Another Jab bites the dust.

[XAIRVTW]

From what Dead-stick has already said from his preliminary inspection ... the engine had done 1370 hrs, 3 topends and was supplied before the thru-bolt upgrade was put into production.Several people here have tried to ask the question in a subtle way .... forgive me for being direct but I can only draw from DS statement that this engine was not compliant with the Manufactures Safety Direction and was still running parts that are known to fail.

 

The attached Service Bulletin... it says it all ... I hope all Jab owners have read this and make sure they have implemented these Safety Directions

 

http://www.jabiru.net.au/Service Bulletins/Engine files/JSB031-1_JSD031-1_Engine_Through_Bolts.pdf

Thats what I was thinking!

 

 

[old man emu]

On the topic of calibrating torque wrenches:

 

We have just had a surveillance inspection of our CASA approve workshop by our local CASA man. The topic of calibrating our torque wrenches came up and we showed the CASA man the calibration certificates we had from an instrument calibration company for the tests done on our torque wrenches. Would you believe that the CASA man rejected the calibrations because the company didn't have the right (to him) accreditation.

 

We have been using this same company for years, and haven't over or under-torqued a bolt yet.

 

It seems that the Sydney office of CASA has a bee in its bonnet, as this problem does not seem to arise in other States.

 

OME

 

 

[facthunter]

I do not doubt what you say. Go for the latest "thing" and forget the big picture. Don't you wonder why you work in the industry? I nearly got started there with examples but stopped myself . They'd go looking for someone rather than rectify anything. Nev

 

 

[Camel]

CamelThere is another thread within the Jabiru section about the availablility from WA of alternate through bolts. These alternates have rolled rather than machined threads (although I have read posts that claim that the assertion that the Jabiru supplied bolts are machined is wrong, and they too are rolled).

 

That said, there is some testing documented in that thread where the required torque is compared to the point of failure and if I summarise (hopefully correctly as Im not a MechEng) the point of failure for the J supplied bolts/nuts was very close to the required torque (arguably too close), annecdotally at times below the required torque, where the WA sourced alternates point of failure is significantly above the required torque. That thread also suggests that interface between J supplied bolt and thread is not correct with the bolt appearing to those in the know, to be oversized leading to early failure.

 

If you havent yet read that thread you should its every bit as useful, perhaps more than this one, and as I recall almost all posts were relevant to the issue.....unlike a fiar chunk of this one.

 

Andy

Andy,

I have read it peviously and am aware of what SP6 has done and admire him for his effort. I would use the bolts except I have a factory built J230 and want to keep original parts as much as possible and only has 150hrs and I have done the nut change. If his breaking point is correct for the original studs then they should never be re used as the amount of force to undo a loctite nut would certainly stress the through bolt. the original through bolt nuts do not use loctite, they are a locking type nut and undo quite easily so I see no problem re using the original in this case.

 

I am a Motor Mechanic from years ago and have worked on cars, bikes, trucks, boats, machinery, very large (huge) stationary plant and aircraft. I believe any motor mechanic that is experienced enough does not need a tension wrench and most mechanic's will understand what I say. So the tension wrench part is not the problem I'm sure, The problem I keep hearing people keep swinging on through bolts with a tension wrench doing more harm than good and possibly think a bit extra will keep it tight which is also bad.

 

The comment from Facthunter is the proof. When wheel nuts have been over tightened by a air tool trigger happy tyre man, the studs stretch, these wheel nut studs will most certainly break from the slightest load applied to the wheel e.g rough road. Wheel nuts tightened by hand by a mechanic will be perfect as he will feel the right tension, nuts tightened by hand by Mr average could be too tight or too loose or uneven, Mr average with a tension wrench is still no assurance that it is right because if his tightening sequence is wrong then it's not torqued even and chances of a problem exist. Truck wheels are more critical but it doesn't mean you need a tension wrench, the sequence and the pretension then the tension is the important part, truck wheel do need to be rechecked later. Seen a lot of broken wheel studs and I sure a lot of other people have too.

 

 

[facthunter]

Camel , Most mechanics overtighten the drain plugs on every car and motorbike that I have ever been unfortunate enough to let them work on. So I do it myself except when it's not possible.

