What to do when bolts is over-torqued?

[dmksails]

I installed the exhaust elbow on my engine with M8 bolts. A table in the manual said to torque the bolts to 30 ft-lbs. Another table says the M* bolts are torqued to a range of 8-15 ft-lbs.

I started to torque the bolts to 20 and got three of them up to that, but the third started to feel funny - like it was getting ready to strip. So I stopped.

Now I'm thinking that the correct torque was 15 ft-lbs max. Do I leave the bolts alone, or remove and re-torque to the correct value?

[MikeHoncho]

take it out and check the threads.

[Wolfe10]

Assuming that it is not the bolt that is stripped, you can you drill it out, tap it to the next larger diameter-- either metric or SAE?


That is one of several options.

[tkeithlu]

If you over-torqued it, which it sounds like you did, you've gone into the plastic range of the bolt, and it needs to be replaced. If you are very fortunate, you may be able to back it out, but more likely it will shear when you try. In that case you need a bolt extractor of the right size, such as:

https://www.amazon.com/Irwin-Industr...s%2C855&sr=8-4

Given that you just put the bolt in, you have a very good chance of being able to extract it without having to destroy the existing threads. If it is necessary to destroy the threads, then the next option is to helicoil the hole:

https://www.amazon.com/Orion-Motor-T...NsaWNrPXRydWU=

The third option is to bore it out and rethread to a larger diameter using an ordinary tap set:

https://www.amazon.com/Threading-Ret...NsaWNrPXRydWU=

Good luck with it. With a little practice you'll find yourself starting to torque something and saying to yourself "Naaa, that's too much for this bolt."

[tkeithlu]

Let me pontificate a little: When you torque a bolt, you stretch it within it's elastic range, so that it clamps two items together. It's a stretched spring. The torque required is a property of the bolt, its threads, its size, and its quality, and has nothing to do with the two items that are being clamped with the bolt. You can learn the proper torques for all the common bolts you use. You can also gain a feel, such that you know what's right for a common bolt independent of a torque wrench.

Any time you enter the plastic range of a bolt, it's game over. Pray it will come back out, and toss it.

Failures in this area are really common when people put tires on cars. Somehow they think that if 100 ft/lbs is right, 200 must be better.

[Rumrace]

I’ve built a few V8’s aluminum and Cast.
All of them managed to break a stud somewhere Fuel and head bolts broke for two different reasons Bolt extractors are great. If the helicoil is in the head /block it’s scrap. I run a tap though everything even after the blocks boiled.
Locktite everything. Blue sounds right for 15-20 pound.
I’d back them off and replace them clean the threads up with a tap. Locktite new bolts to 12 pounds even then to the spec.
On motors I always retorque. Especially Heads intake exhaust valves. On cast iron blocks a head bolt torqued to 180 will 160 after a break in 2-5 minutes. Aluminum you can get bolts sinking into the part from being over torque

[Adelie]

Originally Posted by dmksails

I installed the exhaust elbow on my engine with M8 bolts. A table in the manual said to torque the bolts to 30 ft-lbs. Another table says the M* bolts are torqued to a range of 8-15 ft-lbs.



I started to torque the bolts to 20 and got three of them up to that, but the third started to feel funny - like it was getting ready to strip. So I stopped.



Now I'm thinking that the correct torque was 15 ft-lbs max. Do I leave the bolts alone, or remove and re-torque to the correct value?

A 8.8grade M8 bolts should be torqued to about 29N-m which is about 21 ft-lb.

A 10.9gr M8 should be torqued to 41Nm which is 30 ft-lb.

I would double check the units in the manuals.

[Ballsnall]

I think you will find it's the internal thread that has failed, not the bolt. Remove it and helicoil it is the best solution.

[RaymondR]

The torque on the bolts on an exhaust elbow is not a critical item.

[Nicholson58]

Originally Posted by tkeithlu

Let me pontificate a little: When you torque a bolt, you stretch it within it's elastic range, so that it clamps two items together. It's a stretched spring. The torque required is a property of the bolt, its threads, its size, and its quality, and has nothing to do with the two items that are being clamped with the bolt. You can learn the proper torques for all the common bolts you use. You can also gain a feel, such that you know what's right for a common bolt independent of a torque wrench.

Any time you enter the plastic range of a bolt, it's game over. Pray it will come back out, and toss it.

Failures in this area are really common when people put tires on cars. Somehow they think that if 100 ft/lbs is right, 200 must be better.

Agree totally with your posts.

I would add that specified torque is dependent on bolt material. A highly loaded assembly will need higher strength bolts. Most hardware store bolts and stainless are quite soft, low yield material.

[boat driver]

Make sure the bolts are of the correct grade. To often the bolts are substandard to the manual if not provided with the part from the manufacture.
on the exhaust side, just be sure when cold and starting it is not leaking. The heat will expand the connection and tighten it up. just do not need gasses inside boat.
If stripped/ fix it now while can get it out.

[tkeithlu]

Good point, Ballsnall. What he's feeling could be the threads in the block giving way.

[Ballsnall]

He said it was the exhaust elbow, so I'm assuming it is threaded to a cast iron manifold, but could be cast alloy. 20ft lb for a M8 bolt doesn't sound excessive. I hate that feeling when you go just a little too tight. You know it but just don't want to admit it to yourself.

[Sailmonkey]

Originally Posted by Ballsnall

I think you will find it's the internal thread that has failed, not the bolt. Remove it and helicoil it is the best solution.

I’ll concur.

It’s likely the thread in the aluminum body of the heat exchanger has failed.

Drill and Helicoil. Then re-install and do t worry about the specific torque.