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Hi KG OK. yeah so yours is a Gen 3.5.....

 

Certainly the move (G4) to the aluminium pots and tightening up the clearances (now having similar Te for both bore and piston) may improve things.

 

Interesting reading the reports- the hydraulic lifters provoked a lateral  vibration mode in the thru bolts  (they are long) and this was cured with dampers in the middle of the thru bolts. and the 3/8 thru bolts were also changed ot a different Tc as to not change too much when they got hot.- otherwise with the thru bolts going wacky- the crankcases fretted and that was bad  bad bad !

 

Said vibration mode did not occur to same amplitude with the solid lifters...That's why the hydraulic lifter engines have a bad rap.

 

 

 

 

 

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This has been very interesting to me both for aviation and motorcycle engines. Thanks everyone.

I'm fairly satisfied now that possible reasons for piston throwing in < gen4, which I  beleive that I now have a good handle on, have been substantially reduced in Gen 4.  (piston temps, piston typ

This is something I had no idea about. Bloody interesting.   http://courses.washington.edu/engr100/Section_Wei/engine/UofWindsorManual/Graphics/Piston%20Assembly.jpg Figure 6- Pisto

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well, so far so good. Except for a bit of " I'd tell you about some gen 4 problems but then I'd have to kill you" stuff i reacon nobody seems to be having problems. Go the 4's.

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Hydraulic lifters often run heavier Valve springs too.  They also mask other problems .Stretched stems or recessing seats. The seats (inserts) can hammer into the aluminium or the seats erode.. To check the range of operation of the lifters you need the oil out of them and bottom the travel and check the play from Rocker to valve stem end. It should be a specified figure when  first assembled and of course if it's got none your valve will not seat properly.. I prefer the manually adjusted ones myself.  Nev

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On 17/01/2020 at 11:05 PM, Kyle Communications said:

I am not saying anything specific ...but....your not looking in the right places maybe...they are definetly better but still problems

 

 

I've started playing with the idea of a Jabiru kit to stay gainfully occupied. I find myself hoping some item of farm mshcinery will break down so I can repair it. They all seem annoyingly resistant to failure at the moment. 

 

I've asked Jabiru about the D motor. They quite correctly suggest that their 7,000 motors plus compared to a company probably under a 1,000 motors isn't a good bet.

 

https://d-motor.eu/

 

However....Side valves remove so much complexity and fuel injection adds the correct hi tech. It's definitely the engine I'd want to build from scratch and I'm still looking this direction as a future project.

 

I'll dig up dimensional comparisons and a more accurate weight comparison if this still looks like an option in future. Questions then I'll relate to how it's going to fit and how air flow will work I'd say as important considerations. Especially when looking at the threads here in the subject of airflow.

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23 minutes ago, Mike Gearon said:

I've started playing with the idea of a Jabiru kit to stay gainfully occupied. I find myself hoping some item of farm mshcinery will break down so I can repair it. They all seem annoyingly resistant to failure at the moment. 

 

I've asked Jabiru about the D motor. They quite correctly suggest that their 7,000 motors plus compared to a company probably under a 1,000 motors isn't a good bet.

 

https://d-motor.eu/

 

However....Side valves remove so much complexity and fuel injection adds the correct hi tech. It's definitely the engine I'd want to build from scratch and I'm still looking this direction as a future project.

 

I'll dig up dimensional comparisons and a more accurate weight comparison if this still looks like an option in future. Questions then I'll relate to how it's going to fit and how air flow will work I'd say as important considerations. Especially when looking at the threads here in the subject of airflow.

Favourable reports of Jab engine running on 98.  Look at what RF is going through due to AvGas.  I'm a machinery person and see added maintenance outside the oil change and other scheduled maintenance times that that away flying time heads off, new valves and rings in a particular cylinder every 25 hours.  Before I built what I have now I emailed Jabiru inquiring if a new wing design was on the design board to allow shorter field ops and of course a slower cruise speed comes with that and Daniel there was good enough to give a  reply, 'No'.  I believe there is a market for a shorter field ops wing in their product line as so airfields are too short for all but the very capable Jab flyers.  I hope Dan designs such a wing one day and I may look at build one as well.  I'd go the Jab engine, consider 95 or 98 fuel and hold of on the 'D'; IMO needs more hours and variety in operation and Jab is well established for local (Australia) support.

