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airworthiness suggestion AS/JAB/1


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A properly designed and installed "O" ring should not require sealant. The O ring material must be able to stand temps in use. That's a no brainer. How would you like to be producing aero engines? Nev

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IAW the Jab manual...

head side is the compound is used to hold it in position, and plenum side is gasket glue used as a seal (NOT used with the o ring) .

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Posted (edited)
Quote

 

The jab top end problems are pretty much all of high temperature making...  and a few mistakes along the way.  (lean burn kits, pushrod lengths) .
to quote Bill Clinton : "It's the temperature, stupid"

anyway, my intention with the AS is to get some preemptive action going into Jabiru maintenance. If Jabiru want success on their Gen4, they are, IMO, going to need to take a more proactive and a-priori  role, together with genuine transparency and openness . There will be more.  This one just happened to be an easy one to describe. No argument can be had on the O ring suitability. 

Edited by RFguy
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Little things matter. If you have a temp. sensor under a plug it will run hotter. No one ever talks of plug heat range. Carbon build up can cause detonation. Some real engines of the radial type run 235 c as max head temp..  IF you or anyone else can accurately define issues in service that's a good thing.  Nev 

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Posted (edited)

In the pursuit of building an lower cost engine (compared to the Lycosaurus, Rotaxia-Expensivia  etc market) , certainly, it is understood that there will need to be an economical approach to the design, economical  parts procurement and economical  manufacture. And they wont have an army of people writing up SBs and ADs. 


My feeling is that high temperatures in the head components are the enemy of an economical design and economical manufacture. 

 

There is certainly factory QC that needs work as evidenced by the O-ring that was pinched, and the nordloc washer that was assembled incorrectly- that needs to come from a change at the top. 

 

 

Edited by RFguy
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Quality control is an issue with nearly everything today. .Look at the abysmal quality of tools. It's not control there it's just made of crap. Some stuff is made so problems will be inevitable and not necessarily on purpose. A hose section might be better than "o" rings at each end with inlet pipes. On another tack, many assembly line s just come together and they start it, stick it on a belt run through the gears and brakes and it goes to the dealer with no further predelivery or adjustments.. IF it can be assembled WRONG it will BE. Murphy's law is as reliable as the law of gravity... Some  designs leave me with the Oh Hell just  what were they thinking of. but that was mainly pre and post war where makers stuck to some fool feature out of stubbornness..  Nev

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QC is so important. We manufacture a valve used for mining sample analysis which uses a 50x3 O-ring on the piston. All of a sudden we were having valve failures. The o-ring supplier had decided that an imperial size close to the metric one was ok to supply without advising us. We now check every o-ring and seal. I would hope that aircraft engine manufacturers have a very robust QC system. It would be interesting to see what the o-ring material spec on the Gen 4 is? Good pick up Glen.

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bet it was  a 50.8 x 3.125 !

 

Hi Kevin. I  have to go up to Granny Smith soon.

Yeah, well that's just ONE of the o rings that the mechanic caught. There must be more. need to go through the parts list int he OH man and see what else. 

 

I have a bunch of these to write up for Gen4 and Gen3.

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It's not all Jabiru's fault.

There are alot of engines without EGT on all cylinders, and given the propensity for inlet leaks in the jabiru setup (its not a bad setup, but it does require monitoring) , and high CHTs, it is  no wonder I read online things like 'my mechanic had to replace my exhaust valves every 250 hours!) ....  * eyes roll *

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Inlet leaks will show up at idle when the MP is lowest( Max vacuum) and the flow lowest  where a leak will have the most effect on the mixture. If the idle is steady you shouldn't get much effect at full throttle. RAAus planes are supposed to  do an idle check before taxiing.  No steady and correct speed idle? Return and find out why. Nev

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and Nev exact idle speed is not an easy thing to assure in my experience, because down at idle the  play in the actuator/mechanism/ cable flex. stiction in cables, stops, etc plays a large part , especially with some variations of twin carby setups. you might push a cable and carb onto the idle stop, with pressure, but then if you remove your hand, that spring pressure from cables bowing, flexing and linkages relaxing from the stress may bring it off the stop.  Pushed and held onto the stop with a single carby, that should be pretty foolproof if the choke is home. 

