3300A : A tale of two (hydraulic) lifters, one with no spring.

[facthunter]

Usually near the exhaust port area. When they lowered the compression ratio... how was that done? Have you come across that in your travels? Nev

[RFguy]

In cases I have seen, not near the exhuast port, the recession is  either side of the lowest head screw by 30 degrees- the one behind the grub screw inside the rocker cover.

 

If you run the numbers of cylinder head screw torque, the likely tension with the likely lube, interface pressure , and the aluminium yield strength at elevated temperature, then the numbers say yes, certainly this will occur. and it does.... at least on pre Gen4. Gen4 very different of course in the top of the engine. 

 

so the head recesses, and then cools down, head contracts a bit ,  the screw gets loose. gas escapes before head up to temp. tension off the bolt when cold, so then people tighten it.. etc etc gas escaping is usually visible around the bolt on the outside once the head is off. otherwise the spray is in a very dark, hard to see place.

 

 

 

 

 

[RFguy]

Nev,

I DID find a note in the Jab overhaul manual about bledout clearances- it isnt in the clearance table.

 

"Normal valve tip to rocker clearance with lifter fully bled is around 1.8 / 1.9mm. With later shortened pushrods of 215mm, add 1.0mm to above dimensions."

 

Now, 217mm / 215 mm is the solid lifter, and 207mm / 206mm is the hydraulic roller lifter lengths

(remember, during Gen3 they found that clearances were marginal and shortened the rods from 207 to 206, which increases the clearance at the valve by 1.7mm)

 

So, they're looking for a fair bit of clearance, I guess gives some room for the spring to bind if there is pump up.

 

They do site this, because there were pump up issues apparently leading to open valves and all the bad things that go with it : 

" For a  solid lifter engine high oil pressure is not a problem (within the set limits) however a hydraulic lifter engine needs mid-range oil pressure to operate properly. "

 

 

 

 

[facthunter]

2 MM is what I would have expected. I've said a minimum of 1 mm.  The less you use the more frequent would be the need to check them.  The whole idea with them is to reduce maintenance   They are extra weight, cost and complexity  and require stronger valve springs, Nev

[RFguy]

With too much bled clearance, I would think that if a valve stem was stretching, the lifter would permit the valve spring to extend, and thus the lifter would have room to compress. How likely is that scenario ? (putting aside that if the valve stem was stretching, you have big problems anyway) . In the case of the solid lifter, there isnt anywhere to go beyond the zero lash.

[facthunter]

That's not the way valve stems stretch Usually the overheated part does and often cracks. Once the oil is there the lifter is hydraulic locked These valves are only going at 1/2 engine speed and a wide gap in the mechanism will only cause a sudden opening and reduced lift and a power loss. Sometimes (if there's room to allow it) the pushrod will dislodge and be bent  another possibility is that momentarily the valve may be pushed further by the way the pushrod ball end contacts the tip of a ball and it will cause piston contact. (More likely in diesels). Nev

[RFguy]

OK

" the pushrod will dislodge and be bent"

- IE comes out of the socket at either end and ends up somewhere else, permanently, or momentarily ??? and thus potentially bent by the camcraft force.

[onetrack]

A pushrod can also be simply bent via overspeeding an engine beyond its specified maximum RPM.

[facthunter]

RF guy..Yes that's it. Sometimes the pushrod is so contained this doesn't happen but a bent pushrod is not uncommon over the years and can result is an oil leak and fire.. (Goes through the push rod tube (cover) Power loss is also likely. A momentary guide seizure will produce the same result. The valve hangs up and the pushrod can fall out,  Some older engines used a supplementary spring on the rockers to prevent this or had clevis ended push rods for the same reason.

  One track, a valve bounce event can do that. The rpms of most aero engines  probably wouldn't get there very often  and an overspeed is a problem in other ways as well and usually connected to a CS prop issue. Nev

Edited May 20, 2021 by facthunter

[RFguy]

time for Jabiru to cop a rocket.

 

My engine was assembled with nearly zero fully bled hydraulic lifters lash. nearly no clearance. I have 207mm push rods.

The engine as assembled could NEVER have met the clearances in the book. Not ever.

With a little head recession at the rear and new bushes, clearances go negative...

 

Jabiru changed their push rods in a short space of time in Gen3 - moved from 207.0 to 206.0  and finally to 205mm at the end of Gen3.

