Jabiru thru bolt failure.

[Jabiru7252]

Head shows signs of "pulling In" What's that? Is it something one can keep an eye on through leak down tests etc?

 

 

[motzartmerv]

Pulling in means that the head changes shape slightly over time and develops a slight lip, normally on the exhaust valve side. Its an indication of heat normally. Once the shape of the thing changes obviously things aren't where they are meant to be and you can get valve problems and other issues. If things change shape then the through bolts aren't clamping properly and things start to break.

 

I dont think a leak down would show it up unless it had pulled in enough to cause a valve to not close properly.

 

The focus of our argument here is that this aeroplane never showed signs of over temp, it was always a cool runner. Out of all the jabs we have had online this one was by far the 'coolest' running of them all, as far as indications go.

 

 

[Guest Andys@coffs]

Jabiru heads are held down by 6 bolts. 5 of them easy to get to and the 6th is not. Remove tappet cover and the tapered grub screw. Reality is that of the 6 the 5 easy to get to bolts don't tend in my experience to need tightening much but that 6th is located right between the valves and therefore closest to the exhaust valve. It does seem to require tightening whenever it is called to be checked. If the bolts aren't lengthening/ stretching then what is it that requires the tension to be reset? It's the head being pulled in. If you don't pull it in I can guarantee you'll have a problem! That you have to pull it in is in itself a problem......

 

Deadstick is that your take on the issue?

 

Andy

 

Merv is it true that a loose 6th head bolt won't show up on a leak down.... I know that if it's loose you will sure see signs of exhaust gasses exiting......but I bet 80psi in a leak down is nothing compared to ignition pressures......

 

 

[motzartmerv]

has anybody, ever had any experience with a 4 stroke engine in any other walk of life that required head re tourqing as part of its scheduled maintenance?

 

Ive spoken to several mechanics who have NEVER heard of such a thing. Im curious has anybody seen an engine like this before?

 

 

[Guest Andys@coffs]

Pulling in means that the head changes shape slightly over time and develops a slight lip, normally on the exhaust valve side. Its an indication of heat normally. Once the shape of the thing changes obviously things aren't where they are meant to be and you can get valve problems and other issues. If things change shape then the through bolts aren't clamping properly and things start to break.I dont think a leak down would show it up unless it had pulled in enough to cause a valve to not close properly.

The focus of our argument here is that this aeroplane never showed signs of over temp, it was always a cool runner. Out of all the jabs we have had online this one was by far the 'coolest' running of them all, as far as indications go.

Merv just so I understand what indications are you talking of? Which cylinder failed and which were instrumented?

 

Andy

 

 

[motzartmerv]

Number 4 failed, and i think thats the one that has the CHT. Im not sure tho. I would have to check that Andy.

 

 

[facthunter]

Merv, It's common with motorcycles of an earlier period, to have to retorque the heads for a while, usually while the head gasket compresses. The Jabiru head does not use a gasket but overheats in the area we are talking about and the metal moves and requires retensioning of the bolt(s). Improvement in the metal specs should help, and/or better cooling. The bolts in the cooler running parts don't change much. Most aero engines using individual air cooled cylinders don't have easily detachable heads, and are more or less permanently screwed together with a large diameter thread on the top of the steel cylinder and heat shrunk at the same time.. Nev

 

 

[deadstick]

Two easily overlooked reasons that the exhaust side gets hot is the exhaust is leaking at the flange or the spark plug CHT ring is leaking. Anyone with the CHT rings under the plug should remove the crush washer to allow the plug to screw down equally with the intake plug. A side effect of head creep eventually is valve leakage as the rod dimensions are closing, meaning the valve is opening earlier or being half off the seat. I wonder if different power outputs per cylinder could be causing the case halves to want to split?or even setting up a harmonic through the crank, ie broken flywheel bolts. Ideas?

 

 

[Dafydd Llewellyn]

I think it comes down to this: If the head bolts show no evidence of losing tension, and the valve clearances (on a solid-lifter engine) do not keep closing, then the operation of the engine is staying within safe parameters. If this is not the case, the engine is operating at too high a temperature for the materials from which is is built. There are also various conditions, as have been mentioned on this thread, that can aggravate to situation, especially leaky valves. Unfortunately, one cannot use the valve clearances as an indicator on other than solid-lifter engines. However, checking these things regularly is a useful means of verifying that one's operating procedures are OK, and this is a far better indication than anything the gauges may show.

