I'm kinda on the same page, except I believe the injector settling into it's seat" is actually just the o-rings compressing over time and needing a little extra torque to keep things tightened up.

Hot torque doesn't make sense to me since things expans when warm. You want the o-rings and head shrunk up as small as possible so as the heat expands things you get a good seal. Hot torque almost sounds like it would be a problem sooner on a cold engine.

At least that's how my thinking goes.

 

I'll play. I replaced my injector orings a while back. I torqued them to 120 in/lbs(well I actually torqued them to 130 in/lbs) .. cold. After some miles(not sure how many) I had this ticking clattering noise again. I went back in when the engine was hot and it took another 3/4 turn to get back to 130 in/lbs. Since then I've put a few thousand miles on it and it is still quiet. There are varying opinions on this but this was my actual experience. YMMV

 

As a friendly chime-in to be helpful....

The O-rings are never "pinched" by anything the injector hold-down bolt does. We are dealing with pressures that are waaay beyond what you would see as a garden hose type washer/O-ring. The O-rings fit outside a cylinder that fits inside a tube, so their wear or decomposition has no bearing on hold-down torque.

The face of the injector is held down flat against a copper washer onto a brass injector cup - two "soft" metals - that is the crucial seal to keep compression pressure, gasses, and temperatures away from your injector O-rings and cups.

In my pea brain, my thought of hot-torquing isn't strictly a matter of the temperature. While the copper washer may be only slightly more malleable (squishes down to its compression limit a little easier) after being exposed to combustion pressures and temps, the other half of the equation is the frequent cycling of the cylinder and injector works the "give" out of the components... and a kiss from the torque wrench takes up that give.

After doing any form of injector work, I run the engine to work everything out before buttoning up. I guess I could wait for a while to allow the engine to cool off before re-torquing and covering... but I am not comfortable with that experiment, nor with waiting that long to button up.

 

If you do the math, the forces generated by the combustion cycle acting to lift the injector out of the hole really aren't that great. How big is the nozzle hole, 1/4"? That's a 0.05 sq.in. hole. I don't know what diesel cylinder pressures go to, but gassers are anywhere from 1200 to maybe 3000 psi. Let's give our engines 4000 psi. 4000 * 0.05 = 200 pounds pushing up. Seems like a lot, huh? A 1/4" diameter Grade 8 bolt can hold about 5800 pounds before deformation. And we've got two of those holding it down.

Yeah, I know, they're metric. But who the heck knows what a Newton is? This is America after all.

All we're really doing with that torque is crushing the copper washer, then pre-tensioning the bolt so those vibrations Rich just mentioned, which will raise and lower the torque cyclically, will not lower it to a point where it's loose.

Another thought, we have a 4.11 bore, that 4000 psi cylinder pressure is 53,000 pounds pushing up, per cylinder!
We're really waaay over thinking this here. Andy's comment earlier about the top bolt - just tighten it down good - is probably safe. Any torque spec is a function of the bolt diameter (and material). Any bolt (or nut) tightened snug, and then another quarter to half turn, is probably at it's torque spec. Fine threads need maybe half to 3/4, or maybe even a full turn. This is why when our Dads were kids, the "spec" for a spark plug was a 1/4 turn past snug - they needed to be replaced frequently enough that folks did it themselves, and the manufacturers knew that nobody owned a torque wrench.

 

1/4" x 1/4" is 0.0625". But who's doing math?

If you don't know what cylinder pressures diesels see (I don't either) then the rest of the paragraph is just a bunch of made-up numbers. I would imagine it's a good bit higher than a gas engine because of the higher compression ratio but I have no idea what the real answer is.

A newton is unit of weight in the metric system. And before someone says it, a kilogram is a unit of mass, not weight. Pounds don't correspond to kilograms, despite what most people think.

 

Sorry Charlie. You may know your Newtons, but a hole is circular: Area of a circle = Pi x Radius x Radius Or Pi x Diameter x Diameter / 4 = 3.14159 x 0.25 x 0.25 / 4 = 0.049
Did a lot of searching, but found nothing (free) that gave info on our engines. Did find a 2013 non-specific study which examined passenger car diesel engine efficiency which stated that typical max cylinder pressures were up to 2900psi, and 1995 paper with a '91 Cummins 14L which only hit 2300 psi. So I don't think I'm that far off.

Maximum cylinder pressure is really only a function of the amount of air and fuel stuffed in the cylinder. The higher CR you mention serves two functions: generate high enough temperature in the cylinder due to compression of the air (over 900°F) to ignite the fuel when it's injected. And to allow a greater amount of expansion as the piston travels down. Typically diesels should have lower EGT's than gas engines, as that expansion allows more conversion of the generated heat energy into rotational energy of the crankshaft.
Spot on!

My senior thesis a long long time ago involved torturing a Ford Escort four banger on a dyno for several months. We ran water and alcohol injection on it, played with EGR, engine timings. Ford (Visteon in Lansdale, PA) gave us a control box to adjust things with the EEC-IV computer. It was really a hoot.

 

200 pounds or 150 pounds... consider this: Let's say there's 100 pounds of pressure on the face of the injector during compression. Skip over the heat cycles, the pressure expansion when the fuel ignites, the boost that changes the math... ignore all of that. You still have a 100-pound pulse striking the face of the injector 16 times every second at highway speeds... with 1000-1200 PSI oil pulses striking the intensifier piston at the exact same rate. Anybody who has coasted down a steep grade in gear knows exactly how much racket the injectors contribute to the engine noise... and that racket is caused by the injector getting the crap slapped out of it at a high rate of speed.

The only defense we have against the injector flying out of there in a strategic retreat is a couple of tiny bolts with 8mm heads. One might consider snugging those down as if to brace for battle.

 

Are you saying we knights should ensure our lances are firmly held and prepared to face the fire? Surely you joust

 

Dang. You win at the maths.

 

I'm sure you've pulled old injectors. The old o-rings are always flattened out, the copper washers aren't compressed too much. Tightening down the bolts on old o-rings seems to magically quiet things down.

And if the o-rings aren't pinched any, why do you have to hit the injector to make sure it gets properly seated? You're not smashing the copper crush washer when you hit the top of the injector with a mallet or your hand to make sure it's seated. So what are we seating against if it's not the o-rings? You know, the ones that have to withstand a few thousand PSI of oil pressure.

 

What about lube?

If you use assembly lube or something else?

Doesn't it keep the orings from being grabby and snagging on the cylinder walls "too much" ?

 

The way I read the original post about O-rings, it sounded as if someone might be under the impression the bolt holds metal against an O-ring, like a washer in a hose. The bolt holds a tube in a cylinder, and the O-ring contact surface is as you mention - sliding down the sides. The quieting of the injector is the prevention of the injector from lifting a tiny bit to make a small slap (cackle). Ever had a cover on a running device rattle? Ever tighten a bolt or put a matchbook in there to make it stop rattling?