 

When I was working full time on car engines reconditioning them, I could judge the torques by hand to within a few percent. The length of each size spanner used to be in relation to a comfortable tightness for the size of bolt.

 

No aero or contracted vehicle repair shop today would get away with even the most capable assembly workers doing it by "feel". You could never "assure" anyone that it would be consistent. You can still get feel, but it's harder with the longer tension wrench.

 

An experienced mechanic will know also that the nut has gone too far, and stop ( and investigate it) when someone else may just keep going till it "clicks". ( strips or breaks).

 

If you pretension them evenly to a fairly low figure, each one should only take a fixed angle of extra tightness to get it to the required setting. This principle is used in some assemblies.. Nev

 

 

[Powerin]

All aero cylinder base flanges look too thin, but surely they have been engineered properly?

That, Nev, is the right question!

(with apologies to "I Robot")

 

 

[Ultralights]

Maybe it could be something as simple as a design flaw, almost, well, all horizontally opposed piston aircraft engine I have seen has a crankcase that is bolted together, and each cylinder barrel is bolted to the crankcase. So the barrel bolts only have 1 job to do, that of holding the barrel on against the combustion forces. Whereas the jabiru through bolts have the dual role of opposing combustion and piston movement forces, as well as the forces involved with the rotation of the crankshaft within the crankcase and the flight loads pulling that crankshaft forward within the block.

 

Maybe the combined forces of crankcase loads from the crankshaft/propellor, as well as the forces of combustion and piston movement within the cylinder barrel are simply to much for the design of through bolts to handle in the long term, resulting in reduced service life of the through bolts?

 

 

[xair1159]

I have seen original style through bolts where a genuine aircraft quality AN 3/8 nut was a rattling fit due to the bolt threads being 10 thou undersize. If this type of assembly was torqued up, the threads change from being symmetrical to a "Christmas tree" profile which not surprisingly fails to hold the load.

 

I recently carried out JSB013 on my engine and the new Jabiru supplied slightly longer through bolt threads were checked and all found to be within tolerance (Class 2). The ARP sourced nuts that are now used are available in a self locking style, just like the original MS nuts, so why are non-locking ones specified? If the MS nuts were good for a couple of thousand engines, why is Loctite 620 now used to glue everything in sight?

 

To avoid distorted barrels when hot, careful torquing in stages is required. With Loctite 620 you have a couple of minutes before the whole lot goes solid and the torque measured is meaningless.

 

On my engine it was necessary to chamfer the nuts to avoid them sitting on the barrel flange radius. The holes in the flange had a good 1.5mm clearance over the bolts which seems excessive and means the chamfered nuts are only bearing on a thin annular area. It also meant the barrels could be rotated slightly and had to be aligned to the block with a square. Are all the flanges like this, or is it just a dud batch like the oversize valve guides?

 

 

[Guest ozzie]

Ok so assuming correct torque values are being used. Couple of other things come to mind. Consistancy of materials used batch to batch and consistancy in manufacture by both machine and operator. Cracking cylinder bases are the ones made right before the tool bits were resharpened?

 

Comment above about the thru bolts load being a combination of crankshaft loads and the want of the barrells going bush every time combustion takes place. - These two forces could be working detrimental to each other compounding the problem. Not enough thru bolts or maybe some extra studs need to be inserted into the crankcase halves for each cylinder to help take some of the load off the studs. Another but quick and nasty way would be a series of welds along the case halves between each thru bolt. Could also be the cases are twisting with the crank motion and working the thru bolts. Does this sound too desperate?

 

 

[damkia]

Ok so assuming correct torque values are being used. Couple of other things come to mind. Consistancy of materials used batch to batch and consistancy in manufacture by both machine and operator. Cracking cylinder bases are the ones made right before the tool bits were resharpened?Comment above about the thru bolts load being a combination of crankshaft loads and the want of the barrells going bush every time combustion takes place. - These two forces could be working detrimental to each other compounding the problem. Not enough thru bolts or maybe some extra studs need to be inserted into the crankcase halves for each cylinder to help take some of the load off the studs. Another but quick and nasty way would be a series of welds along the case halves between each thru bolt. Could also be the cases are twisting with the crank motion and working the thru bolts. Does this sound too desperate?

IIRC consistancy of manufacture would be good due to the parts being CNC machined.

