Damage caused by 12 point nut AD

[turboplanner]

Nah Tubz, Deadsticks fingers have worked on other aero engines from different manufacturers, the aircraft have done equal work, in the same environment undrr identical workloads with no issues. Its a fact that often gets overlooked but all these L2's and lames that 'people' keep blaming for these bloody things popping, also work on other types...:) Are we suggesting that the jab engine is such a precious, temperamental engine that there are few qualified, trained aero engineers that have the goods to keep them running? Ive watched and helped deadstick do his maint. He doesn't do a thing without the written schedule or procedure and is Anal (to say the least) in double checking evrything.

That's not consistent with the theoretical wanderings here.

 

If an AD is accused of causing damage rather than improving safety, that's verfy serious and there needs to be some logical proof which will stand up to scrutiny.

 

 

[ozbear]

Hi all, one of my jab 230 engines was stripped for assesment post overtemp at jabiru, the crank showed signs of scoring and damage, the engine shops opinion was that the 12 point nut AD caused it! The reason being that they seem to think there was some chaffing of the crankcase prior and the more effective 12 point nuts clamped the cases harder against the main bearings causing the clearance to close up. The bottom end on these are supposedly able to hit 2000 hrs so something has gone wrong for this one to be showing abnormal wear at 700 me thinks, the cam and lifters were worn abnormally as well...

I doubt the new nuts caused the problem i think it maybe that once the cases had fretted and you fitted the new nuts it may have crushed the bearing shells to reduce the clearence but if the cases were that bad you should have had a huge oil leak before the nuts were fitted.To me it sounds more like a lube problem.Did they check the tunnel sizes after strip down.

 

 

[facthunter]

Ozbear has the relevant point. If a motor has fretted any machined surface there is no way you should just replace a stud/nut etc and retorque it and expect the engine to have the correct crush on the bearings, and correct bearing clearance and alignment. The engine MUST be dismantled and any damage rectified, then reassembled. Nev

 

 

[deadstick]

Ozbear, Nev, I agree with what your saying and it was the opinion of Jabiru that the cases may have fretted prior to the AD being C/O but they could not be sure.

 

This engine provided no evidence of fretting and the through bolt and nut install was done with the barrels off and a bottom end inspection with no defects evident. It is a hard one to dignose without speculation as I didnt strip it, I can only go on what the engine shop said, The engine was running rock solid up until it was cooked (240c) noting that the airflow over the crankcase would be pretty stagnant I suspect that the case halves would have been very hot as well, I am talking with the insurance company today and hope they opt for a complete replacement, but the assesor did mention the words normal wear and tear when referring to the bottom end, You could be forgiven and given the benifit of the doubt if you were to say that the scoring was a byeproduct of the overtemp and case swell but it is an insurance company... LOL the quoted rebuild cost is damn close to the cost of a replacement so fingers crossed...

 

Not sure if its true but I did hear a rumour some time ago that the bottom end isn't dynamically balanced, would be interesting to have this done if not and see if theres any notable difference in reliability....

 

 

[facthunter]

Some of these engine become quite stiff when hot. This would make me think there is some misalignment happening.

 

Regarding balance the shaft is machined all over so the actual ballance error should not be of a high order. Having a rebalance will make it feel a lot better but the actual loads internally won't vary much in real terms. If I were building an engine up and there was a reputable shaft balancing firm around it would still do it but you have to watch where some remove the metal to achieve the balance. I've seen some put a row of smallish holes where a better fix would have been to grind a mass of metal off in the general area that has less structural effect. Cases should be able to be refaced and align bored again as long as the deck heights are not outside limits..Nev

 

 

[ianboag]

As far as I know through bolts have always had loctite used.An old problem was that the threads would strip very close to torque limit too.

And of course if the threads are near the strip point, it is possible that the threads bend a little in operation. When this happens the bolt thread gets a "Christmas tree" look. Not just a theory - we have seen it.

 

When the thread bends, the clamp relaxes a bit. That lets the case halves move relative to each other and they fret. If they have fretted enough, then a subsequent reef-up will nip the crank. Again not just a theory - we have seen it. Replaced the through bolts, torqued to 30, nipped the crank. Engine no turn and all that - a 700 hour 3300.

 

The other thing that happens if the clamp releases is that the pots move round a bit. You can tell this is happening if there is oil weeping round the cylinder base. If this happens, the through bolts are bent back and forth and will eventually snap at the pot/crankcase boundary. The cause is bending-back-and-forth-fatigue. If you look round the net a bit on the failure of bolted clamp joints that's more or less the universal cause.

