Hot rear cylinder example Jabiru 3300A-2575 and plenums

[Thruster88]

Just now, RFguy said:

There is a Gen 1,2,3 piston upgrade available now, also....

Have they been tested or is that your job Glen?

[RFguy]

ha ha. No. 

They have sold one batch, something like 150 pcs.. Next  batch is due next Feb some time. 
I understand they are uprated like the upgrated Gen4 pistons. Not sure what they got for the skirt slot, or not. 

Edited January 22, 2021 by RFguy

[facthunter]

I would NOT be running a slotted piston in an application like this. They are never used in anything but a touring type motor and are likely to collapse and lose size if overheated, Short conrods make it less suitable as there's more side thrust on the piston. Nev

[RFguy]

the Gen4 HD pistons (the 'upgrade') have solid skirts and more meat in the skirt. no slots. 

I am not sure if the new HD pistons for gen 1,2,3 have slotted, split or solid skirts.

at a guess I'd say they are solid.

 

these new pistons have been around only since December 2020.

 

image below from older overhaul manual :

 

Edited January 22, 2021 by RFguy

[RFguy]

there is of course more discussed on this topic in this previous topic. However a new HD piston for Gen1,2,3 didnt exist then. :

 

 

 

[facthunter]

It's probably  adaptable if the clearance is increased because of the steel material not expanding with the piston, as much as the alloy does The steel is also a worse conductor of heat. Retro fit would permit a degree of honing to be done to the cylinder to achieve the required clearance, WHATEVER that is.... It would have to be established. Resizing the piston is not practicable with such a small amount to be removed as it is oval and tapered.  You might also find the ring gaps excessive . Nev

Edited January 22, 2021 by facthunter

[jackc]

On 18/01/2021 at 1:04 PM, RFguy said:

OH, and engine is 370 hours, which, from what I can gather up from all the information everywhere is a pretty common figure for needing a top end overhaul.  Jab recommend new head bolts  when doing a top end.  Jab parts are cheap.

Do they use stretch head bolts, single use?

[RFguy]

"Do they use stretch head bolts, single use?"

They are not designed to be !   For a full 'top end overhaul '  where pistons get replaced also, it is recommended they are replaced . This is not a full 'top end overhaul' though. It's 'replace stuff that is either cheap or damaged'. 

If they've been in for a while (370hours) and hot hot and corrosion accelerates, and having corrosion it can be difficult to torque up correctly.

Aluminium isnt my favourite metal for having a hot screw in ...if you'll pardon the expression.  (although the steel screw does thread into a Steel bore) . . I have a set on hand. $120 for the whole set I think, 36 bolts n washers... If they all come out and look good, I'll have a spare set (to sell or keep) 

 

Edited January 22, 2021 by RFguy

[RFguy]

OK all you old engine people : So got the heads off,  I will post some pics when  have downloaded them.

 

What is the yellowing on (some of) the gunk on the piston tops ?, generally on the lower and rear side

From what I could tell yesterday, It looks it is where oil migrating up into the carbon and gunk when engine off on the lower side of the cylinder blots up into the carbon on top of the piston. There was a small pool  (a few drops)  of oil on the lower side on #6

 

Hot cylinders had far less gunk buildup than the cooler cylinders, which I guess you'd expect

 

Couple of cylinders have ring gaps lined up perfectly....

 

All I would think a fair amount of clearance. around 6 to 10 thou at a guess by rocking with my thumbs at TDC

 

Exhaust valves on generally dark brown  in the middle, lighter brown on the outside.  hotter cylinders have darker middles.

 

Closed valve  heights look OK, plenty of seat there. will know better when I get the chambers cleaned up a bit. 

 

Will take exhaust valves out and Xray  and microscope them at the lower stems .

Might make up a head valve vac tester

 

 

 

 

 

 

 

 

 

Edited January 24, 2021 by RFguy

[RFguy]

continued from above
chamber, CYL 2 cool one, CYL5 (hot one) ,, and the yellow ish #5 piston

Edited January 24, 2021 by RFguy

[kgwilson]

What fuel has been used for the engine?

[RFguy]

"What fuel has been used for the engine?"

100LL

Edited January 24, 2021 by RFguy

[kgwilson]

That would explain excessive deposits. It is hard to tell but are the pistons valve relief models?

[RFguy]

yes, there is a valve relief just visible.

MarkD hypothosises that the yellow is Lead Chromate - the chromium comes from the bores. The 4140 is 1% chromium....

Oil analysis might be interesting.

 

 

 

[Kenlsa]

RF,
Yellow/whiteis result of avgas residue. All our club Jabs are on 98 now due to avgas residue forcing us to lap valve every 150 to 300 hours 

Wire brush it all off with a brass rotary brush on a drill- piston tops (you may scrape them with a small steel ruler to reach inside if the cylinder is still attached to the case) and combustion chamber- and the lower stems of the valves.( This will be very hard and may need machining -see below)

 

if you see small grey spots on the valve and/or face -that is lead.

 

use a factory go/no go gauge on the valve stem guides. It may be too tight. If so BEFORE deciding on new guides, ream out the hardened carbon with the factory reamer (I forget the size), you will be surprised what come out! then recheck the go no go gauge again. You almost never need new guides.

 

take the valves to get recut at an engineering shop.$10 per valve.

 

Then lap in the valve to the seat (again, you probably don’t need to cut the seat as well, just a lap will be ok)

 

Oil and reassemble , with same collets to valves DONOT mix them up. then torque to 24lbs.

fly for 5 hours and then re torque....

 

ring me if you want

Ken

 

[kgwilson]

And change from Avgas to Mogas 98.