Edited by RFguy
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Yes some have crap linkage set ups. IF  it's slow it may stall on  a glide and If too fast you float too far but it has analytical aspects as well.  Check for leaks? Turn the engine backwards with throttle closed and squirt soapy water near joins. Do the same with your head joint. turn slowly over compression. Look for bubbles. Nev

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I was referring to the Jab . I know some engines have a vac pump that doesn't like reversals  but some engines have been known to kick back when very hot. What happens with a 912?  Nev

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On 14/06/2021 at 6:01 AM, facthunter said:

A properly designed and installed "O" ring should not require sealant.

Jabiru seems to not know that. They have used sealant on the rubber rings of the intake manifold from day one. They also use sealant on the oil pump that also has O rings installed, both on the back plate as well as the housing. On the Gen-4 engine they have changed the back plate design to include a second O ring (found in the CAMit engine), and they removed the sealant requirement there.

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22 hours ago, facthunter said:

What happens with a 912?  Nev

Apparently, it draws air in to the oil system and then pumps that air into the lifters, which can be troublesome to clear.

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field report - Time required to change 4 o-rings is approx 2 hours for someone who has done one, and or is well familiar, or 3 hours otherwise.

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Posted (edited)

in my recent hydra lifter experience,  the air may be bled out of them quickly with cam high. the air is quite compressible. You can compress a lifter full of air down  to spring bind with your finger. 

 

- Not the same as getting full of oil.

 

the ingress of oil , which will occur quickly, will compress the air, and may go out with the bled down. 

If there is piston/valve clearance I cannot forsee any harm if it sorts itself out on the first few revs on start.  might make a bit of noise.

I don't know what spring pressure Rotax's run , yet.

but-

The Jab springs could compress the air in the lifter piston chamber  to spring bind.....no big deal. 

so (guessing) I cannot see the rotaxes not compressing the air , either. 

 

Edited by RFguy
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I would ALWAYS Prime( Pre-pressurise) the oil system before running a new Motor particularly, from an outside pressure source. If the motor is  slowly turned all the lifters will be filled and there won't be any clattering lifters. This process will not oil the bores or the camfaces but will reduce any delay of normal oil supply to a minimum. New camshafts are often moly coated to prevent scuffing. Nev

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Posted (edited)

when I started my rebuilt, I cranked it with the starter for about 15 seconds (without plugs in) . saw oil pressure rise into green. then did it again 2 minutes later. 

started first revolution with plugs in , no clattering.

(The engine was bathed in oil everywhere prior to that) .

but, Nev that is a good point about external pressurization. before a piston moves.

of course, in my case main bearings etc still had oil because the crank etc and big ends had not been touched.

 

my mazda mx5 (1998)  , run on synthetic oil. When the oil got > 3000km old, the engine top end would clatter for 3 seconds on start.  change oil time. 

Edited by RFguy
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Some of that can be due to the chain tensioners.. Oil shouldn't deteriorate that quickly. Additives can be used up. Longlife" ED" oils just have more additives. Sort runs can cause oil dilution. Thermostats should always work. Nev

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1 hour ago, facthunter said:

Some of that can be due to the chain tensioners.. Oil shouldn't deteriorate that quickly. Additives can be used up. Longlife" ED" oils just have more additives. Sort runs can cause oil dilution. Thermostats should always work. Nev

Off topic - MX5s are notorious for hydraulic lifter ticks, it's like a built in oil warning system 😆.

If it's low on oil - it ticks

if the oil is getting old - it ticks

if the engine is cold - it ticks.

After 98 they switched to solid lifters.

MX5s also run a timing belt, no chain. It's a non interference engine.

(we have 2 in the family a '90 and a '91)

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Lifters that are built into the rockers etc on multivalve engines are far more likely to tick than the "normal"ones fitted to pushrod motors  which are rare now. Variable valve timing mechanisms could cause noises too..  I was being "general" about causes of ticking.. 

 It can be caused by oil being low as air may be introduced then. Oil itself doesn't wear out. Additives may get used up and polymers can break down and some dilution can occur. Sludge in hydraulic lifters can make them unreliable. Some of the really small ones have a seal that could harden if the motor overheats and they will tick forever after that. Nev

 

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