Thread lubricant and torque values

We are rebuilding a little cat vblock for school. We’re putting one of the heads back on today. There’s a 5 step torquing procedure in the book. So we’re on step 1, on the 4th bolt in the pattern, and something (not the torque wrench) pops. It is the threads in the bolt hole in the block. Came out very prettily: the first 4 threads in one piece nicely with the bolt.

So we go hunting, and find 3 more holes that already have suffered the same fate and have not been repaired. So we’re getting the repair kit ordered. Ok, what caused it?

I have an idea: The book says use moly-coat on the bolts. I didn’t like the look of the “moly” we have in the shop, and there was still some left on the bolts, so I didn’t apply any more. The can doesn’t say “moly” or “molibdinum” anywhere on it, but it looks like moly-coat and it is for the same purpose. What I didn’t like was that it had separated into the silver paste and a thin yellow oil. When I mixed it up it still seamed too thin and lubricious to me. So my idea is that the last group to work on this thing used this “moly-coat” or what-ever-it-is on the bolts and, because it was too lubricious, they got it over-torqued.

Whatcha reckon, guys?

You are an engineer when you think…
that when people around you yawn, it’s because they didn’t get enough sleep.

[QUOTE=Lee Shore;194990]You are an engineer when you think…
that when people around you yawn, it’s because they didn’t get enough sleep.[/QUOTE]

or its because they’re deckies who wandered down below the waterline where they don’t belong, unless they’re carrying a plate of cookies. As my First often says with great solemnity:

      • Updated - - -

[QUOTE=Lee Shore;194990]You are an engineer when you think…
that when people around you yawn, it’s because they didn’t get enough sleep.[/QUOTE]

Welcome to the most glorious invention of man, the heli-coil!

I highly doubt the quality of, amount of thread lubricant of any kind would cause the threads in the block to come loose. General rule of thumb if you have at least one nominal diameter of thread engagement the bolt will part long before you pull threads off either the fastener or out of the tapped hole.

Did you check the hole depth, did someone leave a little surprise there for you, like they broke off a bolt and left it in there so your bolt was bottoming and pulling on too few threads? Are you using the correct length bolts yourself?

Sometimes past sins just add up though and you’re stuck holding the tap wrench as it were.

Viva la heli-coil!

[QUOTE=KPChief;194993]Welcome to the most glorious invention of man, the heli-coil!

I highly doubt the quality of, amount of thread lubricant of any kind would cause the threads in the block to come loose. General rule of thumb if you have at least one nominal diameter of thread engagement the bolt will part long before you pull threads off either the fastener or out of the tapped hole.

Did you check the hole depth, did someone leave a little surprise there for you, like they broke off a bolt and left it in there so your bolt was bottoming and pulling on too few threads? Are you using the correct length bolts yourself?

Sometimes past sins just add up though and you’re stuck holding the tap wrench as it were.

Viva la heli-coil![/QUOTE]

I’m using the same bolts that we took out of it. I will double-check the hole depth tomorrow. I looked at them all carefully with a flash light today… seemed ok.

My other idea is that they weren’t torqued enough, and they broke while the engine was running?

heli-coils are pretty great, its true.

This is a lab activity? Does that mean this poor machine has been raped and hammered together like 1000 times? Threads are mechanical devices, they wear out…sounds like that’s what happened.

[QUOTE=johnny.dollar;194998]This is a lab activity? Does that mean this poor machine has been raped and hammered together like 1000 times? Threads are mechanical devices, they wear out…sounds like that’s what happened.[/QUOTE]

The poor machine has been torn down every year for a few years. But probably not 1000. There’s a long list of problems that we found with our inspection, so it hasn’t had an easy life. Few places where the steel has turned blue, one piston looks like someone tried to take it out with a pry-bar, bearings scored… not a pretty sight. Makes me kinda sad.

using oil on threads vs not using oil: use of oil effectively increases torque by a percentage ratio as to bolt dia.
There are also those who believe that getting the head hot enough, long enough will carbonize the mentioned oil and help trap the bolt in the hole. I would NOT use a helicoil for a head bolt unless you can find grade 8 or better helicoils as it’ll loosen but even then, as KP mentioned: clean the threads, count the good ones and see if it’s practical to use what’s left. I’d like to use something on head bolts but always end up just cleaning both threads and maybe wiping the bolt with a rag with some light diesel film or something on it.

