Head bolts - reusing or replacing?

They can be reused if the amount of stretch is within spec. The FSM discusses how/where (on the bolt) to measure.

 

A practicing Pastafarian, I see.

 

Attached is the head bolt inspection specs, from Repair Manual. A caliper (or micrometer) is prerequisite for measuring the bolt diameters in particular. They're not difficult or expensive to acquire.

That said, if you’re DIY and really don't want to have to do the job twice (should the old bolts crap out), new bolts may be best. I suspect dealerships doing head gasket repair would just replace them. It's an added expense (born by the customer), but arguably safest for both parties. Attached TSB has part no for the bolts.

 

I bought new and didn't use them My original bolt just looked better but I have a new set sitting here I think they were $18 or something and they don't look cheaper bad it's just my factory bolts look better so I reuse them turns out my piston was bad anyway so the engine was toast and they're sitting on the floor now and a new engines in the car with new head bolts and all of the original head bolts and all of that never been taken apart in America I hope it works out for you

 

My time and work is more important than the cost of new head bolts. In the aircraft industry, in many places, it is a certification fail.

May died when the Galloping Ghost crashed at the 2011 Reno Air Races on account of a failed reused lock nut.

https://en.wikipedia.org/wiki/2011_Reno_Air_Races_crash

 

Yep that's an absolute fact I rebuild the engines in my planes all the time and in a plane engine. Lycoming whatever. And then the z engine it's probably a good idea I wouldn't disagree with that I just reused mine because I figured with the car the engine and all something else was going to happen and it wasn't going to be the head piston failed and the rod broke I should have replaced the engine first lesson learned with this open deck stuff

 

Pops was there with one of the pipers citation or Cherokee I can't remember

 

Yet another materials science question that if studied properly could reduce a great deal of waste in vehicle maintenance. But only if we had solid data that proved reusing cylinder head bolts won't cause headgasket failure. Meanwhile in a hyper consumptive society just about any excuse you think up for consuming more is valid... Ironic that the collapse of civilizations that came before us was mostly due to failures in their supply chain and here we are having strange problems with our supply chain and the idea of materials science research to help reduce waste is still not a well-funded interest/agenda.

 

The first thing I ever learned in Materials Science 101 is that a structures fail with the degradation of the first molecular layer.

All head bolts are made of a high carbon steel alloy. All alloys are an aggregate of crystals that go through the austensic and martensic transition process when they are hardened, quenched and annealed.

Based on the industry accepted kinetic molecular theory, every crystal in the alloy is bound by ionic or mechanical bond. Over time, just by being in the environment, all of the molecules are in vibratory motion. This motion works over time to weaken and tear the bonds apart. Repeated heating and cooling as well is longitudinal stress applied by tensioning or torqueing constantly is pulling these bonds apart.

On that account, the functional life of a bolt is can be reasonably determined or calculated as MTBF or mean time before failure.

One bad bolt can have a domino effect of bring down the entire structure. A crack in one eyebar bought down the Silver Bridge at Point Pleasane, West Virginia on December 15, 1967.

The old ones can be recycled.

 

Brake rotor replacement comes to mind; everyone likes shiny/new.

In Vancouver there’s Happy Stan’s: takes all the odd ball stuff more conventional recycling depots (like ReturnIt) won’t touch. First learned about them when I had a kaput lawnmower to deal with.

 

The NTSB is loaded with reports of aircraft failures that happened on account of over torqued and reused bolts that have resulted in catastrophic failures and crashes.

There are two kinds of stress related failure and all materials have a drgree of ductility or hardness. Ductile failure is gradual as the material stretches before. The other is brittle fracture where the material snaps. Head bolts experience the later. Pottery and ceramics are brittle materials. The degree of brittleness is termed frangibility.

 

I guess the thoughts are if it's rolling down the road on four tires it's not going to fall on anybody essentially at least not out of the sky maybe off of bridge I guess that's why FAA maintenance isn't done on cars maybe it should be that would be an expensive proposition oh my goodness

 

Aircraft or even Nasa spacecraft standards are not flawless. The consequences of failure is more significant. The space shuttle scattered itself twice. Personally I think proper torque and tightening procedures on a Prius head bolt are more important than brand new. Especially when you consider the variability of the auto mechanics doing it.

 

How is spewing out a bunch of hypotheticals and fancy jargon say anything? Real science does tests and takes samples specific to the forces and conditions involved.

In this case, do the forces involved in engine use ever come near the threshold that would make a head bolt worth replacing. The answer of course is Mendel's post about a standard test for that bolt size, just as there a standard torque and standard measure of when that bolt will likely fail.

But in real life there's endless variables for a head bolt and if you had a statistically valid sampling of 500 Gen3 that used the old headgasket bolts during a head gasket job and 500 Gen3 that used new, you'd be able to calculate what's most likely and then start testing further to prove it. Of course by the time you're done everyone will be driving electric vehicles and your research will be meaningless...

 

There is no doubt the 500 new head bolt installs would prevail. I believe in education and certainly a few of us need to understand the physics and material science involved.

I would rather have a rebuilt engine with new pistons and rings over a simple head gasket repair. Given the resources and desire, I would chose a brand new engine instead of a head gasket or a rebuild. But resource allocation is an important factor for most of us driving and maintaining ten year old econoboxes. Otherwise we would be driving brand new Tundra 4x4s made in San Antonio.

 

Thank you everyone. I used the spec sheet shared by Mendel. I measured all 10 head bolts in 5 minutes. The length of each bolt and and the diameter of the first inch or so of threads. All were well within the standard spec. The spec states to only replace if necessary so I will be reusing mine.

Also I am a certified A&P mechanic after working on AV 8B Harriers in the Marines, in the aircraft industry you follow the manual you don't just buy extra parts when you don't need to.

 

I saw an in spec head bolt reused last year. A couple of weeks later it loosened up blowing the new head gasket. It was a bad day for the mechanic as he did the job again for free..

 

The head bolts were cheap enough when I was ordering parts from even suppliers like Fel-Pro no problem like $34 for the set or something A&p mechanic or not for $34 in my vehicle I might opt for the $34 but on a 2ZZ Atkinson engine I'm going to probably opt for a lot of nothing.

 

new is for you then my brother. I just wanted to comment so people with less mechanical knowledge aren’t shamed into buying $35 Chinese parts on Amazon when the factory head bolts are still within spec. After 14 years of head gaskets on these cars I think the spec might have changed if reusing them was causing tons of failures. Just like the other bulletins. Lots of other cool stuff to replace like the thermostat, pcv valve, timing tensioner, bringing the head to the shop for testing, it all adds up.