 

I need 6 rods, and they are rather special in the ends, and they want $73 each. 

I object to having to buy new rods where the engine would have NEVER met spec in the clearances when it left the factory...

I'd be happy to go halves. Nope.

 

I'll see how I go with shortening the rods, they've been pressed on red hot.  Other than that, my patience with Jabiru is finished.

 

Oh , I removed and fully dissassembled all 12 hydraulic lifters, a time consuming task. heads off again. 

only found ONE piston spring missing out of total of 12 of them. 

 

-glen

 

 

 

Edited May 31, 2021 by RFguy

[RFguy]

Hi Nev

the rods have large ball ends,  I think 10mm ish  (3/8"spheres  and hollow 5/16 tubes, have to measure the half sphere (ball end) up carefully. 

 

different to the solid lifter  rods.

there are a few shops that can make these up custom. or I can work harder on shortening (it is done, but it isnt easy) . 

 

 

 

 

 

Edited May 31, 2021 by RFguy

[facthunter]

Grip the difficult end in a chuck or collet on a lathe and use a slide hammer device on the other end. Shorten by the exact amount and press together. . Nev

[RFguy]

yeah. agreed. I tried that, didnt budge. Others have suggested to add some heat to the ball end and try again...

[facthunter]

Don't think heat on small diameters will have much effect. You'd only ruin the heat treatment of the ball also. Carefully grind the end so it comes away and make new ball ends. What a drama. Have you upset the man in Bundaberg?  Nev

[RFguy]

Hi Nev. No, have not upset them. ( To my knowledge.....) I am disappointed with Jabiru for not offering some discount or providing them FOC as this is their fault, they shipped an engine that  did not meet the rules in their own book !  But now, as you can see, I have lost patience. And I have stopped being nice. Which started with that missing hydraulic lifter spring.  And a lack of documentation about all these push rod length changes.... 

 

MarkDs going to help me with his bigger lathe and bigger slide hammer...

 

Grinding the ends down (only need a 1mm) is also an option. 0.5mm would do it...... if I can satisfactory treat and polish them .Not something I have experience with but others have ...  

 

[Thruster88]

3 hours ago, RFguy said:

time for Jabiru to cop a rocket.

 

My engine was assembled with nearly zero fully bled hydraulic lifters lash. nearly no clearance. I have 207mm push rods.

The engine as assembled could NEVER have met the clearances in the book. Not ever.

With a little head recession at the rear and new bushes, clearances go negative...

 

Jabiru changed their push rods in a short space of time in Gen3 - moved from 207.0 to 206.0  and finally to 205mm at the end of Gen3.

 

I need 6 rods, and they are rather special in the ends, and they want $73 each. 

I object to having to buy new rods where the engine would have NEVER met spec in the clearances when it left the factory...

I'd be happy to go halves. Nope.

 

I'll see how I go with shortening the rods, they've been pressed on red hot.  Other than that, my patience with Jabiru is finished.

 

Oh , I removed and fully dissassembled all 12 hydraulic lifters, a time consuming task. heads off again. 

only found ONE piston spring missing out of total of 12 of them. 

 

-glen

 

 

 

The way I see it any of these engines assembled as yours was is a ticking time bomb. Time for jab to step up. No clearance valve heats up, valve drops off, bang no power. 

[RossK]

Report it as an incident/defect with RAA, there should at least be some follow up from RAA with Jabiru.

YLIL maintenance guys have been logging their Jab engine issues on the RAA Accident and Defect register.

[facthunter]

It IS a safety issue. What's the repair history from NEW and the date of build ? Likely Mr Stiff didn't build it and the firm who probably did are not around.   IF you shorten the ends you'll go through the hardness almost guaranteed.  What about the eccentric rocker bush(es) I suggested some time back .5mm there equals 1.00  in effectiveness. Nev

[RFguy]

Hi Nev,yeah.... eccentric rocker bushes . maybe. but I need alot more clearance than a eccentric bush could provide without increasing the diameter of the bush casing in the rocket itself. would be easier to buy pushrods I suspect now its all together and built up. 

 

[RFguy]

Date of build - Mid 2013. 370 hours. No service requirements beyond book  maintenance.

about the time AVGAS jabirus tend to start caking up with avgas by-products and other carbonized products. 

 

At ~ 25 hours before I got it, #6 exhaust didnt make leakdown and the valve seat was lapped. (reduces clearance ! ).The clearance trouble was clearly starting. valves and seats all are in excellent condition.

