Another Jabiru engine bites the dust

[Oscar]

Oscar, out of interest why is the gasket clean off so hard? Is it some sort of epoxy? I ask as a motorcycle rebuilder who has cleaned plenty of gasket material off but obviously not in such a fastidious way. Do you examine under a strong lens to see? Are you scraping or doing it with a rag and solvent? I admit I'm just curious.

pm: it's a Loctite product - Loctite 515 - and it sets as hard as rock after a while, I'm not aware of any useful solvent. Yes, scraping (very, very carefully, plus polishing with 1200-grit paper on a piece of machined flat steel bar) - and strong, low-angle light shining on a fully cleaned and dried surface continually shows up small patches of micron-thin residue that you can only see by the slight discolouration of the alloy.

 

It also sets damn quickly on assembly - the Jab. manual states:

 

Note that the entire crankcase assembly process including the fitting of pistons and cylinders MUST be completed in one continuous operation otherwise the Loctite 515 Sealant around the crankcase join can dry before uniform pressure is applied to the crankcase join by the cylinder nuts, so make sure that you have all parts ready for completion up to this stage (i.e. pistons and cylinders) before starting to fit the crankcase halves to the crankshaft.

 

7.8.1 Crankcase Joining

 

 During the joining process various sealants will be used which will cure quickly – therefore it is essential that the overhauler be thoroughly organised before starting this task and that they work efficiently and quickly once they have begun. If too much time is taken the engine must be disassembled and re-cleaned, ready for another attempt at joining.

 

Watching the CAMit engine shop guys assembling Jab. engines is an experience - two-person job, both of whom have done it hundreds of times - move so damn fast you'd think that the engine was coming together in a step-motion photography replay. Less than 5 minutes from everything on the bench to the cases (and cylinder barrels, obviously) bolted together. Every part is laid out ready before they start, every tool is placed ready for instant use.

 

It's absolutely a two-person job, and they need to be experienced people - Jab. really ought to say 'Don't try this at home - we really, really mean it! in the manual. If the 515 dries anywhere before the assembly is torqued down, you can't get full, even pressure on the sealing compound, and it will later crack and some fall out, which backs-off the pre-tension on the studs, which then allow the cases to fret and shock loadings to occur to the studs and bang-oh, another stud failure.

 

 

[Teckair]

Well, I suppose somebody has to ask it . . . Why did it fail? Because this thread is a waste of electrons until that information is available.

First post mentioned a cracked cylinder stud.

 

 

[facthunter]

If it hardens before clamping the crush won't be correct for the bearings either. Most of those compounds go off quicker with higher temps. Nev

 

 

[AVOCET]

all the crankcase joining I've been involved with ( at jab engine shop ) ,after a good clean & inspection ,we're assembled DRY and then after tensioning a small amount of green locktite wick- in is run along the join ( along the top . not to much or it leaks in and can run down and make engine tight ( have seen this ).

 

this makes sure that the mating crankcase halves are clean and dry before tensioning

 

mike

 

 

[turboplanner]

pm: it's a Loctite product - Loctite 515 - and it sets as hard as rock after a while

So let's say you do a perfect fit before it has cured: Loctite 515 has the same expansion factor as the through bolts has it?

 

 

[facthunter]

The film is so thin, if it is clamped in time it's expansion rate wouldn't matter. Nev

 

 

[turboplanner]

It would to me.

 

 

[facthunter]

It is almost a metal to metal situation, or the crush would have to allow for it's thickness. Nev

 

 

[turboplanner]

We've both put a lot of motorcycle engines together do you use Loctite 515?

 

 

[facthunter]

NO. Used Yamabond for years, but not lately. I use a good appropriate temp rated silicone, and use it sparingly to both clean faces. What you use depends a lot on the condition of the mating faces. On some I use a thread of cotton along the face. Some products will "wash out" of parts where another surface meets and the fine corners may be a bit damaged, or there will be a step where the faces of two parts meet, and a third part has to fit over both parts. Requires a little art work with an extra gasket, in affected areas. Nev

 

 

[gandalph]

Alf,

 

You have absolutely no reason to apologise and every reason to vent.

 

Stay strong mate, stay strong!

 

Regards

 

Charles

 

 

[gandalph]

It's absolutely a two-person job, and they need to be experienced people - Jab. really ought to say 'Don't try this at home - we really, really mean it! in the manual. If the 515 dries anywhere before the assembly is torqued down, you can't get full, even pressure on the sealing compound, and it will later crack and some fall out, which backs-off the pre-tension on the studs, which then allow the cases to fret and shock loadings to occur to the studs and bang-oh, another stud failure.

