Damage caused by 12 point nut AD

[deadstick]

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...

 

 

[Old Koreelah]

Could it be that instead of addressing the cause of bolt failures, the factory have tried to prevent the symptoms.

 

Stronger thru bolts and nuts may not break, but band-aid solutions invariably lead to more problems; in this case buggering up other parts of the engine that were working quite well. Why not stop detonation?

 

 

[cooperplace]

Could it be that instead of addressing the cause of bolt failures, the factory have tried to prevent the symptoms.Stronger thru bolts and nuts may not break, but band-aid solutions invariably lead to more problems; in this case buggering up other parts of the engine that were working quite well. Why not stop detonation?

stop detonation? you have a few options: higher octane fuel (people already use 110, can't go much higher); lower compression ratio, but this will cause a drop in power and/or an increase in fule consumption, but having said that, maybe only 0.2 is enough; cooler heads, ie, water cooling, been discussed a lot already, and lastly, different design heads, which is well beyond us average engine users. Thoughts?

 

 

[Blueadventures]

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...

Not good to hear. But should be able to quickly prove or disprove the theroy using plastigauge on the bearings. Clamp down with original nuts and then undo and measure the plastiguage and repeat with the 12 points. Although the plastigauge not necessarily accuarte soulf give an indication of the bearing clearance. Any engine will do.

 

Best wishes

 

Mike

 

 

[jetjr]

The new nuts are done to the same torque as old ones, not sure how they can clamp case halves together tighter

 

 

[Bruce Robbins]

The new nuts are done to the same torque as old ones, not sure how they can clamp case halves together tighter

Hi JetJr,

I agree. It's not the new nuts causing the problem. I know of shops replacing broken through bolts in the past, only to find that the engine wouldn't turn over after torqueing up the new bolts. I did the 12 point upgrade before they were officially released, and have had three engines with broken through bolts.

 

My opinion is that the old nuts were losing torque, thus reducing the clamping force, and allowing the crankcase to start fretting , and the bolts to experience cyclical loading leading to metal fatigue and failure. Deadstick, if the mains closed up after replacing the nuts, then you were on the way to a broken through bolt.

 

Can't see how the crankcase halves closing up would affect cam followers, unless it was damage from debris in the oil, or more likely low oil pressure in the galleries due to the larger clearances in the main bearings and crankcase halves due the fretting. The standard Jabiru oil pressure pickup point will not always show up low pressure in the gallery either.

 

Bruce

 

 

[Yenn]

Detonation can be caused by poor quality fuel. Can you guarantee that the mogas you buy is good quality. the only time I had a problem with detonation was when i couldn't get avgas. I ran on mogas for several months before I had a problem, I think it was just a bad sample of fuel. No re mogas for me.

 

 

[Guest Maj Millard]

The standard mogas doesn't seem to be causing any detonation problems in the 1000s of 912s that it is used in !....not the fuels' fault ...........................................Maj...

 

 

[Guest Maj Millard]

Anyone who has ever built up a decent bottom end will know the term 'bearing crush'. Particularly important when setting up the rod-cap bottom bearings onto the crank, but also very important on the main crank bearings themselves.

 

The VWs were very good at achieving this, and the bottom end of a 912 is not a lot different. Get that 'crush' correct and the bottom end will go for a long time. Don't get it right and... well.????.....It really is one key to longevity.....................................................Maj...

 

 

[turboplanner]

After reading all this I'm almost certain the cause is the fingers.

 

 

[jetboy]

The original MS nuts are friction locking and have a torque setting of 30 lb/ft

 

The 12 points have no friction locking and are assembled with loctite 620 to 30 lb/ft

 

loctite 620 is classed as a lubricant similar to light machine oil for assembly purposes.

 

ARP will specify what the difference is to torquing 'wet' vs 'dry'

 

anyone spot the difference to the finished job here?

 

I wouldnt speculate if this had any bearing on the engine in question. After all, it had an overtemp, all sorts of things can go bad

 

Ralph

 

 

[deadstick]

Turbo, not sure what your saying? Please clarify.

 

 

[Sloper]

Goodaye all

 

There are different torque setting for oiled and no oiled bolts, make's a huge difference.

 

If memory serve's me correct about three times the value.

 

Most manuals state when oil is to be used, but most people are thinking they are doing the right thing in dousing things in oil or using CRC.

 

So unless specified use clean dry bolts or nuts.

 

regards Bruce

 

 

[Sloper]

Also l should add there should be crush tubes between the cases to prevent the casing being pulled together to hard and changing the bearing clearences.

 

If the bolts had been over torqued they should be checked and if required replaced.

 

Proberley should raplace them anyway on a AIRCRAFT engine.

 

regards Bruce

 

 

[jetjr]

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.