 

I have submitted a report to RAAus.

 

 

Edited June 1, 2021 by RFguy

[facthunter]

It might be worthwhile to follow it up with a phone call to determine if they understand the problem.  Less and less people would. Nev

[RFguy]

I wonder how many problems Jabiru inherited from third party builders without knowing or , or insufficient random audit and QC ... Either way, someone assembled this engine with minimal/zero clearances on 11 out of 12 tappets. That's pretty hard to ignore unless you are under some directive to ignore it. and then there was the lifter with no piston spring that had a couple of mm tappet clearance....

[facthunter]

There's only one builder of Jabiru engines  at that time  and towards the end, He and Rod didn't agree on the direction to go with engine  development. Both firms were at Bundaberg at the airport.  Nev

[RFguy]

I think the Jab Gen3 design is OK. Putting aside flywheels falling off. 

 

If you're going to make a cost effective engine without one person per part on QC then any problems that occur the mfr has to be onto those problems like a tiger, and use the knowledge and experience of the group of owners PUBLICLY  to assist in the continuous refinement.  As a manufacturer of very hi-tech requipment, I can tell you that owners are not completely stupid, you still have to put dots on a map and see if you have a cluster despite what you think and think you know is right or wrong.

I like everything perfect, but I know when it cant be, but there are sometimes good reasons for this, and other times there are pig headed reasons for imperfection.

 

Gen 3 : Keep the heads cool, oversize the oil cooler so the oil NEVER goes over 80C. maybe a thermostat is required,  use an additive on avgas to reduce buildup , attack some of the cooling problems with addition bits of aluminium sheet for CHT never even above 160C where it is monitored, cooling means less BAKED on stuff in guides etc,  monitor valve lash, CGT, EGT, set head screw and thru bolt torque/tension  appropriately, dont run over 3000 RPM WOT ,   and she'll probably be right. 

 

Gen3 : 

1) I would have liked to have seen solid pistons from the beginning and a better focus on keeping the head temps down through much better fin design on the heads and bores.  Piston strength w.r.t. resistance to detonation has a very high temperature dependence. Have you seen the cylinder pressures during detonation ?! holy je3u3, that's enough to permanently stretch those thru bolts and break hot pistons in one  single detonation event.  

2) Since the engine is knock sensitive,  I would like to have seen a knock (detonation sensor) with a sticky light that requires engine inspection after it occurs.

3) I would like to see an approval for an additive like Decalin for AVGAS to reduce buildup. I am SURE that many problems are the rate of avgas muck buildup in guides and and around seats. Getting head temperatures down 20 to 30 deg C would reduce the baking on and  trouble in the guides and stems and around seats.  

4) I'd like to have seen better controlled machining of interior circlip grooves. They're all over place in my engine. 

5) The CHT sensors are really showing an optimistic state of the real temperature under the heads. They need to be moved.  The CHT values recorded on top are total fantasy.

6) but cooling cooling cooling. That cooling setup hasnt got a hope of working well because of the wide open barn doors between the pots steal airflow in the fins .  

7) The oil gets so hot in the head in the rocker compartment it really trashes the oil and the 25 hour change might even be optimistic. Maybe even more often.

-8) I would like to have seen a much more preventive approach by Jabiru on the known problems- like valves not running with enough tappet clearance. Why did this engine not get a letter from jabiru and a packet containing some shorter rods ? Why dont we know about the changes to to the pistons in the Gen 4ABC versions ?

 

Edited June 1, 2021 by RFguy

[RFguy]

What Jabiru SHOULD have done in my opinion with this push rod/tappet  clearance problem is written to everyone and generate a public SB

"Hey everyone, we have become aware of a few issues with low bled-out tappet clearance in gen3 enignes SN.... to ....  that could lead to valves not closing and eventual failure in flight . We would like you at the next opportunity measure these clearances and send us the information you gather . If your bled-down clearances on a tappets are less than XYZ, we'll send you a shorter, replacement rod that must be installed in the next 25 hours " etc etc

and

"Hey everyone, we have had a single  report of a  hydraulic lifter missing an internal  component  (spring) . If  bled-out lifter tappet clearances are > 2mm, at the next overhaul, the lifters must be checked for correct operation. "


 

 

Edited June 1, 2021 by RFguy