I believe that some shops are using a Hylomar aerosol as a non-setting case jointing compound and achieving very good results. It has a much longer working time than 515

 

 

[Guest Maj Millard]

Not Jab bashing ...way beyond that now, besides I don't need to now as everybody else does it for me.

 

As I 've said before I believe the very basis of the Jab engine problem is the CNC machines alum case. Highest rate of thermal expansion with steel cylinders hanging off it, and a hot steel rotating crankshaft within, trying to get out. No wonder through-studs break and valve trains can't keep up ..!

 

There is a good reason most aero engine cases are either cast or forged alloy....IE: Lycomong, Contenentals and Rotax

 

Alf...with you all the way there mate...and feeling your pain..

 

End of my rant ................Maj....

 

 

[Oscar]

So let's say you do a perfect fit before it has cured: Loctite 515 has the same expansion factor as the through bolts has it?

Turbs, you miss the essential point of pre-tension of the studs: if correctly designed and tensioned, you don't get expansion of the gasket material because the studs themselves do not expand sufficiently to allow movement, that is taken up by the pre-tension. However, if the pre-tension falls off for any reason - and there are several reasons why this can occur, incorrect preparation /assembly being two of them - then a chain of circumstances starts to happen that can lead to: hammering of the studs (especially in the case of even mild detonation), bending of the barrel bottom flange, bending of the stud end and rapid fatigue cracking of the stud. There are some even more non-intuitive problems that can happen: as I said earlier, the whole case securing thing is a 'system' and the 'weak link' principle applies.

 

That when everything in that system is correct, Jab engines will hold together is evidenced by the fact that a fair proportion of them DO hold together. However I'd be daft to suggest that there is not way too much evidence that if anything in the system isn't right, then they damn well don't. Not even the most one-eyed Jab. 'fan' ( and I rather assume that I am seen as such) could deny that and seriously believe what he or she is saying. I certainly wouldn't and don't. The extremely finicky nature of the basic Jab 'system' that requires everything to be absolutely dead-on, balls accurate (it's an industry term ) is the reason why CAMit have completely modified that system. The crankcases themselves are fine for purpose - it's what holds them together that is their Archilles heel.

 

I believe that some shops are using a Hylomar aerosol as a non-setting case jointing compound and achieving very good results. It has a much longer working time than 515

Funny you should say that... and it makes sure the oil passes between the crankcase halves properly without loss of oil pressure (I'd be worried to use AVOCET's dry assembly technique, for that reason). It for sure takes some of the pressure off (little assembler's joke there) when a circlip goes 'Jesus Christ' on one when assembling the engine and it flies off into the corner of the workshop and hides under the edge of the linoleum. (Ask me how I know? - go on, live dangerously.) BUT: it does need to be used in conjunction with the proper techniques and tools. Once again, kiddies: DO NOT TRY THIS AT HOME, and then come crying back that Hylomar is a crap case sealant. There is no such thing as a mechanic in a can, and if one knows about all of the effects of using WD40, mostly one bans it from one's workshop.

 

 

[Oscar]

Not Jab bashing ...way beyond that now, besides I don't need to now as everybody else does it for me.As I 've said before I believe the very basis of the Jab engine problem is the CNC machines alum case. Highest rate of thermal expansion with steel cylinders hanging off it, and a hot steel rotating crankshaft within, trying to get out. No wonder through-studs break and valve trains can't keep up ..!

 

There is a good reason most aero engine cases are either cast or forged alloy....IE: Lycomong, Contenentals and Rotax

 

Alf...with you all the way there mate...and feeling your pain..

 

End of my rant ................Maj....

You are so completely wrong about the crankcase material and manufacturing technique as being a problem, that it is not just risable but woeful. Your fundamental lack of understanding of engine manufacturing and metallurgy is so far wrong as to place you in the realm of the wilfully ignorant. Possibly, you would be better to stick to commenting on parachute packing - I have no doubt you are extremely well qualified there.

 

BTW, it's Lycoming and Continental - but at least you got Rotax correct. In the circumstances, one out of three is about par for your course.

 

 

[Dafydd Llewellyn]

Not Jab bashing ...way beyond that now, besides I don't need to now as everybody else does it for me.As I 've said before I believe the very basis of the Jab engine problem is the CNC machines alum case. Highest rate of thermal expansion with steel cylinders hanging off it, and a hot steel rotating crankshaft within, trying to get out. No wonder through-studs break and valve trains can't keep up ..!

 

There is a good reason most aero engine cases are either cast or forged alloy....IE: Lycomong, Contenentals and Rotax

 

Alf...with you all the way there mate...and feeling your pain..

 

End of my rant ................Maj....