 

Material consistancy would be the next thing to look at, but I would guess as some of the aircraft engines are certified then there would need to be reasonable QC on the materials with documentation of standards to be met with each batch.

 

Whether the end products designed/manufactured with appropriate materials could be a factor.

 

In military applications I am actually aware of various parts only having a safety factor of about 5% over what they are used at, to maximise the lightness of each part. (old man was RAAF mechanical engineer - his team was the one that "added lightness and simplicated" the F-111 swing wing mechanism for the yanks years ago )

 

Once again, no expert, or Jab basher, simply someone interested...

 

 

[facthunter]

A combination of through studs and ones that just go into the crankcase are a feature of the lyc's and continentals. This is a necessary evil. with those designs because of their main bearing location. The jabiru had seen fit to have a mainbearing between each throw of the crankshaft. Whether this is necessary in a motor that only puts out 20hp out of each 550 cc cylinder could be debated but it enables more through studs. Through studs take all of the tension across the motor and are generally considered a good thing from a purist design point of view. I suppose you could add extra short studs but they wouldn't have such a good load path. 4 should do the job if the flange and cylinder wall is adequate.

 

You couldn't rule out case flexing. Cases that have individual cylinders hanging off them don't have a lot of rigidity. Some Radials have steel cases. Nev.

 

 

[Guest Maj Millard]

I have always suspected that the billett CNC machined alloy case on the Jab engine is half the problem. Forged or cast alloy a different story. You've got thermal expansion problems and internal stresses in the billett that are upset or released when the case is machined.

 

Years ago when operating a CNC mill for two years, we recieved boxes of alloy extrusions to be machined for Helwett Packard. The tolerences on the final part were very tight, and during the first machining operation a lot of metal was removed, from the extrusion. Right there we would reject and lose about 30% of the extrusions, due to released internal stresses which would twist the part and throw it way out of wack. Finally it got to the point where we rejected more than we made and the boss told HP that we didn't need to do them any more. Their reply : you will do them or we'll own the company in the morning !....we kept doing them but doubled the price !!........................................................Maj...

 

 

[facthunter]

I would imagine that all the cast and forged cases are heat treated to stress relieve and improve tensile strength. There could easily be residual stresses in the CNC'd cases. I don't know what jabiru do there.

 

As an example of what stresses can exist in aircraft parts, there is a large forged or cast ring in the fuselage of the Vampire Jet and I saw one of them cut. when it was cut through, it sprung so as to displace the edges of the separated metal by 3". (Three Inches) How much stress was in that part before any flight loads were put on it? Nev

 

 

[gandalph]

You guys have got it all wrongRod mentioned at Natfly that if we are having a problem with the engines (I am) it is because we are all operating them wrong.

He said that the reason we are having overheating problems with the 3000 engine is because they are tuned to operate in Bundaberg and we have different weather in Melbourne.

 

I wasnt sure whether he was recommending that all Jab drivers in Melbourne relocate to Bundy or that we get Jab to retune them for Melbourne weather

 

Any suggestions?

 

Cheers

 

Bryon

[facthunter]

Yeah OK. But you have different weather once you get airborne. It gets colder and thinner. Nev

 

 

[turboplanner]

Yeah OK. But you have different weather once you get airborne. It gets colder and thinner. Nev

You have to get out and change the jets there too.

 

 

[Bandit12]

I am so not going outside to change the jets when it is cold!

 

 

[Vev]

Nev,

 

An interesting observation was shared with me when I spoke with the South African Jab distributor 2 years ago ... he said that Jab engines flying out of Johannesburg would easily make TBO +, yet engines in Durban were only making 350 hrs and needed a topend refresh. He said the altitude at JHB is approx 5250 ft and the thinner air made the Bing run rich (despite pressure compensation) and therefore cooler, whereas in Durban, at sea level, engines that are run at around 2800rpm ran lean and got very hot, which consequently cooked the top ends. The lesson learnt was to run the Jab engine hard and ensure that the Bing was operating on the pointy end of the needle jet to get the fuel into the engine.

 

He said the best indicator is EGT and it must remain below 1250 deg F as anything above this means a shorten life on the top end ... Don at Jab in QLD also says "fly them like a fighter" and never baby them. If you operate the engine around 2800 rpm you will cook the top end without you knowing it as your CHT will indicate normal temps but your EGT's will be 1400 deg F and making a mess of the top end.

