Another Jab bites the dust.

[pudestcon]

I don't do it enough pre-take off Motz.

 

I will now!!

 

Pud

 

 

[old man emu]

The thru bolt that failed at the base of the nut has clear fret lines.My opinion only: the shims under the barrels idea introduces another weak spot, it has sealant on both sides (not good for rigid torques) and also offers a heat sink for differing thermal expansion properties.

 

Not sure I support the detonation theory as the cast pistons are poor quality and more of a weak link, I lean towards a resonance issue with these bolts. JMHO

Humble opinion be buggered. This is the first post I have seen in this discussion that points to the problem. It is the sealant at the cylinder/crankcase interface that is the problem. You torque down onto a soft surface. When the engine runs each time there is a combustion stroke, pressure is applied to the piston to move it, and pressure is also applied to the cylinder head (Remember Isaac Newton?)

 

The pressure on the head pulls the cylinder away from the crankcase, but that motion is stopped by the cylinder base bolts and the through bolts. That puts stresses on the cylinder base flange, and the cylinder adjacent to it. Repeat this sequence of pull and release thousands of times (eg 2400 RPM = 600 sequences per minute per cylinder) and is it any wonder that eventually something has got to give?

 

We put our cylinders on clean and we don't have any problems with the engines we have done this to. Engines that are assembled by Jabiru after overhaul etc are the cause of our problems, and economic loss to the owners.

 

If a Jabiru suffers detonation, it coughs, farts and blows clouds of black smoke, as well as failing to develop full power. As for hot spots in the head caused by carbon deposits, when was the last time you heard of an engine, be it aero of automotive, being pulled down for a de-coke? The fuels we use are a heap better than they were 50 years ago, and the carburrettion of a modern engine is so much better.

 

If you want to have your cylinders hold together to TBO, get rid of the crap at the cylinder/crankcase interface.

 

OME

 

 

[motzartmerv]

Good onya Pud. Make a quik plan, at every airfield you fly from. Run it through your head as you line up. That way, when the band stops playing you will be actioning the plan, rather than writing the plan as you go..

 

Cheers..

 

 

[facthunter]

How can fitting hydraulic lifters break thru bolts?

It would be hard to say they do, but there is a case that can be made where the loads ARE increased This would only be if the loads on the valve gear are upped by say, stronger springs, less lash, or quicker lift. I can't see it being of a great order of magnitude. Nev

 

 

[Tomo]

If you want to have your cylinders hold together to TBO, get rid of the crap at the cylinder/crankcase interface.

OME

Sounds like a pretty reasonable analysis OME, I wonder why the silastic is there in the first place? Other then to stop it leaking oil...

 

 

[turboplanner]

Great post on EFATO Motz.

 

I know of about 4 RA pilots who are now dead, not because of terrain, but because they screwed up a perfectly good landing.

 

I've been guilty of it myself once when an instructor pulled the power just above the strip. I was focused on clearing the top of B Doubles on the highway and finding a path clear of trees, and without the instructor's reminder to put the nose down, in congratulating myself at crossing a highway and finding a paddock in a few seconds, I may well have pancaked us into the mud.

 

The statistics I posted do underline a very good standard of discipline during the many forced landings, because only one injury was listed.

 

However, many pilots would not react well enough in the same circumstances, and over the years many pilots have tragically (as against stupidly) died when there was absolutely no way they could avoid a solid object, even when touching down at shortfield landing speed.

 

So, while trying to diagnose the design issues, albeit without metallurgical reports and other test data, might be interesting for all of us, consider this:

 

Last week what I believe was a new record, $8 million, for a Public Liability payout was set. This was a case where someone allegedly ate a contaminated food item at a Fast Food Outlet and suffered brain damage. (Note, the Fast Food Outlet is appealing the case, raising some issues of concern, so this will not necessarily be the end of it).

 

So this gives us a new benchmark to worry about.

 

I would expect the Manufacturer to be giving some serious thought to his long term position, given the regular number of failures.

 

I would expect flying schools to be doing the same.

 

I would expect any pilot who may decide to take up a passenger to also be giving some thought as to whether he or she has PL Insurance to cover the $8 to $10 million which may be needed if the passenger claims on the pilot. (Particularly if he knew, or ought to have known there was an ongoing problem which could strike at any time).

 

If you have $10 million PL cover as part of your home insurance, I'd recommend a call to the insurance company to see if this covered flying light aircraft; you may have to take out a separate policy.