Maj, you've raised this issue before, and it is just as invalid now as it was then. Attached are the data for 5083 (the Jabiru crankcase material), 6061, and cast 355 alloy (which is what Lycoming and Continental use). At (say) 90F, the expansion coefficients (alpha in the graphs and the table) are: 5083: 13.2 x 10^-6 inches per inch per degree farenheit; 6061: 12.65 x 10^-6 inches per inch per degree farenheit; 355 : 12.4 x 10^-6 inches per inch per degree farenheit. Note that the values make no distinction whatever between wrought or cast forms. So the difference amounts to a bit less than 6.5% between 5083 and 355, with 6061 in between them. This hardly justifies your assertion.

 

 

 

 

[deadstick]

Oscar: have some decorum mate please, have you ever rebuilt one? I'm sure you have seen one disassembled before! Some of your assessments thus far have been slightly left of fact but some closer to accurate. I admire your passion but not your tone or superiority complex! Obviously its hard to be humble up there on your golden throne with all your books and publications telling you what its like down here with the aircraft engineers working with this product! Qualifications mean nothing against experience mate but I'm sure you have read that somewhere hey?

 

Its hard to take you serious with posts like the above!

 

 

[gandalph]

Maj, you've raised this issue before, and it is just as invalid now as it was then. Attached are the data for 5083 (the Jabiru crankcase material), 6061, and cast 355 alloy (which is what Lycoming and Continental use). At (say) 90F, the expansion coefficients (alpha in the graphs and the table) are: 5083: 13.2 x 10^-6 inches per inch per degree farenheit; 6061: 12.65 x 10^-6 inches per inch per degree farenheit; 355 : 12.4 x 10^-6 inches per inch per degree farenheit. Note that the values make no distinction whatever between wrought or cast forms. So the difference amounts to a bit less than 6.5% between 5083 and 355, with 6061 in between them. This hardly justifies your assertion.There you go again! Spoiling a good rant with facts.

[M61A1]

pm: it's a Loctite product - Loctite 515 - and it sets as hard as rock after a while, I'm not aware of any useful solvent. Yes, scraping (very, very carefully, plus polishing with 1200-grit paper on a piece of machined flat steel bar) - and strong, low-angle light shining on a fully cleaned and dried surface continually shows up small patches of micron-thin residue that you can only see by the slight discolouration of the alloy. It also sets damn quickly on assembly - the Jab. manual states:

 

Note that the entire crankcase assembly process including the fitting of pistons and cylinders MUST be completed in one continuous operation otherwise the Loctite 515 Sealant around the crankcase join can dry before uniform pressure is applied to the crankcase join by the cylinder nuts, so make sure that you have all parts ready for completion up to this stage (i.e. pistons and cylinders) before starting to fit the crankcase halves to the crankshaft.

 

7.8.1 Crankcase Joining

 

 During the joining process various sealants will be used which will cure quickly – therefore it is essential that the overhauler be thoroughly organised before starting this task and that they work efficiently and quickly once they have begun. If too much time is taken the engine must be disassembled and re-cleaned, ready for another attempt at joining.

 

Watching the CAMit engine shop guys assembling Jab. engines is an experience - two-person job, both of whom have done it hundreds of times - move so damn fast you'd think that the engine was coming together in a step-motion photography replay. Less than 5 minutes from everything on the bench to the cases (and cylinder barrels, obviously) bolted together. Every part is laid out ready before they start, every tool is placed ready for instant use.

 

It's absolutely a two-person job, and they need to be experienced people - Jab. really ought to say 'Don't try this at home - we really, really mean it! in the manual. If the 515 dries anywhere before the assembly is torqued down, you can't get full, even pressure on the sealing compound, and it will later crack and some fall out, which backs-off the pre-tension on the studs, which then allow the cases to fret and shock loadings to occur to the studs and bang-oh, another stud failure.

If the sealant you are using is going off before you join the cases, you're not using 515. 515 is an anaerobic polymer and will only go off once you deprive it of air (and quite some time after, unless you heat it). In fact I've had tubes of it open in my road machinery toolbox for several years with no ill effect.It's not something I'd use on any narrow sealing surface, as it isn't very flexible. Never found it difficult to remove though, so maybe it's not 515 you're removing. I've had much better results with Nev's Yamabond or the equivalent Threebond. Mind you, I don't play with Jabirus.

