrench is for a 3/8" bolt which should not be 120 ft-lbs - 120 in-lbs would be more like it but that does seem a little light.

I am not familiar with the application but is this a light torque load where it is locked some other way and they are more for shear - if so 120 in-lb would be where I would put them.

Do you see what felt like it was stripping?