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22 minutes ago, Blueadventures said:

Favourable reports of Jab engine running on 98.  Look at what RF is going through due to AvGas.  I'm a machinery person and see added maintenance outside the oil change and other scheduled maintenance times that that away flying time heads off, new valves and rings in a particular cylinder every 25 hours.  Before I built what I have now I emailed Jabiru inquiring if a new wing design was on the design board to allow shorter field ops and of course a slower cruise speed comes with that and Daniel there was good enough to give a  reply, 'No'.  I believe there is a market for a shorter field ops wing in their product line as so airfields are too short for all but the very capable Jab flyers.  I hope Dan designs such a wing one day and I may look at build one as well.  I'd go the Jab engine, consider 95 or 98 fuel and hold of on the 'D'; IMO needs more hours and variety in operation and Jab is well established for local (Australia) support.

Excellent. I'm a long way off future project so will keep looking with interest at developments. 

 

I have 3 aircraft now that will end up in farm hangar in Melbourne. An unexpected development.

 

Between them I have 

 

Airbike... Take off and land in bugger all feet. 100 to 300

Quicksilver with floats. Nice option over Westernport.

Nynja. Capable little aircraft and I'm not far from going to meet both it and Vince.

 

Current thinking long term they'll be a single aircraft ikit build. Can fly and enjoy and build while not getting into too much trouble hopefully. Look forward to visitors once runway sorted.

20210220_090307.jpg

20210216_071359.jpg

Screenshot_20210220-090159_Chrome.jpg

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11 minutes ago, Mike Gearon said:

Excellent. I'm a long way off future project so will keep looking with interest at developments. 

 

I have 3 aircraft now that will end up in farm hangar in Melbourne. An unexpected development.

 

Between them I have 

 

Airbike... Take off and land in bugger all feet. 100 to 300

Quicksilver with floats. Nice option over Westernport.

Nynja. Capable little aircraft and I'm not far from going to meet both it and Vince.

 

Current thinking long term they'll be a single aircraft ikit build. Can fly and enjoy and build while not getting into too much trouble hopefully. Look forward to visitors once runway sorted.

20210220_090307.jpg

20210216_071359.jpg

Screenshot_20210220-090159_Chrome.jpg

The Nynja's are soon at 600kg and will be factory build option as well.  Will make a good trainer flying wise and maintenance wise.  Just showed the 600 undercarriage drop test being done on facebook a few days back.  My next project is too get another Skyranger and fit 'Puddle jumper' floats as the Whitsundays and other areas a good to visit with on water landing ability.  Just realised thread drift, apology in advance.

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Gen4 has a few years now on them . theyve changed out a few things , pistons amongst others. there have been a few recalls for various parts.  cooler running engines may be a bigger caking up issue than previously. Jab are still not giving people the right thru bolt tightening advice  ...(that is, contrary to thru bolt mfr) .

 

 

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A Jabiru Gen4 engine quit today, and the J170 landed safely in a paddock. No further details available. So yes, the Gen4 engine is not immune to failures. Just not being reported like many other engine failures. 

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What do you reckon is the minimum strip length for a Jab, blueadventures? At the farm here, I made a 550 meter long strip which I landed on a few times, but I have been spoiled by the much longer strips at Gawler.

My experiments showed that 300 m was long enough , but that allows nothing much for errors, and so I was scared of the 550m.

I know of a strip which is 250 m, and I guess that this would do provided there was nothing higher than a farm fence at each end, and it was into the wind.

 

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As for that failure- well we know minimum so far - was it catastrophic ? (piston failure , valve) etc or did it have a fuel delivery issue ?. 

 

so a little more about the hydraulic lifters and vibration modes from a few posts back.  A couple of months on and my own top end refurb ...my own investigations and calcs  says this excited vibration mode reports by jab I think was mostly BOLLOCKS and GUESSING.