 

As to the use of Loctite- I am a but confused - Lycoming and Conti say that through-bolt nuts should be lubricated (engine oil is suggested) before they are torqued up .... go figure ... what would they know?

 

 

[turboplanner]

I'm with you Ian, another reason the threads can bend and produce that Christmas tree look is if the tangs don't extend all the way to the heels of the thread. If you are torquing up tang to tang on part of their full length then both tangs can gradually stretch and bend in operation. Someone has posted with detail of this in the past.

 

 

[facthunter]

Lubricating threads changes the torque setting to camping relationship totally. Loctite may well act as a lubricant. Torque should correspond to a load which equals a particular stretch. Marine parts often apply a load and then move the nut to a seated position at the valid stretch point. This sort of principle would favour the idea of applying a small torque to seat the nut and then a subsequent number of degrees of rotation to stretch the stud which is probably a better way to go. It's not approved, but a better method which would stop overtorquing which is part of the problem in some cases.

 

Thread distorting would be caused by poor thread fit or underperforming materials or overtorquing Nev

 

 

[ozbear]

Ozbear, Nev, I agree with what your saying and it was the opinion of Jabiru that the cases may have fretted prior to the AD being C/O but they could not be sure.This engine provided no evidence of fretting and the through bolt and nut install was done with the barrels off and a bottom end inspection with no defects evident. It is a hard one to dignose without speculation as I didnt strip it, I can only go on what the engine shop said, The engine was running rock solid up until it was cooked (240c) noting that the airflow over the crankcase would be pretty stagnant I suspect that the case halves would have been very hot as well, I am talking with the insurance company today and hope they opt for a complete replacement, but the assesor did mention the words normal wear and tear when referring to the bottom end, You could be forgiven and given the benifit of the doubt if you were to say that the scoring was a byeproduct of the overtemp and case swell but it is an insurance company... LOL the quoted rebuild cost is damn close to the cost of a replacement so fingers crossed...

Not sure if its true but I did hear a rumour some time ago that the bottom end isn't dynamically balanced, would be interesting to have this done if not and see if theres any notable difference in reliability....

Yes i have found that insurance companies are great as long as you dont want to make a claim.

I think its possible the oil could have broken down due to the heat as well.

 

 

[jetjr]

I have old original engine repair manual for solid lifter 3300 and Im sure loctite is specified. Torque spec has altered BUT this is to do with stretching of threads. THIS is the reason for the new nuts as others have said.

 

It was quite easy to strip a few through bolts during installation. The new nuts had a few more threads contact and reduced this problem. Lots of debate previously on IF this is satisfactory fix however replacing with longer through bolts was the final fix and they stuck with new 12 pt nuts. Also lots of discussion on thread fit and rolled vs cut threads. We are assured they are all rolled and fit is OK.

 

The installation of new nuts was "sold" as easy job, as is many of the repair tasks on Jabiru engines. This is far from the truth and requires experienced people - not necessarily highly qualified or experienced on other makes but of course this is mostly a good start. I have seen some appalling work done to these by respected LAME's

 

Jabiru documentation is not perfect and extra monitoring and procedures may be needed beyond indicated. Thats why open discussion like here is so valuable interested in fixing rather than inflaming perceptions and problems. Deadstick, your Alert and thread title really not productive IMO.

 

The real issue here is that the bolt breakage, thread stretch in service and some other issues too were not a problem until hydraulic lifters were introduced.......why? Back then overheated heads needing torquing up were the key grumble. More fins pretty well fixed that.

 

From my experience even a mildly overheated Jab engine can have some serious problems, let alone > 240 deg. Not surprised if case could be deformed.

 

 

[deadstick]

Jetjr, thread title wasn't meant to be derogatory or misleading in fact they are the words of mark in the engine shop. I am sorry if it gets your goat so to speak but certainly was not my intention. I really don't get it hey, frustrating as it is if there is a requirement to check for case chafing prior to carrying out the AD then it should be included in the proceedure, I am bound to not deviate one inch from the manufacturers spec and I never have. When I had the barrells off I checked the bottom end as much as I could without performing a full bottom end and there was not one indication of anything sinister. I really hope the larger through bolts and roller cam sorts out the issues and I wait patiently, I have invested a lot of my $ in both my jabs and really need a solution for longevity in the training environment, cause no one will fly them I am currently considering fitting a Rotax to the 160 but am stuck with the 3300 in the230 due the LSA regs

 

 

[ianboag]

The real issue here is that the bolt breakage, thread stretch in service and some other issues too were not a problem until hydraulic lifters were introduced.......why? Back then overheated heads needing torquing up were the key grumble. More fins pretty well fixed that.

