I keep breaking bolts

[SPYDERLK]

I wouldn't consider grade 8 bolts brittle. The grade rating is a measure of material strength not hardness. I machine and drill them all the time. DP hit on a point though. If the bolts are snaping in shear or tension, a larger bolt or more bolts should help. It's the same reasoning used to hold a head on an engine. You won't find any 1/4" bolts on a diesel engine.

You said grade 8 bolts are to brittle, they aren't. They are ductile just like lower grade bolts. They just don't show deflection or deformation until much higher stress levels. And reference books are there for just that, reference and specifications.

A misread id say. The brittle - ductile thing is a matter of degree. Regardless, mechanical strength generally goes up with hardness - with accompanied loss in ductility.
larry

 

[Bill_C]

Young's modulus is unaffected by heat treatment, however. Since the elastic limit for Grade 8 bolts is higher than Grade 5, the Grade 8 bolts can be stressed more than Grade 5 and still remain elastic.

We don't want the bolt stressed beyond its elastic limit...at that point it doesn't really matter whether the bolt deforms in a brittle manner or in a ductile manner, the fastener has failed.

 

[DieselPower]

A good example of just how ductile or elastic grade 8 bolts or metric grade 12.9 bolts are are diesel engine head bolts. I have factory head bolt stretch gauges in my tool box for Cummins cylinder head bolts. If they have stretched to much then they have to be replaced. Cummins knows they stretch and have to be checked before reuse. International 444E cylinder head bolts are another good example. I have done a lot of cylinder head jobs on these engines and when trying to reuse the original head bolts they just keep turning and turning and turning, never reaching the torque spec. Upon removal I have seen that they had grown as much as 3/4 to 1 inch in length and gotten rather thin at the stretch point.

 

[radman1]

DieselPower

You know more about bolts than I know about life. My head still hurts from reading your quotes.

 

[bottleworks]

How about, as suggested earlier, making sure the bolts are at proper torque before modifying?

A chart is attached.

 

[sixdogs]

Others have mentioned it but I would make sure the pulling forces are lined up with what is being pulled. I had that complex problem once with an invention. Photos sure would be helpful here.
Good thread with lots of smart guys. Everyone should have a torque chart somewhere close by.

 

[Stumppuller]

Ok, it seems like the folks on this discussion line know what they are talking about, so here goes:

On one of the other tractor forums there was much discussion about re-torquing head bolts. One guy says to loosen them all & retorque, another says to just apply the spec'ed torque to the bolts; a company statement for one of the big diesel makers says not to touch them as they are stretched to proper limit at the factory; my dealer (Jinma) says they used to re-torque the engines, but stopped doing it & have had no problems so far. I don't know of any modern car that requires re-torqueing, but I don't know that much about diesels.

My only suspicion is that this tractor is essentially 50's-60's design technology and the head gasket may indeed squash down with use.

What are your opinions on this?

 

[DieselPower]

It really depends on the exact engine. Different head gaskets, head materials (steel or aluminum) and the actual design of the bolt. With head bolts it's best to stick with the factory torque procedure. Some use an entirely different process called torque and turn. The bolts are torqued to a rather low torque setting and then turned a specified amount of degrees past that. Many heavy truck diesel engines are this way.

 

[banjodunn]

The style of tensionig you are talking about diesel power is called "torque to yield" anything with this style of tension it is usually suggested to replace the bolts instead of re using as they have been streched to their limit while in service and may not achieve correct tension if done as specified

 

[DieselPower]

That's where the factory strech gauges come in. If the bolts have exceeded the max limit they have to be replaced.