Broken Kubota head bolts?

[illflem]

Servicing a B5100 today wiping up some spilled oil when the head of a head bolt wiped off. Upon further inspection four in a row were broken off, just the stock paint was holding them in place. They were easy to replace. Tractor has always ran fine, head's never been off. Who knows how long they've been broken.



Any ideas on what could have caused this?

 

[rrausch]

Bill, my guess is the bolts themselves were probably not hardened properly.

I've seen this with a lot of non-USA made bolts. It has gotten to where I will NOT buy a foreign bolt if it has to hold something important.

 

[Bob Horrell]

Ditto what rrausch said. Bolts not proper grade but torqued to specs of hardened bolt. Heating and cooling of the head will end up popping the head off.

 

[rrausch]

Chinese bolts are the worst. I'd rather use American bailing wire than a Chinese bolt. I had a bad feeling back when the U. S. got rid of almost all of it's steel producers. A local steel mill out here, in Fontana, was shipped lock, stock, and barrel to China. They bought the whole shebang and shipped everything out, right down to the trash cans!

 

[A Johnson]

Bill, I have done several ISC head bolts just as you described, in transit buses. Some times the head gasket was seeping coolant sometimes they are fine. Supposedly the cause was the head gasket, I assume letting condensation in, doesn't make sense to me. Anyway I was going to report it to Cummins factory and I found out it was an old problem fixed by changing the head gasket design. These bolts all had pitting in the fillet area of the bolt and a lot of rust just above the threads also. FYI



A Johnson

 

[illflem]

Not a bit of seepage on it. Sort of doubt the bolt quality is the problem, Kubota's are generally very well built.

 

[rrausch]

Were the bolts degraded & pitted with rust right where they broke?

 

[Ray Torresdal]

bolt quality

It sounds like hydrogen-embrittlement - I don't know the specifics of the problem, but know we've made many fixtures over the years to test for the problem. Torque the bolt down, and after a few minutes to a few hours, the head pops off. I'm sure there are engineers on board who can discuss this more intelligently than I!!!



Ray

 

[illflem]

Originally posted by rrausch

Were the bolts degraded & pitted with rust right where they broke?

Not at all, just the first one that came off with a rag had rust at the break, the intact paint protected the others and they were clean. I'll also add the tractor is 17 years old.

 

[rrausch]

Time to call in the metalurgists.

 

[fbaurley]

Has the engine ever been torn down before. Sometimew the retorqueing of certain fasteners such as rod bolts may already been stretched beyond their elastic limit may cause them to fail. Thats why a lot of engine rebuilders say to go back with new rod bolts and main bearing bolts. I think the head in a diesel, could be much the same way.

 

[illflem]

Engine's never been apart.