 

You're going to look a goose if you get up in Court and say you've never heard of the engine failures before, because the plaintiffs lawyers will most likely have obtained an order to inspect the administrator's files, which may well show more failures not already printed. (Our own Insurer's lawyers did that to us, years ago; they look after their own pockets as

 

well)

 

Rather that speculate on what the engine problem is, it might be a lot smarter to start talking about protecting the pilots from injury, and severe financial risk.

 

 

[turboplanner]

Sounds like a pretty reasonable analysis OME, I wonder why the silastic is there in the first place? Other then to stop it leaking oil...

Call me stupid, lucky or a magician, but I successfully ran race engines for many years using silicon on those mating surfaces. I only ever found one through bolt fractured, and that may have been caused when the con rod bent into a U shape and shoved the piston out the side.

 

However, the silicon is applied with a finger, not a caulking gun and is a very thin film.

 

On bolting up, the compression contact is not with the soft silicon but with the high areas of the mating metal surfaces; the silicon fills up the occasional low spot, and defect.

 

The engine will still work without it, or similar sealants, but there is a chance of a small leak through a low spot or fault.

 

So if you want to go there in discussing issues, the discussion should really be, is too much silicon used?

 

 

[av8vfr]

So if you want to go there in discussing issues, the discussion should really be, is too much silicon used?

Aaahh I had a girlfriend like that once....

 

 

[motzartmerv]

Aaahh I had a girlfriend like that once....

You can never have too much..:)

 

 

[RKW]

You can never have too much..:)

Sometimes less is more..... what am I saying??

 

 

[motzartmerv]

[old man emu]

Call me stupid, lucky or a magician, but I successfully ran race engines for many years using silicon on those mating surfaces. However, the silicon is applied with a finger, not a caulking gun and is a very thin film. On bolting up, the compression contact is not with the soft silicon but with the high areas of the mating metal surfaces; the silicon fills up the occasional low spot, and defect. The engine will still work without it, or similar sealants, but there is a chance of a small leak through a low spot or fault.So if you want to go there in discussing issues, the discussion should really be, is too much silicon used?

A good point made here:

 

If silicon is applied sparingly, it will level off the low spots or defects, and stop those niggling small oil leaks. However, from the photo posted here, it looks like the silicon was applied with a butter knife.

 

What gets me is that these surfaces should be made without low spots or defects, so the need for a filler is eliminated. Perhaps a bit more attention to quality control is required from this manufacturer.

 

OME

 

 

[turboplanner]

What gets me is that these surfaces should be made without low spots or defects, so the need for a filler is eliminated. Perhaps a bit more attention to quality control is required from this manufacturer.

OME

You would be amazed at how rough production engines are.

 

When setting up a race engine I use a sheet of glass and gemstone grit mixed with oil, and it takes some time just to get even surfaces.

 

Manifold castings need to have substantial flash ridges ground out, which the average motorist never notices.

 

 

[M61A1]

A good point made here:If silicon is applied sparingly, it will level off the low spots or defects, and stop those niggling small oil leaks. However, from the photo posted here, it looks like the silicon was applied with a butter knife.

What gets me is that these surfaces should be made without low spots or defects, so the need for a filler is eliminated. Perhaps a bit more attention to quality control is required from this manufacturer.

 

OME

I would tend to agree about the excess of silicone, it can be quite dangerous, if it gets in the wrong places it will block/restrict oil galleries.

 

 

[nong]

I have only had one in flight failure of a Jabiru. It was a broken through bolt and cracked cylinder combo on a 2200 hydraulic.

 

The instruments were shaking like mad but the little Jabby stayed afloat (two up) the fourty miles to home base and even gained a thousand feet along the run.

 

As a matter of interest, the oil pressure and temps stayed rock solid even though most of the oil was pumped overboard. We could not find more than half a litre of oil in the system after the event. Thats good to know.

 

 

[facthunter]

Some of the roughness is intentional It helps to hold certain gaskets in place. Some of these gaskets have a surface treatment that bonds to the metal surfaces.

 

Too much silicone is one thing. Leaving the parts apart for too long is another fault. If the parts are assembled whithin the specified time the excess squeezes out as you tighten. I often use a light spray of high temp. silver paint on gaskets like exhaust flanges.

 

Having shims/packers under cylinders is not a real good idea. Nev

 

 

[petetheprinta]

Pardon my ignorance, but why don't they use common old gaskets like for example the head gasket in my car.?? but obviously of a suitable material and thickness. Or just plain gasket Goo.

