New Through Bolt SB JSB031-3

[Guest Maj Millard]

You still have all the loads in the valve train and the piston rings to cause friction and make the determination difficult. It's pretty silly to use drag as an indicator, where practically no clearance must exist to cause some and a situation where a positive clearance of probably .002" is needed for the engine to run safely. Nev.

Yes that would be a very 'backyard' method to check for bearing crush for sure. On Lycomings and Continentals the bearing bosses or blocks pretty much set the required bearing crush as the come together. Being cast or forged alloy there is not going to be lot of give in the case itself, and with correct torqueing of case through- bolts things are fine. GA engines have relatively large clearances anyway to allow for thermal expansion. Anyone who has put a VW bottom-end together will recall the methods used there where the rods projecting from the case are left to drop or descend when torqueing the case bolts. When the drop is correct and all are the same then the bearing crush is correct. (iE: they descent at a slow rate to the table rather than dropping instantly to the table.). As basic as this sounds it worked very well for many millions of VWs over many many millions of miles. VW cases are magnesium and are known as being pretty damn robust.

 

 

[facthunter]

A lot depends on the bearing metal used.. Older engines used Babbit metal and being low friction, didn't need a lot of oil flow to cool it so small clearances were the order of the day..Later high load bearings were copper lead with indium overlay. Eventually most went to alutin which needs more clearance to allow more oil flow. The engine has to have the correct clearances for the materials used. Nev

 

 

[ianboag]

When this stuff started hitting the fan a year or three back, a group in NZ found that torquing beyond 30 ft-lb produced a serious risk of stripping (or at least bending) the BOLT thread. Turned out that the bolts were undersize. The "12-point" ARP nuts helped because they were longer, but it's a pretty ugly "solution" ....

 

So the group found some 10mm ARP bolts (for a Suzuki motorcycle) (closest off-the shelf to 3/8"). Torqueable to 80 ft-lb before the stud broke. This info was all passed to Jabiru at the time and totally ignored. After considering the crush issue, 35 ft-lb was selected as a suitable (?) torque. Seems to be working .....

 

The 3/8" vs 7/16" bolt thing is a bit strange. The key factor is the amount of clamp force holding the joint together. This is proportional to the torque, no matter what the bolt diameter. A 3/8" bolt and a 7/16" bolt torqued to 35 (or 40 or whatever) ft-lb will have essentially the same clamp force. The only difference is that the stress in the bigger bolt will be less. If the smaller bolt can take the stress, who cares?

 

Just one other point - ARP (fasteners are what they do) stress that bolts should be torqued using their proprietary lubricant - if you look on their website, the logic (based on a LOT of testing) is pretty convincing. They compare it with dry torquing and "lube oil" - Loctite does not get a mention.

 

 

[jetboy]

heres a photo of the different bolts

 

http://www.jabiruworkshop.com/through-bolts/

 

 

[ianboag]

I don't know that much in terms of Aero engine crankcase halves. But if fretting occurred I would suggest the crankcase may be a right-off unless the mating surfaces can be machined and the crankcase tunnel line bored.Are you permitted to machine and then line bore aero crankcases?

I had it done about 15 years ago to an O-300 in the 172A I then owned. While doing a strip (ex prop strike) a crack was found in the crankcase. I was told that crankcase cracks in old O-300's were not unknown. It was welded up, then the case was machined and line bored. Didn't cost an astronomical sum as I recall.

 

 

[facthunter]

Welding is a separate issue. Since most cases would be heat treated originally to achieve a higher tensile strength in appropriate alloys, welding will weaken them as they are not able to be heat treated again once machined as they would warp too much. It is fairly common to weld many aviation crankcases none the less.

 

Facing of mating surfaces is often done combined with re machining of bearing and camshaft tunnels to get the correct crush dimensions. There is usually a minimum deck height that has to be observed, or the case is scrapped.

 

Loose fit of threads is BS, as you have no idea how easily they would strip the threads. There are stipulated depth of thread (thread length engaged) verses diameter given for most common metals where the stud will break before pulling out the thread. All of that is invalid if the thread is a.loose fit.