We first saw the thread strip/stretch thing on a solid lifter 3300 that had done about 500 hours ......

 

 

[ozbear]

In the automotive industry today head bolts are torqued with a degree wheel which gives a truer tension also the bolts act kind of like a spring .I think with the jab what is needed is a bolt that will give but come back as we'll to cope with the expansion and contracting cycles.

 

 

[ozbear]

Ozbear, Nev, I agree with what your saying and it was the opinion of Jabiru that the cases may have fretted prior to the AD being C/O but they could not be sure.This engine provided no evidence of fretting and the through bolt and nut install was done with the barrels off and a bottom end inspection with no defects evident. It is a hard one to dignose without speculation as I didnt strip it, I can only go on what the engine shop said, The engine was running rock solid up until it was cooked (240c) noting that the airflow over the crankcase would be pretty stagnant I suspect that the case halves would have been very hot as well, I am talking with the insurance company today and hope they opt for a complete replacement, but the assesor did mention the words normal wear and tear when referring to the bottom end, You could be forgiven and given the benifit of the doubt if you were to say that the scoring was a byeproduct of the overtemp and case swell but it is an insurance company... LOL the quoted rebuild cost is damn close to the cost of a replacement so fingers crossed...

Not sure if its true but I did hear a rumour some time ago that the bottom end isn't dynamically balanced, would be interesting to have this done if not and see if theres any notable difference in reliability....

With the balance I don't think it's far out they seem to run too smoothly.Having said that the former owner of my 2200 had it balanced whilst it was being rebuilt due to a prop strike.

 

 

[jetjr]

We first saw the thread strip/stretch thing on a solid lifter 3300 that had done about 500 hours ......

First one Ive heard of , dont know of any broken bolts from earlier engines do you?

Re balance, I dynamically balanced a few props and difference was significant - I thought they were pretty good before.

 

 

[Ultralights]

i cant help but think, if the engine was designed better, then bolt breakage, stretching and other forms of fastener failure should never even be an issue. it sounds to me as 90% of the bolts load limits are reached when the engine isnt even running.

 

 

[turboplanner]

Or exceeded before the spanner finishes turning.

 

 

[Ultralights]

i would bet a quite substantial amount of anything, that, if the case was redesigned, so it was held together by its own fasteners, like, well, pretty much every other airborne boxer piston engine, then the load on the fasteners holding against the combustion forces, and keeping the cylinder barrels attached, would last a great deal longer, i would say, to stated TBO quite easily.. and it would not be a difficult redesign of the case either, just a leave a half inch thick flange around each half...

 

it appears to me, there simply isnt a bolt capable of doing what the single through bolts are being asked to do, 2 opposing cylinders, and crank case forces. to get a bolt to do that kind of work, and last. would require something made of a titanium or similar alloy.

 

 

[turboplanner]

i would bet a quite substantial amount of anything, that, if the case was redesigned, so it was held together by its own fasteners, like, well, pretty much every other airborne boxer piston engine, then the load on the fasteners holding against the combustion forces, and keeping the cylinder barrels attached, would last a great deal longer, i would say, to stated TBO quite easily.. and it would not be a difficult redesign of the case either, just a leave a half inch thick flange around each half...it appears to me, there simply isnt a bolt capable of doing what the single through bolts are being asked to do, 2 opposing cylinders, and crank case forces. to get a bolt to do that kind of work, and last. would require something made of a titanium or similar alloy.

If the system is all OK the combustion forces are not huge - I don't have the pressure figures for a Jab engine but would expect it to be under 135 psi.

 

Here's a photo of a head bolt and nut from an old Honda 500/4 with the breathing opened up, valve timing change for much bigger gulps of air and a lot more spark with the end, producing an output of around 100 brake horsepower, and operating around a quarter of the time at 12,000 rpm. With this degree of breathing and high compression pistons, compression is around 150 psi - well above a normal production engine.

 

The head stud is about 8 mm diameter, the nut six point. The stud goes down through the head and cylinder casting and screws into an aluminium thread in the top half of the casing.