 

I think that if the hydra lifter engines truly do have a bad rep, then its more like what Facthunter suggests :

"Hydraulic lifters often run heavier Valve springs too.  They also mask other problems .Stretched stems or recessing seats. The seats (inserts) can hammer into the aluminium or the seats erode.. "

 

maybe, although I have not read or heard of stretched stems and recessing seats.

 

-glen
 

 

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Keep reading and you will. The surface cracks on the stems near the  main part of the valve are due to overheating and stretching. Seats often hammer into the ports. You will feel a slight raised bit in the aluminium in the port when it happens.  "Recessing"  of seats is the usual reference to wear or erosion of the seat area whether it's in the parent metal (iron steel or bronze) or an insert.   The impacted seat movement is usually restricted to aluminium heads  as it's relatively soft especially when overheated. IF an aluminium head has been heat treated during manufacture to make it harder and improve it's tensile strength, operating at above the max temp recommended  (even just once) will negate the heat treatment process. IF there ever was one done. Nev

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OK, yes I have seen that described in the Jabiru overhaul manual- the stretches etc.

 

My belief is that the pistons that WERE used, ( at least until the new HD pistons for Gen3 and Gen4 this last year)

that is - the use of motor vehicle pistons designed for water jacket cylinder bores,  and designed to run at a much lower temperature were a 'sub-optimal' choice,  given that the strength of 4032 alloy, falls by 50% between temperatures 170 and 220 ish C. 

Drops 4x from 100>200C. drops another 2x from 200>300C ! and about 2x less fatigue strength 150 to 210C, and it keeps on falling, so that's the basis for my consideration of using water jacket rated pistons.

 

250-300 C under the crown is not unusual for a water jacket cooled petrol engine.  top of the skirt aroundsrings  200 to 250C... So, you can see, we are in the danger region for the alloy  if you add another 70 to 100 degC to the temps. 

 

The burned oil I found on the undersides, the  carbonized oil on the support of the crown from the skirts, for W100   must have been > 210C ...

 

The new pistons, according to info from Jab are custom made variations with heavy meat in the skrt/rod crown region where pervious pistons had failed.  The new 'HD' version are also available for Gen3 from Dec 2020/Jan 2021. $195 a piece .

 

my two bobs worth. Keep oil ytemps to the minimum required for operation. Keeping oil temp low is low hanging fruit for overall engine temp.  The Gen4s with heavy alloy barrels should be much better at getting the heat out of pistons. 

 

 

 

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I agree about the non suitability of the original pistons as the heat  flow from crown to skirt is deliberately reduced to permit closer running clearances. I've never seen this copied in any aero or competition piston. because the rings run hotter and the piston is weaker.  It's designed for relatively LOW specific Horsepower CAR engines and is structurally unreliable. The offset gudgeon pin is a  problem you didn't need to have as is the change of circlip section (Murphy's Law)

 In the car engine there were no circlips as the pins are a press fit in the conrod small ends. Having to machine the circlip grooves is another QA problem that would normally be done during the original machining process and more likely to be consistent in quality..Nev

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Nev, I concur and I think it goes something like this ;

 

1) piston gets hot.

2) The piston skirts protrude a long  way out of the bottom of bores  in the Jabiru

3) The side thrust motion of the crankshaft tries to push the piston a little in shear just after BDC 

4) The hot piston that becomes ever-so-slightly plastic ,  deforms 

5) piston skirt / lower jams  ..... the rest is history. 

 

high clearances may make this more likely if the the piston can become out of vertical. (cold engine?) 

equally, tight clearances may provoke the jam (hot engine with low clearance and low strength pistons)

 

But I think the ability for the piston to get out of vertical is more of an issue unless the pistons get really hot. Then the clearance issue will untimately kill it ! (comments ??? ) 

 

This together with my numbers on cold thru bolts mean I think one has to be careful at the extremes of operating range (cold - high clearances, and loose through bolts) and hot (reduced piston strength, thru bolt excessive tension leading to permanent stretch beyond yield) 

-glen

 