[QUOTE=jimrr;195026]using oil on threads vs not using oil: use of oil effectively increases torque by a percentage ratio as to bolt dia.
There are also those who believe that getting the head hot enough, long enough will carbonize the mentioned oil and help trap the bolt in the hole. I would NOT use a helicoil for a head bolt unless you can find grade 8 or better helicoils as it’ll loosen but even then, as KP mentioned: clean the threads, count the good ones and see if it’s practical to use what’s left. I’d like to use something on head bolts but always end up just cleaning both threads and maybe wiping the bolt with a rag with some light diesel film or something on it.[/QUOTE]

The manufacturer is on your side with the helicoil issue: they say no. But its only a shop exercise engine, it doesn’t do work and its never under load. Its not started often or run for long, so we’re going to try it with the helicoils. Finning wants us to bring in one of their techs to do the fix. Instructor is like: a-no.

I wouldn’t use oil when the book asks for something less lubricious: that’s just asking for it. Nor would I put them in dry when it asks for moly-coat. I’m a procedures following kinda girl, most of the time.

I think KPChief was right about the reason why they broke. I took a more careful look into the bolt holes and the broken ones have a shiny little dimple at the bottom of each hole. Someone used too long a bolt, or too thin of a gasket I reckon. Kids today, eh?

Back in the days of the 398 and 499’s Cat had a problem with some of the head bolts and they came out with a fix that was something like a Helicoil but a lot more robust. Too many brain cells have gone bye bye for me to remember what they called them but we had to do this to several blocks back in the 80’s and 90’s. After installing them we never had any more problems but I can see if they have been tearing it down all the time as a teaching tool, they might have just worn out.

[QUOTE=Tugs;195042]Back in the days of the 398 and 499’s Cat had a problem with some of the head bolts and they came out with a fix that was something like a Helicoil but a lot more robust. Too many brain cells have gone bye bye for me to remember what they called them but we had to do this to several blocks back in the 80’s and 90’s. After installing them we never had any more problems but I can see if they have been tearing it down all the time as a teaching tool, they might have just worn out.[/QUOTE]

Ours is just a humble 3208. Interesting to know that the big ones were having a similar problem, once upon a time.

I would rocomend using Time-Serts they have been used for head bolt repairs and are stronger then helicoils in my opinion.

www.timesert.com

2nd the time-sert or any other brand of insert. Or, if you have machine shop…make your own.

I’ve never used time-serts, or heard of them. I’ll keep my eyes open for them. For now we have the helicoils and that’s what we’re using. Class is over next Friday, and Monday’s a holiday, time is a limited resource.

Out of curiosity, has anyone here made their own thread inserts as johnny.dollar suggests? I have basic competency with making chips, but I’m not exactly what you’d call a machinist. Sounds like a tricky project, so small and precise and out of a hard alloy. If I could learn to do that, I’d feel like such a useful mammal.

[QUOTE=Chief Seadog;195064]I would rocomend using Time-Serts they have been used for head bolt repairs and are stronger then helicoils in my opinion.

www.timesert.com[/QUOTE]

This looks a lot like what we used on the 398 and 399 Cats back in the 80’s.

I’ve done a fair amount of single point but making thread inserts will undoubtedly require a specific dia. jig, dealing with the hardened steel issue is quite doable, or, as is hardening it after you make it but the jig, (for me) would req. some looking.

Regarding helicoils in head bolt holes. We have empirically determined that it is not a good idea. But you all knew that, already.

(I love shop class so much)