 

I agree that it should not be necessary to keep doing this, but it seems to be necessary for Jabiru engines; they do not have much margin in hand. The same principle applies to the through bolt tensions. If nothing's changing, it's OK; if things are changing, it's not likely to run to full life, unless you alter your operating procedures to stop things from moving.

 

Whether it is possible to operate the engine so that the head-bolt tensions etc do not change, depends upon the adequacy of the installation of the cooling system, and on the mixture not being too lean; these engines have (like any air-cooled aero engine) to run well on the rich side above about 75% power.

 

I am not addressing questions of workmanship here; but people should perhaps be aware that there appears to be some evidence that there can be a difference in reliability between engines that have been modified subsequently to their original assembly by CAMit, and those that have not. CAMit was once building over four hundred engines per month; but that led to an overstocking of engines by Jabiru; and some of those overstocked engines may have been modified to later build standard, not necessarily always by CAMit.

 

 

[deadstick]

Hi dafydd, I recently experience lifter pump on my 35 he old 3300. I had a minor leak past one of the valves on the leakdown, turned out the lifter was not leaking back. Speaking to Ian at Camit he believes it because the lifters are not designed to lay on their side. Are you able to bait us with some of your test results yet?

 

 

[Dafydd Llewellyn]

Hi dafydd, I recently experience lifter pump on my 35 he old 3300. I had a minor leak past one of the valves on the leakdown, turned out the lifter was not leaking back. Speaking to Ian at Camit he believes it because the lifters are not designed to lay on their side. Are you able to bait us with some of your test results yet?

No, still wrestling with instrumentation and other gremlins. The engine I'm currently running is a solid-lifter 2200, with some but not all the CAMit mods. I doubt I'll get an opportunity to look at an hydraulic-lifter or roller-follower engine; Ian is concentrating on solid lifter engines (of course, he has a conversion kit to convert a hydraulic lifter engine back to solid lifters.)

What I'm seeing so far is mainly to do with the pressure drop from above the barrels to below them, and the head temperatures that result - and at present I can only get to 27 inches manifold pressure (I'm at 2000 ft AMSL here) and 2800 RPM with the available propeller. The engine in the cell has the old coarse-pitch fins, so not representative of the later "microgroove" fin heads. The engine has under-cylinder baffling, so that's different from a typical aircraft installation, too. I had a lot of trouble getting the oil temperature up into its correct range, because in the test cell, the engine is uncowled, with the cylinders supplied from a blower, and the whole of the sump is exposed to the full propeller blast; I had to insulate it to get proper circulation through the oil cooler, which allowed me to get the oil temp under control. So the test cell is still in its teething stages and the data so far aren't sufficiently general or proven for me to start making pronouncements.

 

However it looks to me, from what I've seen so far, that it would be preferable to locate the oil pressure sender on the main oil gallery downstream of the oil filter and oil cooler; and also to locate the oil temperature sender on the return side of the oil cooler. The engine limits would need to change to reflect these locations; but Ian's mods to the oil pressure relief valve have resulted in the test engine holding its oil pressure very well.

 

I've installed a large Roots supercharger on the cell, driven by a 5 HP 3-phase motor, and I'll be running a test on the supercharger lubrication system this afternoon, if that works, I should be able to get sea level conditions this weekend. However indications are that the cooling blower needs modification to its intake and blading to supply sufficient air for full throttle running; with the old heads, I don't want to exceed 180 C (calibrated).

 

 

[spacesailor]

:drive:Running too lean!. A post from another world.

 

61.mpg @ 105.mph!

 

spacesailor, ps, four bolt heads, air cooled.

 

 

 

[turboplanner]

That's not necessarily running too lean spacesailor. The power required to overcome wind resistance is exponential with speed, and if you can minimise frontal area and maximise streamlining you will not have to worry about leaning the engine and can use more fuel to cool the combustion chamber.