 

 

[turboplanner]

Turbo, not sure what your saying? Please clarify.

the fingers which hold the $2 spanners

 

 

[deadstick]

Turbo, so its the engineers fault that installed the 12 point nuts? is that what your saying?

 

 

[turboplanner]

I'm suggesting that the policy of doing your own maintenance on certain sections of an aircraft should be removed.

 

The next question is the standard of training of L2's and LAMEs.

 

Then we could look at whether it means anything if nuts and bolts only seat by a percentage of the tang available.

 

But unless a nut has AI (artificial intelligence) I couldn't see a 12 pointer compressing material any more than a six pointer at the same torque and installation method.

 

 

[deadstick]

Turbo, I have done all the maintenance on this engine! Standard aviation practices have been used in line with jabiru's maintenance manual! Not sure where your going but the jabiru engine shop themselves stated that the 12 point nuts do a better job providing clamp down force per tq unit than the old military standard ones. As far as the scoring on the crank goes i was of the opinion that it perhaps is a by product of the overtemp causing the crank cases to expand closing the clearance, my opinion only but their belief is that its from the 12 point nuts. I am at a loss as to why you would blame the engineer? I have been in the aviation industry for over 18 years with 12 of those as a LAME, not sure why you would question the level of training to become a LAME its pretty solid and robust as far as I am concerned. As far as $2 tools goes, bit of a generalist statement dont you think? I do have one or two tools that cost that much, but hey so what? My torque wrench is calibrated regularly and prior to the install of the 12 point nuts was set and checked against a master acratorque as i and im sure you are fully aware that jabiru were blaming the broken through bolts on the "fingers" as you put it. Are you afilliated with jab? My experience dealing with them over the years has all the hallmarks of your responce "oh it must be the engineer", cant be the product! In fact there was a lot of buck passing all of which was disproven by the history on the 12 channel egt/cht guage. Fact is mate the engines are weak, they require constant maintenance, the support from the factory is poor and the quality control stinks all of which i have witnessed and experienced over the years, hey the engine shop is that snowed under and there are no new engines available which to me assists in supporting any reliability claims. Statistics around the south coast of NSW are not favorable to support any claims of reliability either. Do you think that perhaps the locktite could be acting as a lubricant prior to flashin off? I think that it a possibility.

 

 

[turboplanner]

The heading "damage caused by 12 point nut AD" is a serious accusation of negligence by the people in the design/manufacturing/technical process and needs to be backed up by some proof.

 

I don't have anything to do with Jabiru, will not fly one and will not let any of my family and friends fly in one.

 

The suggestion that loctite might be acting as a lubricant before going off I don't think is an issue or we would see it in the thousands of other applications. In racing applications I prefer UNF threads because of the shallower angle, but I don't think the angle is causing nuts to vibrate loose.

 

 

[deadstick]

Turbo, the heading is not an accusation it's a quote from mark in the engine shop at jabiru!

 

The old ms nuts were out of round self locking the new 12 point nuts are not, hence the lock tight. Don't think these nuts or the new we're ever vibrating loose as the torque seal line never moved. The movement of the case halves might be due to a not so interference fit on the dowels, since been addressed by jab.a few of your comments to do with nuts, bolts, torques leaves me to believe you don't understand the concepts involved fully ( not an attempt to upset you). You can indeed achieve different clamp forces at the same Torque of the nuts.

 

 

[turboplanner]

Maybe I don't understand the "concepts" but you're the one with the stuffed engine. I spent 12 years putting race engines together, sometimes in motel carports - engines which produced about double the manufacturer's outputs, with high cylinder pressures, and in some cases aluminium U bolt threads and didn't have this issue. I did start by lapping the cases with oil filled abrasives on a sheet of glass to get a perfect mating and a few procedures like that, but fundamentally I just bolted the engine together using loctite and precise torquing pattern and ratings.

 

I'd be interested in hearing how your clamping forces vs nut torque figures are obtained.

 

 

[deadstick]

I'm sure you would champ, you can read about it like I had to when I was undergoing that lackluster AME training (that you eluded too earlier) in order to get my license so I can certify that it's not my fingers causing the problem. Yep I do have a dead engine being repaired under an insurance policy! Your point? Lol 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.

 

 

[motzartmerv]

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.

 

 

[DonB]

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.

Hello to all. This makes me wonder how many people performed the change to the 12 pt nuts while the cylinder heads were still attatched to the cylinders, valve springs compressed trying to push the block apart. If done one at a time there should be no disturbance to the case halves but I cleaned and checked the heads and valves while performing the nut change. It is my understanding that the sealant between the case halves could be compromised allowing internal oil leakage, lower oil pressure if the cases split the slightest amount during the process. To me, possibly, it's not the 12 pt nut torque but the proceedure some may use to perform thier work that gets them in trouble. I am saying this generaly speaking, not towards anyone in this group!!