 

They don't give any problems in this very heavy duty high compression application

 

So if you double that force to replicate the power stroke of a boxer, and assume just for this exercise the same number of studs/bolts for the two engines, you theoretically have twice the tension. For your theory then, someone might like to look up the strength for 8 mm and 16 mm high tensile bolts and nuts. I'd expect 16 mm to be either less or more than twice 8 mm. You can disregard the threaded stud vs U bolt for this exercise. The Honda nuts, however are full thread depth and appear way stronger than what's been shown on here for Jab, so you can disregard the Jab nuts too for this exercise. If the result is that 16 mm bolts and nuts provide double the clamping force, then there's a chance that there will be a bolt and nut combination made with the strength to do this job. It would be interesting to see what arrangement and bolt system is used on the Subaru boxers.

 

 

 

[geoffreywh]

my continental 0-200 has thru' bolts, if I remember rightly 9/16" dia...........(14mm) . the thru bolts hold the main bearing caps and cylinders . But the crankcases have a complete " round the edges" set of about 20 x 1/4" bolts. Very difficult to do with machined cases!...The 3300 has very basic design feature that has proved faulty and is almost impossible to correct....I don't ever remember of hearing of a case of broken cylinder thru' bolts in the 0-200, BUT that is not to say it has never happened...

 

 

[facthunter]

The small bolts around the edges don't do much but stop oil leaks on the 0-200. The through bolts carry most of the loads, and clamp the mainbearings of which there are more in tha jabiru's The uneven expansion of aluminium cases and the steel "studs" is responsible for some of the stress, but the extra load to cause failure comes from something like detonation which gives pressures way above all normal anticipated ones. Since the problem seems to have been exacerbated with the introduction of the hydraulic lifters and the studs and nut assemblies are beefed up since it is possible that the hydraulic lifter valves are not seating as well as the adjustable ones and may overheat.

 

It is a realatively simple calculation to determine if the tensile strength is adequate in the studs for normal loads, in this type of engine. Nev

 

 

[bexrbetter]

rebuilding engines in a motel carport, and you have a go at others swinging $2 spanners lol sounds like your justified on that one also.

I have some Queensland and Victorian Rally titles aquired with my power units and I have done my fair share of rebuilds in motel carparks too so don't be too hard on that point!

 

But anyway, on topic, from my experience the combination of cam, lifter and crank scoring all adds up to oiling issues, wrong specific, too hot or filter issues. I thought you Flying guys always sent your oil samples out to be tested every change? Strongly suggest cutting your oil filters in half every oil change too to check for materials that shouldn't be there but also to check the quality of your brand oil filter, are they cheating you with a smaller filter area inside and is the bypass valve crap?

 

Loctite does not change the nut or bolt torque values FWIW but oil on the threads will.

 

 

[deadstick]

Here is an extract from an independent report into the damage from the old through bolts to new 12 point nut AD on my engine, interestingly the engine never gave a single indication of fretting even when the bottom end was inspected during the through bolt AD.

 

Crankshaft, Crankcase & Bearings

 

Examination of the Crankshaft and two alloy Crankcase halves and the matching main bearing shells contained within the crankcase halves revealed unusual wear to the crankshaft bearing journal surfaces in the nature of a significant score mark at the centre of each journal. This score mark matched the centre oil grove in the mating bearing shells. There was also evidence of fretting on the mating surfaces of the crankcase halves. Jabiru has suggested that the adverse wear to the Crankshaft journals has been caused by an initial loss of torque on the through bolts resulting in fretting of the Case halves and a loss of material. Hence when the through bolts were subsequently replaced with modified bolts in accordance with a Jabiru Service Bulletin the increased torque setting

 

on the bolts caused a reduction in main bearing clearance. The preferred bearing clearance is 0.05 mm. The minimum clearance is 0.04 mm. The measured clearance at disassembly was found to be less than 0.04 mm and that this has resulted in excessive bearing preload and wear to the crankshaft journals.

 

 

[Ultralights]

i wonder if this might be part of a root cause for the failures?

 

http://www.advancedpilot.com/articles.php?action=article&articleid=1838

 

 

[damkia]

I still think the issue is more to do with differential expansion, what is torqued OK and within metallurgical limits, may not be so well controlled with the amount of expansion of the aluminium alloy vs the steel through bolt on a hot day under full power. Unfortunately there is no real way of measuring "in flight torque" of the nuts/bolts/studs. One marker would be if the bolts loosen after a while from crush defects in the alloy pots.

 

What may be useful is to mount some "ping detectors" commonly found on EFI cars and associated electronic sensors/data recorder on or near the offending area to sense if there is any residual resonance of the through bolts. This would prove/disprove whether fatigue from resonance is partly or wholly to blame.