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The "W" slot piston distorts the skirt to reduce it's ovality  when the crown expands being the hottest part. AND. This forces the oval skirt to be more round and REDUCES the size at right angles to the piston pin and takes the skirted part with it as the only piece connecting the two  sections are the piston pin bosses. The whole idea relies on there being a marked difference in temperature  between the crown and the skirt and works much better in a liquid cooled engine where the bore surfaces are significantly cooler. The forced distortion puts a significant extra load on the pistons structure but is for reducing Noise when the engine is cold as it can have a lot less initial clearance on the piston skirt than a solid forged one does. I suspect very few engine builders know A LOT  about this. Some skirts have INVAR struts cast into them around the gudgeon boss area and fail eventually as a result due to the stresses imposed.  Nev

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Nev,, thanks for the explanation on W slot style and the likely and expected deformations.

 

FWIW  the consistency of the machining of the circlip slots in my 2013 pistons I would say was below par. wasnt flash.

 

If there was an AD on these, I think the only practical solution would be replacement of the pistons with a FORGED 4032 type. 


The Gen4 with its big Aluminium bore cases is likely to  provide much better cooling capability than the steel bores if they get the airflow.A much better opportunity to get the heat away. It's probably also more sensitive to hot/cold sides leading to asymmetric deformation  compared to the lower (~ 13 at 200C) thermal expansion cro-mo bores. I dont know the thickness and hence ability to move heat from the hot side to the cold side. 

 

With Gen4, I wouldn't be so concerned about getting high piston temps if the bore cases can be held on their surface below 120C.

 

 

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The steel bores are much more likely to have uneven temps as the conductivity is not good as well as practically no real effort is made to guide the airflow around the cylinders. I've seen them with a blue patch  about 1/3rd the bore circumference adjacent to the exhaust valve common to what you get with aircooled sidevalve engines. The ovality and taper on most pistons doesn't take account of that type of heat pattern. Nev

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got it.  Well,  mine will get a fair bit of air ducting work before it flies far. 

This bizzo with some engines being hard /stick to turn over just after shutdown , it has in various forums been put down to a hit piston into the bore that cools a little. I am surprised. But it's also a red flag in my book .  If so I wonder if it is a ovality problem on the cooling bore/piston combo, or there is minimum clearance on all piston sides... 

My engine cold has 'plenty' of clearance from my experience with vehicle engines , and is about the same as initial/ as built spec. 

The piston going from 20 C to 200C expands by 0.0035 so if was 97.5mm, is now 97.84mm

 

Correspondingly, the bores over that range would go from 97.8mm (example)  to 98.04mm.

but the bore outside  wont get as hot as the pistons because they have 1) oil in between 2) high thermal resistance. 

So the bores wont expand so much. maybe 70% of that- maybe 97.96mm. Maybe Trouble !

 

So, on those numbers- if the bores cooled down fast, they woul certainly cauise a clearance issue with hit pistons

But the hot piston should very rapidly lose its energy ...

 

Maybe that is where some of the running  trouble is coming from with piston breakages- hot pistons, cold bores. 

 

-glen

 

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If all of the piston is the same temp (engine shut down for a small period) the original amount of ovality will resume but overall the piston becomes bigger than when the engine is cold and would be expanded more than the steel barrel is. That would easily explain the stiff when hot engines. The only other possibility is crankcase distortion Alignment. I'd go for the pistons being the problem.  In any case the engine should NOT become hard to turn  due to any increase in friction. Nev

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Which means before goign crazy on cooling the bores all the way round, one has to be confident the piston temperature is down also, otherwise cooling the bores may have unwanted repercussions.  You would 'guess' that cooler bores would mean cooler pistons and less piston expansion.

 

I wonder if the amount of clearence/slap can be heard and detected to get some handle on this.

 

I wonder if a sudden rush of cold air in the front could cause a fast cooling of the bores and generate a jam 

 

 

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Not in a W slot engine which depends on a good temp difference in the piston  for the idea to work. When the crown is much hotter than the skirt it forces the ovality to reduce and thereby counters the normal expansion process which seems pretty cunning except a lot of ovality reduces skirt contact with the cylinder wall and is then cooled less effectively. than if it was near round.   Nev

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