 

 

[facthunter]

A hummel is pretty clean and Its usually a 1/2 VW. Motor. Nev

 

 

[jetjr]

the head pulling in was common in older thick finned heads and was regular to need tighten heads every service - basically they ran hotter more easily. Being solid lifters then the lifter clearances needed adjustment, a pain but you knew what was going on. Also got to have a look inside valve covers for brown oil stains or guide leakage.

 

Whole issue of thread locking procedure and tension accuracy too comes into it, corrosion, loctite etcetc all alters force and strong arms and weak torque wrenches also no doubt played a part.

 

Plenty of heads leaked badly, Rotec LCH was born from this problem and probably solves it but so would getting all heads even and cooler.

 

the CHT ring sensor was common then even on EMS setups and bled exhaust gas leading to crazy readings and fiddly leakdowns. Getting crush washer off is easier said than done ive found, and then still leaks.

 

Some indicate head material isnt best selection but is easy to work with so theres a trade off.

 

With "new" hydraulic lifters and no mention in books the potential issues with heads pulling in and subsequent valve train problems unless you have seen it or visit forums like this you wouldnt know. I believe theres not much tolerance in the springs as far as binding goes??

 

Uneven CHT and EGT are the killer, some heads run close to limits all the time and well above occasionally, others far to cold for efficient operation. Small air dams and things can solve this with lots of trial and error. Without full monitoring youd have no idea. shouldnt be the case but it is. Variations change at differing rpm too, without info youll never guess whats going on.

 

The published limits are often all out the window as CHT rings give poor readings depending on airflow. (Camit I see have new shielded CHT available now). I know for example my CHT are all reading low though experiment, my limits are now 170 on the EMS.

 

Im concerned to hear of flywheel bolt breakages - thought this was solved. Heard of another last week on 6 cyl which is unusual. What prop was it running?

 

 

[deadstick]

JJ it was running a jab prop, correctly tensioned. It is a strange outcome, I mean the bolts just stress fracture and not from shear loads as the dowels are still in place.

 

 

[jetjr]

Ok was it the wood or new composite?

 

Used to be debate on 4 cyl re more rigid props and harmonics, 6 cyl never had the issues and I was surprised to see Jab went for composite on 4 cyl.

 

Seems Rod can change his mind.

 

Flywheel problems may not be at all related to prop just interested. Most agreed dowels would fix it inc CASA engineer types.

 

 

[deadstick]

Prop was the standard wooden laminate, torqued and tracked . I am suspicious that there is some sort of longitudinal harmonic.

 

 

[jetjr]

How do you track that down and deal with it?

 

Will vibration transducer see it?

 

Rotorcraft ones can do two directions?

 

.......guess its hard in engines not going

 

 

[motzartmerv]

I reckon you can hear it.!!

 

 

[deadstick]

I reckon you can hear it.!!

A top end Chadwick helmeth vibrio comp would find it. Would be interesting to mount a transducer vertical, lateral and for-aft to see what the vibration spectrum is. I recently spent a lot of time balancing the prop on my j160, I got it down to .01 ips, very noticeable and at the time I was considering putting another transducer at the rear of the engine and comparing the vibration results, might have a play around next time I've got it hooked up.

 

 

[facthunter]

I think we are talking about torsional vibration here. 4 cyl motors (flat or in line) are worse than most other configurations. You would detect it with a strobe and it will be RPM sensitive.. Prop mass will move it, but we are not talking of balancing props or even the engine in the normal sense. Nev

 

 

[Old Koreelah]

I fill in my paperwork while sitting in the cockpit as the engine warms up. Going back thru my flight plans shows that lately the writing is getting increasingly wobbly. Is that an indication of balance issues- or old timer's disease?

 

 

[SDQDI]

I fill in my paperwork while sitting in the cockpit as the engine warms up. Going back thus my flight plans shows that lately the writing is getting increasingly wobbly. Is that an indication of balance issue- or old timer's disease?

No comment:happy:

But on a serious note if you compared your handwriting elsewhere that was done while not in the plane maybe a diary or the likes you might be able to see a pattern.

 

 

[facthunter]

Hypoxia. The engine is idling too fast. Nev