 

Lubricated threads use different torque figures to achieve the same force. A finer thread will need less. than a coarse one What you are actually trying to do is achieve a predetermined and repeatable amount of bolt stretch, to apply the force you need. Nev

 

 

[ianboag]

Loose fit of threads is BS, as you have no idea how easily they would strip the threads. There are stipulated depth of thread (thread length engaged) verses diameter given for most common metals where the stud will break before pulling out the thread. All of that is invalid if the thread is a.loose fit.

Yep. We had threads stripping on several engines at torque values of 30 with a calibrated torque wrench and knowledgeable user. The first time it happened, Jabiru blew smoke at the issue and sent a set of bolts. They were no different. Some of the bolts that did not strip just bent the threads so they looked like a Christmas tree. I think Roger in the UK was right ... there was a batch issue with through bolt threads. There were some undersize bolts ....

 

Lubricated threads use different torque figures to achieve the same force. A finer thread will need less. than a coarse one What you are actually trying to do is achieve a predetermined and repeatable amount of bolt stretch, to apply the force you need. Nev

Predetermined clamp force. For a given clamp force (torque), bolt stretch will be less for larger diameter. The ARP guys are all over this and how things are all standard and reproducible with their proprietary lubricant.

 

And let's not forget the benefits of looking at the cylinder base. If there is any oil weeping, the joint is moving and things are heading toward the fret/fatigue/pop situation.

 

 

[facthunter]

Ian I'm all for a low initial stipulated seating torque, followed up by an angle applied. It wouldn't be hard to do a set of tests and compare outcomes to what the specified torque equivalent was. Far less variation in the method I'm advocating in practice,, I would think. It's used in many other applications.. Nev

 

 

[jetjr]

Is it a concern that replacing/upgrading/tightening through bolts will just move the weakest link elsewhere, like cylinder bases?

 

I havent seen it but CASA talk of cylinder cracking and someone reported this being the case after through bolt breakage.

 

Not sure of the merit of increasing bolt size if the smaller one isnt over stressed.

 

Although I have heard mention of twisting/ levering force on bolts in which case it may help.

 

CAE have different bolt setup and heavier bases on cylinders I think?

 

 

[facthunter]

Increased base flange thickness will aid general stability in that area but you are always fighting weight A failed critical component like through stud, would place a lot of stress in other areas. ALL items likely to be affected should be checked afterwards.

 

If you move the weakest link somewhere else, it is only done by raising the strength till that happens, so you have an advantage still . This is different from concentrating a lot of stresses in one place, which will shorten it's fatigue life. If you did it in a wing spar it would be poor design and heavier than it need be as well..Nev

 

 

[ianboag]

Ian I'm all for a low initial stipulated seating torque, followed up by an angle applied. It wouldn't be hard to do a set of tests and compare outcomes to what the specified torque equivalent was. Far less variation in the method I'm advocating in practice,, I would think. It's used in many other applications.. Nev

Why bother reinventing the wheel? ARP say - use our lubricant and know what a torque value means. It's what they do .....

 

 

[facthunter]

Because I believe a more consistent result is possible and believe it could be demonstrated.. Not so dependent on calibrating tension wrenches so accurately either. Nev

 

 

[deadstick]

I have had one engine, a 3300 where the mains gaps closed up after the first through bolt ad was carried out. The post mortem on the engine revealed unusual wear in that the crank had worn and not the bearing, the mains gaps were measured and a few were at 4thou the minimum clearance at the time. Independent examination and report indicated that the increased clamp of the 12 point nut and new through bolts had closed the gap slightly. Having read the new SB I See that Jab have developed a check for the turning friction of the crank now? looks like a hip shot to me! so many factors can effect this measurement!!!

 

 

[facthunter]

I would think turning friction is inappropriate. The clearance should never be so small that friction could increase. The bearing should always have positive clearance, so you are never going to measure any change unless it is far too close a running fit. It's a dud idea. Nev

 

 

[ianboag]

Because I believe a more consistent result is possible and believe it could be demonstrated.. Not so dependent on calibrating tension wrenches so accurately either. Nev

Have a look at http://arp-bolts.com/p/arpultratorque.php

 

 

[robinsm]

How does a company justify, finding a fault with their engines, puting out an upgrade and making the owner pay for it? They screwed up, they pay, or so I thought...

