Jabiru top end problems

[motzartmerv]

Hi. We had the heads off our 170 today. The tops of the potts were coated in the usual coke and crap. The engineer cleaned it all up.

 

 

This engine has done about 500 hours.

 

After we cleaned all the crudd off the tops we found this...

 

 

(the arrow is stamped on the piston, I didn't draw it in)

 

And this...

 

 

Can you see the scar on the lip of the cyliner?..

 

3 of the 4 cylinders had a scar in the same position.

 

The valves showed no scaring at all, but the engineer informs me that the valves are made of an incredibly tough metal where as the cylinders are a cast alloy.

 

Bare in mind I am not an engineer. I have some questions to you learned folk.

 

The early solid lifter engines didn't seem to suffer thru bolt failures (correct me if im wrong)

 

Have any of you noticed this scaring on your engines?

 

Have any of you seen this sort of thing show up on a post thru bolt failure inspection?

 

Could this be the problem?

 

I should add that this engine has not suffered thru bolt failure. It did however suffer partial power loss and was accompanied by a heavy landing at 52 hours. The reason for the power loss was never given to us. Jabiru never inspected the engine (we discovered today) but only repaired the damage (fuse crack) and sent it back.

 

Please dont take this as a Jabiru bashing thread, and Im only looking for informed opinions on what we can see..

 

If you happen to have the heads off your engine at the moment, can you please clean the coke and inspect the top of the pot, Im curious weather this is a 'normal' Jabiru engine thing.. We will have a 3300 engine apart tomorrow and will be looking for similar injuries.

 

Thanks

 

 

[turboplanner]

Where are the valve edges, with the valve open, in relation to the scars?

 

 

[motzartmerv]

Right over them.

 

 

[turboplanner]

Hydraulic lifter travel too long? Piston needs indentation cut?

 

 

[xair1159]

I think it's generally accepted that the hyd engines are not performing as well as the old solid ones and Jab have had various combinations of high and low leak tappets and cams to try and improve things. Based on my own experience, tappets do pump up and prevent valves seating fully which could also cause your problem.

 

Do you think there is any chance the barrel base shims might be fitted to do more than just reduce the chances of detonation with poor fuel? Or am I just a bit cynical?

 

Longer barrels or shorter push rods would stop valves meeting pistons, but I don't think the 0.5mm change is going to do a whole lot to the compression ratio. There have also been a good few exhaust valve failures with heads dropping off - perhaps bending the head of the valve slightly every rev. has something to do with it.

 

The real cure would be to sort out the tappets and their oil supply as they have in Jabiru RSA.

 

PS - think I would fit new valves.............

 

 

[turboplanner]

Motz, if you are interested in re-assembling temporarily, putting a patch of plasticine about 3 mm thick where the valve will make contact with it, turning the engine over to open the valve, then pushing the rod of a vernier caliper through the plasticine to the piston surface and measuring the valve clearance from the piston, I'll look up a benchmark I work on for race engines (which need a little more clearance than operations under 3000 rpm).

 

That should tell us whether the constant valve problems are caused by regular contact, although the bruising does look very light.

 

 

[skeptic36]

I see Ben Mcguinness, the bloke who built the Waiex featured in this months Sport Pilot, bought a factory reconditioned 3300 for it but changed the hydraulic lifters back to solid and modified the plenum chamber prior to installation.

 

Regards Bill

 

 

[damkia]

Longer barrels or shorter push rods would stop valves meeting pistons

Not really as the rocker ratio (ratio of push rod to rocker fulrum, and valve to fulcrum) is what defines how far the valves get sent down the guides assuming that there are correctly set clearances and the hydraulic lifters are working correctly (correct leakdown rate). Changing the camshaft throw would be another fix to achieve the same result.

 

Changing the push rod or cylinder barrels alone would also change the camshaft timing, once again changing the camshaft to adjust for that would put you back at the beginning.

 

 

[turboplanner]

Not really as the rocker ratio (ratio of push rod to rocker fulrum, and valve to fulcrum) is what defines how far the valves get sent down the guides assuming that there are correctly set clearances and the hydraulic lifters are working correctly (correct leakdown rate).Changing the push rod and cylinder barrels alone would also change the camshaft timing, once again changing the camshaft to adjust for that would put you back at the beginning.

What are your thoughts on the markings on the pistons?

 

 

[damkia]

What are your thoughts on the markings on the pistons?

I would be looking at the valve guides to see if the valve stems are "soft" sticking in the guide art some point, not a hard jam, but a reduction in valve velocity leaving a bit of extra slack in the valvetrain and allowing impact with the piston. This could be from wear of the guides and a valve "rattling" down the guide, or too close a tolerance causing sub critical valve seizure issues in a hot engine.

 

The marking on the piston and lip of the cyl would seem to be an errent valve.

 

The arrow on the piston is used to indicate the correct orientation of the piston when rebuilding, Normally points to the front of the motor.

 

Please note edits to text.

 

 

[motzartmerv]

Thanx guys. Tubz the heads are back on already but might try the plastacine on the 3300 today. I'm just wondering if this is a common problem hidden by the coke buildup.??

 

A few members have recently changed the nuts as per the AD but used the same bolts. This complies with the AD but the bolts are recessed into the new longer nuts and the end 2 or 3 threads are not used at all.. I'm wondering what the point of changing the nuts at all is.?

 

 

[jetjr]

The new nuts use the last few threads of the old bolt

 

A video on Hydraulic vs solid valve springs and valve clearance and spring bind in Jabiru's

 

 

 

[facthunter]

A few points, not necessarily in correct order or importance.

 

The cylinders are NOT cast they are billet machined 4130 or similar.

 

If you are going to change from solid to hydraulic or vice versa the camshaft profile is different. You would have different ramps on the solid one.

 

You can assume zero running lash with the hydraulic lifters. They can pump up at high revs but these engines are not doing high revs. If they do pump up there will be a loss of performance. as the valve(s) become unseated and the engine will run roughly

 

The hydraulic lifters rely on engine oil pressure to supply the oil that enables them to automatically adjust the "clearance" and may respond to oil pressure changes.

 

Hydraulic lifters can also fail to hold up and the valve may fail to lift. I have experienced this personally on a Cont O-300 and it nearly caused a serious accident, so as a result I would prefer to have solid lifters (if there is an option), in an aircraft engine.

 

Another advantage of solid lifters is that, when adjusting the tappet clearances, if one of them is behaving strangely,( changing rapidly) , you detect it and do something about it earlier. The hydraulic lifter masks the problem till it's operating range is exceeded. That is assuming that it was set up properly in the first place.

 

Valve geometry..... The only thing that affects the correct angle of the valve to the stem , and hence valve guide wear, is the height the valve stem end. ends up at when the valve is seated, and the position of the pivot point of the rocker in relation to it. Provided the runningclearance is correct, at the contact point of the rocker to allow it to operate, none of the other dimensions matter . Cylinder length push rod length etc as long as you can get the adjustment... Hope that makes sense. Nev