correct torque, clutch bolts

[Richard McDonel]

I recently pulled my transmission, and to my own embarrassment noticed that when I installed the clutch last year, I had used a variety of bolt grades.   Now that have the opportunity I figure that I should attach it properly.  The clutch is held by 8mm bolts, and I began to install new 8.8 grade pieces.  But I'm confused about torque.  The FSM calls for 17.4 to 18.8 ft/lb, which to me sounds pretty loose.  I then went into the Haynes and Clymer manuals, and they're all about the same.  The  chart I use for general torquing reference calls for 30 Nm, or roughly 40 ft. lb for 8 mm, 8.8 grade bolts.  

Can anyone tell me why, in a fairly heavy duty application such as clutch-to-flywheel, we'd be looking at half the normal torque?

 

Thanks,

 

[Zed Head]

We can all only guess, but one possibility is that since the requirements are exactly known and cannot be exceeded, it's safer to set a mid-range torque value to reduce the possibility of damage.  The springs of the pressure plate are the limiting factor for force on the bolt heads.  Plus, the bolts have leverage since they're out on the perimeter of the plate so don't see as much force as they might in a different application.

Just a guess.  I've never heard of anyone having a pressure plate come loose.  I wouldn't exceed the spec.  Imagine your aggravation if you strip a thread or break a bolt trying to be safer.

[Patcon]

I wonder if the low torque helps prevent warpage so the clutch engages smoothly

[Diseazd]

You are dead on with the torque numbers....17.5 to 19 ft. lbs. Avoid at all cost the urge to give it a little more uummmph! Broken bolts in the flywheel suck. 19 lbs. at the very most....and just torque once!

[Richard McDonel]

Thanks for sharing your thoughts fellows.

As bolts that aren't torqued properly tend to come loose, I'll install fresh bolts, with lock washers  and lock-tight, but keep the torque under 19 ft. lb.    Sort of like wearing both a belt and suspenders.

 

Cheers

[zKars]

Fresh lock washers (and correct torque) are your best moves to keep that clutch on there. Lock tite is overkill in this application. Won't hurt a thing if you use it. 

[Patcon]

16 hours ago, Richard McDonel said:

Thanks for sharing your thoughts fellows.

As bolts that aren't torqued properly tend to come loose, I'll install fresh bolts, with lock washers  and lock-tight, but keep the torque under 19 ft. lb.    Sort of like wearing both a belt and suspenders.

 

Cheers

Blue lock tite only!

[Richard McDonel]

Patcon,

Understood! Job's done.