 

 

[facthunter]

Gaskets that have "give" in them load the corners of the cylinder flange and require retensioning in service. Metal to metal is the preferred way but often there is a large thin sectioned "0" ring to seal it against leakage, provided. Nev

 

 

[kgwilson]

Wow, I have finally read all 194 posts and discarding the Jab knockers & other ill informed comments this is one of the most informative threads I have read especially as I am planning to install a Jab 3300 engine in my Sierra.

 

The early comments on the use of silastic between the crank case and cylinders concerned me some and OMEs bosses comments rang a truer note to my engineers mind than some others. With highly sophisticated CNC milling, grinding and polishing techniques now common practice in mass produced engines there should be no need to add any substance to fill any low spots. They just should not be there if the quality assurance and control procedures are maintained throughout the manufacturing cycle.

 

There have been some comments about the quality of Jabiru components, rebuilds etc. Perhaps this is the area that really needs addressing. While a direct drive, air cooled Aero engine may be low RPM, high torque & lightweight, it is highly stressed most of the time. Those in flying schools will also suffer from shock cooling from time to time especially after students go solo as they will forget things when they are full on just getting up there and returning to Terra Firma. The quality of the design, its components and manufacture must be world best practice as we can't just stop & call roadside assist.

 

 

[Guest Andys@coffs]

You would be amazed at how rough production engines are.When setting up a race engine I use a sheet of glass and gemstone grit mixed with oil, and it takes some time just to get even surfaces.

Manifold castings need to have substantial flash ridges ground out, which the average motorist never notices.

Im struggling with the discussion on the amount of silicon.If its a little bit and uncured then we achieve metal on metal, with a bit of silicon around to fill in the odd micro gap as the bolts are tightened. If its a lot then we achieve exactly the same outcome to almost a molecule...The only difference is that there will be a heap of unused silicon outside of the joint, or perhaps in galleries where not intended, where the galleries dont exist then the amount used will be irrelevant wont it?

 

What am I missing? The inference is that somehow the silicon stays in the joint and seperates the metal surfaces? I can see that occuring only if the material cures before being the bolts are tightened?

 

Again, what am I missing?

 

Andy

 

 

[turboplanner]

Nothing Andy, your assumption is correct.

 

However some engines, as Nev said use an "O' ring, and some people run a ring of silicon around to save cost and let it cure before assembly, and some people run a ring of silicon around the flat surface between cylinder flange and base, and let it cure. That's what has quite correctly drawn criticism because you can't squeeze out the cured silicon, so you don't have a metal to metal contact.

 

If you do use silicon you HAVE to know it will not bleed out in enough volume to break loose and find its way into oil galleries.....and that requires a lot of practice.

 

Personally, I've reverted back to "Gasket Goo" which is a lot more controllable.

 

 

[facthunter]

Hot day and some of the other stuff can go off quickly. Extra bits of silicone ruin many a japanese motorcycle engine. Block up the fine oilways in the cylinder head. Nev

 

 

[turboplanner]

Some of the roughness is intentional It helps to hold certain gaskets in place. Some of these gaskets have a surface treatment that bonds to the metal surfaces.

The dangers of passing on 15 words out of 20 years engine building experience on a forum!

 

What I should have conveyed was that I was grinding out high spots to give a more even metal to metal surface. That still leaves enough indentations, 1/2 casting bubble holes etc to hold gaskets.

 

Phil Irving, in his book "Tuning for Speed" explained which areas of an engine should be as near perfect as possible and in which areas you can be as rough as bags.

 

His book is currently on sale on Amazon with one seller advertising it "from $850.00" and another "from 203.70"! So it's not cheap, but I built my four stroke up from it, and got a Second in one of the heats from our Victorian Championships without any prior experience, so it pays for itself. You might remember that Phil design the Repco Brabham Formula 1 engine which one the World Championship in its first season.

 

Having said all that please to not take to your aircraft engine using snippets of what you read on this forum. It takes a LOT of bitter experience working on engines before you reach the stage where the newly installed engine starts first time, puts out the expected power, and achieves the life cycle you expect. In racing about two thirds of the engines never reach that stage.

 

 

[fly_tornado]

in the bike business there is yamaha bond #5 which is this ultra fine silicon adhesive. its designed for sealing machined surfaces.

 

 

[turboplanner]

With highly sophisticated CNC milling, grinding and polishing techniques now common practice in mass produced engines there should be no need to add any substance to fill any low spots. They just should not be there if the quality assurance and control procedures are maintained throughout the manufacturing cycle.

Certainly that's what happens in drag racing and sprint car racing - many components are replaced and machined from billet, but open up a production engine and you'll be amazed at the crudeness. That's why it is still possible in the 21st century to take a basic production car engine and extract around twice its original power.