 

Personally I fitted EGT's and use this to set my throttle position when in cruise ... so far my top end looks as good as the day I put together and all of my temps are sitting down in the lower quarter of the heat range :-)

 

Cheers

 

Vev

 

 

[David Isaac]

Vev and Nev,

 

This all adds credibility to the detonation argument.

 

Has anyone done a correlation of the particular cylinders that keep braking through bolts and any relationship to high EGTs on those particular cylinders?

 

We know that the mixtures across the cylinders varies considerably because almost all you talk to speak of widely varying EGTs. If this is so, it would have to be consistent across all engines of the same type and therefore the same cylinders would be running lean, which logically could lead to the same cylinders detonating and breaking through bolts and cracking cylinders.

 

The fact that the cylinders are cracking and at the same time breaking the through bolts leads to a problem beyond weak flanges and indicates massive forces that could be argued consistent with detonation. I still don't think many on this forum understand how destructive detonation is in an engine and I am NOT talking about pre-ignition ... there is a world of difference, I have been on the expensive end of detonation in boat racing engines I have built over the years and boat engines are very similar to aero engines in application.

 

Regards,

 

 

[turboplanner]

Good point, if we could get a consistent failure on the same one or two cylinders, that may point in the gas flow/manifold direction

 

 

[turboplanner]

Also, I wouldn't mind seeing actual head temperature (metal) reports rather than EGT which is a little downstream from where the action is taking place, and can be quite different.

 

 

[facthunter]

Vev, the trouble with the logic of that is that at 5.000 plus feet the engine never gets near rated power on WOT. It's probably 25% below. Also a CV carb at that height will not get the dashpot piston anywhere near the point ( right up) where it is enrichened by the bottom part of the needles taper being employed to affect the mixture. strength.

 

It's probably that the engines operating at the higher and colder altitudes are not working hard at all. They are effectively "derated" and last longer because less power less heat and virtually NO chance of detonation ( if that is the cause) because the volumetric eficiency is down and so is the effective compression ratio. Nev

 

 

[facthunter]

On the mixture variation thing, Jabiru have spent a lot of time and development on that. They seem to have very streamlined branches in the manifold trying to get even flow and distribution. I'm not sure this can be made to work perfectly. Many engines have this problem. The Rotax has it too but the heads run cool so it does not cause a problem apart from a wasting of fuel.

 

The Continental 0-200 has an updraught carb and a X shaped distribution which has almost equal length pipes on the inlets and the carb feeds straight into the middle of the X. I'm sure this works better, but the sump is in the way on the Jab. for something like that to be done. Nev

 

 

[Guest ozzie]

Wondered how long before the updraft carby would be mentioned. The central positioning only has to benefit the even balance of fuel air mix between the cylinders.

 

Running the exercise of the jab engine around and around it always seems to come back to the induction system as the major player in the game.

 

 

[Vev]

On the mixture variation thing, Jabiru have spent a lot of time and development on that. They seem to have very streamlined branches in the manifold trying to get even flow and distribution. I'm not sure this can be made to work perfectly. Many engines have this problem. The Rotax has it too but the heads run cool so it does not cause a problem apart from a wasting of fuel.The Continental 0-200 has an updraught carb and a X shaped distribution which has almost equal length pipes on the inlets and the carb feeds straight into the middle of the X. I'm sure this works better, but the sump is in the way on the Jab. for something like that to be done. Nev

Nev,

 

I am sure you are correct over all with this thinking.... as I am aware that making some small mods (wont bother with detail here and now) to the manifold you can improve things a lot. However I also think the Bing has its problems too, that is, if you tilt the carby a couple of deg to one side when mounting it you can get a better EGT balance and less variation.... the US jab distributor worked this one out. Additionally if you pull the carb heat on you will get an almost even EGT temp across the instruments, however not a good idea to do under power owing to an increased risk of detonation. Lastly, increasing the emulsion air bleed holes in the Bing on the 6 cylinder will also improve EGT balance too. My point is the carby has some poor atomisation characteristics too and fixing the EGT balance is a combination of both.

 

The other interesting thing is the African Jab guys have developed a fuel injection ... I believe it gives much better EGT balance and 30% more power.

 

Cheers

 

Vev