 

 

[AVOCET]

Turbs, you miss the essential point of pre-tension of the studs: if correctly designed and tensioned, you don't get expansion of the gasket material because the studs themselves do not expand sufficiently to allow movement, that is taken up by the pre-tension. However, if the pre-tension falls off for any reason - and there are several reasons why this can occur, incorrect preparation /assembly being two of them - then a chain of circumstances starts to happen that can lead to: hammering of the studs (especially in the case of even mild detonation), bending of the barrel bottom flange, bending of the stud end and rapid fatigue cracking of the stud. There are some even more non-intuitive problems that can happen: as I said earlier, the whole case securing thing is a 'system' and the 'weak link' principle applies.That when everything in that system is correct, Jab engines will hold together is evidenced by the fact that a fair proportion of them DO hold together. However I'd be daft to suggest that there is not way too much evidence that if anything in the system isn't right, then they damn well don't. Not even the most one-eyed Jab. 'fan' ( and I rather assume that I am seen as such) could deny that and seriously believe what he or she is saying. I certainly wouldn't and don't. The extremely finicky nature of the basic Jab 'system' that requires everything to be absolutely dead-on, balls accurate (it's an industry term ) is the reason why CAMit have completely modified that system. The crankcases themselves are fine for purpose - it's what holds them together that is their Archilles heel.

 

Funny you should say that... and it makes sure the oil passes between the crankcase halves properly without loss of oil pressure (I'd be worried to use AVOCET's dry assembly technique, for that reason). It for sure takes some of the pressure off (little assembler's joke there) when a circlip goes 'Jesus Christ' on one when assembling the engine and it flies off into the corner of the workshop and hides under the edge of the linoleum. (Ask me how I know? - go on, live dangerously.) BUT: it does need to be used in conjunction with the proper techniques and tools. Once again, kiddies: DO NOT TRY THIS AT HOME, and then come crying back that Hylomar is a crap case sealant. There is no such thing as a mechanic in a can, and if one knows about all of the effects of using WD40, mostly one bans it from one's workshop.

just for the record; it's not my technique ,

that's the way jab does it , or did it .( last engine revisit was 2 1/2 years ago.

 

I wasn't aware that it's being done as you say ,or camits assembly procedures .

 

my engines that were done in front of me all had the cases inspected for flatness and were either accepted or rejected .

 

cheers mike

 

 

[motzartmerv]

For the less educated amongst us, what is the thermal expansion rate of the cylinder heads, compared with the crank case?

 

 

[Oscar]

Indeed, I have rebuilt one. Under the watchful eyes, the instruction and with the help of the guys who build them all, not only showing/checking what was done but explaining all the things that matter about what was being done. With the guy who manufacturers them telling me what to do and teaching me how to do the machining and inspecting my work (and even hand-making some tools just to do my old engine to the new specifications). With the guy who measures each and every engine for production checking my work every step of the way, in a laser-measuring-equipped, temp. controlled measurement room. In the build room that every new Jabiru engine comes out of, using the tools, the benches, all the gear that every new Jabiru engine is assembled with. Following the assembly procedure from dedicated bench to dedicated bench, starting with the crankshaft and down to the electrics.

 

Got a 'tick' from those guys. Would I suggest I can do it for a living? - NO. I learned so much from the experience of being there, I know how precise and experienced one needs to be, and I certainly know that I don't have that level of experience, nor do I have the very specialised tools for doing certain procedures correctly..

 

Not from books, little Noddy, but from DOING IT. Spent two weeks learning and doing it, making mistakes, learning from them, fixing them.

 

So suck it up, princess, and get off your own high horse.

 

 

[motzartmerv]

Fellas, lets keep it cool hey?

 

 

[SDQDI]

............ You fellas are going to get/keep me fat at this rate

[Oscar]

just for the record; it's not my technique ,that's the way jab does it , or did it .( last engine revisit was 2 1/2 years ago.

I wasn't aware that it's being done as you say ,or camits assembly procedures .

 

my engines that were done in front of me all had the cases inspected for flatness and were either accepted or rejected .

 

cheers mike

Mike - dry assembly is absolutely NOT the Jab recommended technique: see: http://www.jabiru.net.au/eula/eula.php?u=/Manuals/Engine/JEM0001-6_Overhaul_Manual.pdf I don't have knowledge of early times, but I understand that the Loctite 515 assembly material has been the recommended technique for a long time - others may well be more knowledgeable than me there.

 

Given the precision of everything required for effective use of Loctite 515, I'd personally hate to have to use the stuff, and quite possibly dry assembly has a lower potential of problems - but given the clamping design, I suspect some form of sealing between the cases is required to ensure you don't lose oil pressure from inadequate inter-case sealing. You'd certainly want no less than perfect case-face flatness to be confident of dry assembly.

 

The Hylomar alternative ( if used correctly) is far more tolerant of anything less than perfect assembly technique. I believe that CAMit will fairly soon produce an assembly manual for their modded engine that will detail what is required; personally, I have that on my list of required reading..