 

 

[jetjr]

Been reading SB031-3, few points

 

SB only effects those in training use, apparently "almost all" through bolt problems on training 2200 aircraft, claim is that repeated large throttle applications somehow related.

 

Even IN training use only applies to hydraulic lifter SN range. 2200 range leaves out a few too??

 

3/8 bolts still use loctite system and low torque, 7/8 use no loctite and higher torque

 

Comment in there on moving to 7/8 bolts to permit more torque without damaging bolts - probably link to thread damage during install Ian is taking about.

 

The big one.......3/8 bolts need to be changed every 500hrs......INCLUDING studs. previous SB excluded studs. I believe this means a case split so its a big deal.

 

Any over 500hrs need to have them changed in next 25hrs.

 

Summary is that at >500hrs, every hydraulic engine (in training use) will end up being swapped to new 7/8 bolts setup via engine replacement or rebuild.

 

I dont understand why there appears to be difference between solid and hydraulic lifter engines in regards to through bolts. If there is a failure link, then that would possibly be worth getting converted when through bolt upgrade being performed wouldnt it?

 

Debate around ARP bolts is irrelevant (no matter how correct) to any training aircraft as they must follow Jabirus instruction to remain LSA.

 

After this CASA action Id suggest any experimental would spend money on other engine options - even though SB doesnt apply

 

They havent found a fault, just an improvement. Secondly this is aviation.

 

Historically they have been good to me regarding parts and upgrades. I also know they have done deals for training owners on changeover and replacement engines in the past.

 

Anyone spoken to them?

 

 

[facthunter]

I wonder how they arrived at the 500 hours life. I would have thought the normal max loads would be temperature related. More like number of cycles. Nev

 

 

[cherk]

 

Been reading SB031-3, few pointsSB only effects those in training use, apparently "almost all" through bolt problems on training 2200 aircraft, claim is that repeated large throttle applications somehow related.

Even IN training use only applies to hydraulic lifter SN range. 2200 range leaves out a few too??

 

3/8 bolts still use loctite system and low torque, 7/8 use no loctite

 

I think that should read".........7/16" not 7/8

[jetjr]

New way bigger bolts........sorry yes 7/16

 

Id expect they will be thinking that the data shows 2200 engines over 500hrs in training represent a significant number of the problems. Reduce them and overall failure rates drop to level CASA accepts.

 

 

[SDQDI]

Lol jet I was thinking "at last some good old agricultural solutions a 7/8 bolt is something a farmer would trust"

 

Actually as a side note building the hornet after being used to fabricating farm stuff was a real eye opener in regards to saving weight while still holding together.

 

 

[hyundai]

I think you all are missing the point CASA is a aviation governing body, If a 747 kept having engine failures they would step in and demand a fix to the problem for our safety. Jabiru have learned a lot from there early days I was down at jab last week and they are testing the new 2210 there is no fly it for a few hours and put it on the market because CASA are looking over there shoulder. I believe they Jabiru will get it right.

 

 

[rankamateur]

When they have sold ten 2210 engines there is still going to be bucket loads of experimental 2200 engines out there, so when they get the 2210 engine right, it is going to have a miniscule impact on public safety in the short to mid term.

 

 

[jetjr]

Mate lots of threads for you to read it seems

 

Casa would act on solid public data, they would act on a particular engine not limiting all RR engins ever built

 

Anyway

 

Jabiru have strong history releasing half tested or upgrades without much tech merit.

 

I am happy you believe this has changed

 

With the void between CAE wider than ever at Jabirus pushing, , im not sure where the rest of the engines wil be coming from

 

 

[Yenn]

So CASA would step in if therew ere multiple 747 engine failures. They have stepped in with Jabiru saying that there are multiple failures and the rate is increasing. They also state that they don't know why. They also state they have confidence in Jabiru.

 

What they will not state is the actual number of engine failures and the cause of those failures.

 

Funnily in the USA the safety record of Jabiru is the best of all LSA aircraft. The statistics in USA sshow that the Jabiru is only marginally worse than the C150, remember that the C150 has been flying since the fifties and all the problems have had years to be sorted out. If you were in the USA to planes to avoid would be Remos Evector and Czech Sport Cruiser, but the Skycatcher, C172 and Tecnam are all poorer than the Jab for safety.