[facthunter]

If you have nitrided valves don't reface, replace. The hardness is not very deep.  Some of those inserts appear to be loose in the ally.   Heat the head and check if they prise out.   That's a lot if deposit. Some of it is often fine dust.  Are there any scratches in the bore? Nev

[Jabiru7252]

I'd be guessing but, I think the club planes needing the valves lapped so often is because they are used for training. If it were to apply to any Jab engine, it would be a very expensive exercise for many owners. Planes used for training get 'flogged' a lot harder than (say) me plodding around in the sky.

[facthunter]

They cycle more and have a greater % of their time at take off power.  They do best in cruise where they are hot enough and not near any limits.  Short trips in DC-6b's used to result in much more servicing issues than long legs did.  Nev

[Bruce Tuncks]

Nev, you have just shown that Mike Busch was correct in saying that hours are not a good indicator of engine health.

I was sufficiently worried about lead deposits that I have changed to 98 octane mogas too. It is too early but soon I will have a good look with an endoscope to see if the engine is cleaner. There used to be a coating ( about 1mm thick) of whitish lead looking stuff mixed up with carbon, and this coating flaked off and I guess went out the exhaust.

On re-reading the Limbach cooling report, it stated that getting the cooling improved was urgent for mogas operations. The implication was that a mogas engine ran hotter, but I have seen no difference.

Further, I don't see any reason why a mogas engine should run hotter. But those Limbach guys would know more than me I bet.

Kensla, is the new gen 4 on 98 doing well?

 

[Old Koreelah]

4 minutes ago, Bruce Tuncks said:

Nev, you have just shown that Mike Busch was correct in saying that hours are not a good indicator of engine health.

I was sufficiently worried about lead deposits that I have changed to 98 octane mogas too. It is too early but soon I will have a good look with an endoscope to see if the engine is cleaner. There used to be a coating ( about 1mm thick) of whitish lead looking stuff mixed up with carbon, and this coating flaked off and I guess went out the exhaust.

On re-reading the Limbach cooling report, it stated that getting the cooling improved was urgent for mogas operations. The implication was that a mogas engine ran hotter, but I have seen no difference.

Further, I don't see any reason why a mogas engine should run hotter. But those Limbach guys would know more than me I bet.

Kensla, is the new gen 4 on 98 doing well?

 

Bruce I too am joining the ranks of those disenchanted with AvGas, especially its lead deposits. I’ve been filling my left wing tank with ULP 98 and switching over after takeoff. Early days yet, but the few one-hour flights I’ve done have been uneventful, except for surprisingly low fuel burn: consistently 10 lph at 2800rpm/90kias when AvGas showed 10lph @ 2650/85kias.

 

After all the discussions on this and the Jab/CAMit forum, I’m also in the process of improving cooling with yet another redesign of my ram-air ducts.

I’ve widened them to encase all the barrels, not just the top 1/3. (I could never understand how those thin fins were supposed to cool hot steel cylinders with no air blast.)

I’ve also installed close-fitting sheet aluminium ducts around and under the cylinders to direct air thru the fins. Three short test flights have been disappointing- no major reduction in climb-out CHTs, perhaps because those hot steel barrels are finally getting some of the air.

 

Obviously their needs to be more “suction” under the engine, so it’s time to remove the biggest air leak of all; the plurry oil cooler, which dumps its air under the engine.

 

The oil cooler will soon have its own dedicated exit below the belly, with seasonably-adjustable door.

[facthunter]

This amount of engine fouling is not good. Those build ups can incandesce and cause pre ignition and engines running on.. Also the top ring land may be carboning up, which could cause ring breakage like in 2 strokes. Water Meth injection on take off may help clean it up. 100LL is a problem for the Industry. Originally a bromine compound cleared out the  left over lead. It caused the white exhaust you don't seem to see now.  Bromine is really toxic. Nev

[Kenlsa]

Bruce,

the Gen 4 that has been on 98 since new has now had its 100 service. Camera inspection shows no/zero/zilch residue on piston or chamber.  L2 took a photo  and the rest of the fleet is now on 98 by now afaik.

 

Note——-the new fuel filter had plenty of muck inside and some was metal (magnetic test) we are loading from drums at the moment until a bowser goes in.

 

Ken

[kgwilson]

I've run my Gen 3 on Mogas since new and it has stayed clean. I have only used Avgas a few times when I've been away as that's all I could get without going in to town and back. I had a few early problems that I thought may have been the fuel but one was airflow and the other was the balance tube came off between the carb & airbox. Simple fixes and it has run like a swiss watch ever since.

[RFguy]

thanks everyone for their input.  a single long reply : 

Nev - you are saying that's alot of deposits for the  370 hours  of 100LL , in your opinion ?

Nev- yeah I noticed I could see the seat rims. They have to of moved surely to be able to see that. I will heat it up and see what we get. 

 

My concern with MOGAS is a higher likelihood of detonation, with hot cyls upping the anti of everything.

Maybe when I get the temps right down. (but then buildup is more of a problem) .

 

There are certainly many hotspots in the system due to cooling issues  no doubt about it. 

 

OldK - remember wrapping the bores has to be tight otherwise the air wont go in the fins, and that will also push up  the pressure drop required up ALOT to get decent airflow - which might end up being a low speed cooling problem... and also you lose your radiation cooling with wrapping Ally. Spray with heat tolerant matt black paint on BOTH sides of the ally.. black anodizing also can be no good at long infra red. 

 

thanks Ken for the input, I have the stem tools. noted on the cutting of the valves (and nev your nitride note)

 

Bruce / Ken- what are you getting for leakdowns on your highest hour Gen 4 engine ? do theey have the new uprated pistons ?