Torque wrench question

Factual Questions
1

Hello Everyone,

I’m relaxing the clutch in my Mazda RX7 and I’m pulling the flywheel to have it resurfaced. The flywheel is held in by a single nut and the torque spec when reinstalling the nut is a whopping 350ftlbs!

Now I don’t have a torque wrench that will go up that high, so I’ll borrow one from somebody. What confuses me is that the workshop manual says that you can take a standard torque wrench, slide over it a 3 1/2’ pipe, Set the wrench for 100ftlbs and when it clicks the nut will be torqued to 350ftlbs.

Now I get it that applying 100ftlbs using a 3 1/2’ lever will generate 350ftlbs at the item being torqued. But how can that work using an ordinary torque wrench. Wouldn’t the wrench click when the nut reaches 100ftlbs regardless of the length of the handle?

I’m confused. Anyone know the answer?

2

You’re right, putting a cheater bar on the end of a torque wrench applies the same amount of force to the nut…I mean, it adds more force for the same amount of input, but if the scale says 100#, it’s still 100#. However, putting the cheater bar between the nut and the torque wrench will multiply the amount of force that the scale says. That’s what you need to do. If you look up Torque Wrench Extenders you’ll see all kinds of various ways to do this (actual correct parts meant for doing this and things you can hobble together with tools you might have laying around) as well as calculators for figuring out what you need to do. Your example was pretty straight forward, but if you happen to have a 4 inch extender on hand and need to apply 135# of torque, the calculator might make that easier to figure out.

3

I saw a few other methods online for tightening that bolt (without a torque wrench), but this one, if the math is correct, is probably the easiest. You don’t even need the tool, just read the directions and you can use your own wrench (either a long one or one with a cheater bar).

4

You should rent a proper torque wrench from a shop for this type of job. Snap-on makes 600 ft-lbs torque wrenches that are like 3 feet long.

5

Correct, but you can still get a reasonable accurate torque up when you have a 100 ft lbs torque wrench to measure yourself … or to calibrate yourself.

When you apply 100 ft lbs, you are applying a certain force at the distance of X …

Now if you blindly apply that same force at the distance of 3X, well you have 300 ft lbs

6

Borrow, or rent a proper torque wrench if possible. If not, as Joey P said, look up Torque Wrench Extenders. Caution: Math is involved.

Please, do not guess on this torque. You do not want the flywheel coming loose while the engine is running.

7

Kewl! I used to have an RX7. If you’re doing the work you may know this already but when I was reading RX7 forums the accepted way to get the nut off was a 1" drive impact wrench as used at a heavy truck garage. Apparently these can get reeeel tight! Just FYI…

8

I agree with this, get the feel for 100# force and then install your longer extension and apply the same force to the socket, you will be very close.

9

Some of the newer 1/2 drive torque wrenches can give you 350# torque. I worked in a garage and we normally just used the 1" impact for things like this. You have to know your gun and how long to stay on it for the approx right torque.