Hot rear cylinder example Jabiru 3300A-2575 and plenums

[Bruce Tuncks]

Don't use a pressure system to transfer fuel from a drum to the plane. Some years ago, a guy wrote up this system he had been using where he put compressed air into the drum to force the fuel out. The editor unwisely published the article and then all hell broke loose. Apparently this is a fire/explosion hazard.

Ken's fuel pump is ok though. I might get one, on account of finding 20 litres quite heavy to lift just so all the time.

 

[Jabiru7252]

Apart from the drive to the airstrip, playing with fuel is probably one of the most dangerous things we can do. 

[RFguy]

I'm still young (50) , tall, strong enough to lift a 20 L jerry up over the wing and pour into a dual filter Mr Funnel (20 seconds fill) .

But would be happier with a drum and a manual hand pump.

Edited February 19, 2021 by RFguy

[RFguy]

Seems that about 400 hours on AVGAS LL100  is about the upper limit before the thing needs decaking.  

Need to explore what others are using for additives to slow /prevent buildup . Need to clean it, if it is possible before the black muck turns to hard black glass like it has under the rings in the grooves.  Beige lead based gunk in the cyl head and piston crown  , guides , and below the contact area of the seats may likely be removed with a gunk cleaner like MEK without disassembly.  (followed by oil drain, flush, and flood same area  with oil to replace the lube in attacked areas on start) 

 

suggest run fresh 98 on a clean engine, not one with  carbon flakey spots everywhere to glow red.  Perhaps fuel return loop to maximise flow and fuel cooling, and vapor lock  especially heatup around the mech pump. back to the header tank would be sufficient.    Maybe the premium 98 fuel like Vortex (which is different to their standard "premium 98") maybe the detergents are useful if fuel is fresh.

 

The coolest cylinders in my engine have the most buildup...MUST avoid idle ground running. MUST bring the engine up to good clyinder temp and pressure as soon as possible to reduce buildup. Suggest open engine up on long descents to get it hot. 

Lead Fouling | Preventing Lead Fouling in Aircraft | Shell Global

Edited February 19, 2021 by RFguy

[kgwilson]

I  think that the continued used of leaded fuel will cause deposits to form no matter what you do. The tolerances in Continental & Lycoming big bore engines appear to me to be considerably greater than those in Jabiru and Rotax engines.

 

When you shut down a Jabiru or Rotax the prop stops within 1 revolution or thereabouts. A Lycoming continues to wind down for a few RPM. I know the technique is different with no mixture control on Jab/Rotax to pull so it is just an ignition shutoff. Flywheel weight will also have an effect but the Lycoming/Continental just seem a bit more sloppy..

 

Perhaps engine tolerances are one of the reasons the Jab engine is more prone to lead oxide build up. 

[RFguy]

AFAIKT, its not where the tolerances are different- the sort of places that are causing problems.  - below valve seats, guides...

 

Most  likely related to higher temps and lean mixture usage in the Lyco... 

 

the rings are getting BLACK hard carbon- no beige buildup

 

like the crowns, chamber, guides, seat bottoms - heavy beige buildup. like having brickwork in your chamber.

 

 

[Yenn]

try turning the ignition off on a Lycoming to stop it. I reckon it will puff and blow for quite a few revolutions.

[Bruce Tuncks]

I remember as a kid that car engines needed decoking at 40,000 miles. That is thankfully a thing of the past, I wonder what changed...  was it the oil or the fuel or the engines?

My Jab has gone 700 hours just fine on avgas and although I'm not planning on doing a decoke, I will be looking in the pots soon with an endoscope.  

 

 

[Thruster88]

58 minutes ago, Bruce Tuncks said:

I remember as a kid that car engines needed decoking at 40,000 miles. That is thankfully a thing of the past, I wonder what changed...  was it the oil or the fuel or the engines?

My Jab has gone 700 hours just fine on avgas and although I'm not planning on doing a decoke, I will be looking in the pots soon with an endoscope.  

 

 

Looking in the cylinder with a bore scope has limited value in my opinion.  A differential compression or leak down test will tell you exactly what is happening with the cylinder. The differential tester is $43 on ebay, do the test often record the results. The trend is you friend. 

[RFguy]

I agree. Trend.

Some endoscopes are better than others, but the proof in the pudding in my opinion,  leak down, and the NATURE of the leak down , along with an oil sample analysis ($30)  tells all.  The nature is important.... golly it can tell you alot. 

 

I am probably telling chooks how to lay eggs, please excuse me - Bruce is your engine able to hold the leak down compression 15 deg either side of TDC at well as at TDC  ? 

 

Mine could not hold leakdown  (40s !) away from TDC on 3/6 pots (ring #2  stuck). But held leakdown at TDC ( ~65 valves leaking)

Wasnt so bad at TDC. and at TDC had  leaks  on  exhaust  valves due to build up (valves and seats  are fine) - buildup preventing closure.

 

Rotating the valves slightly in situ didnt change the nature of the valve leaks. 

 

at tdc, piston crowns   are up against the buildup and carbon around the top of the bore/head interface. , I think sealing !

 

 

 

 

Edited February 20, 2021 by RFguy

[Bruce Tuncks]

Wow good point rfguy. Yes I know that you should do leakdowns at more than one spot. But I have been lazy so I just have done the one reading.

On the deposits stuff. ... my book ( Skyranch ) says to get a lab test of the deposits stuff.  I have asked about oil analysis on Jabiru engines and have been told that there is not enough database for this. Skyranch has 50 years of experience with Lycomings and Continentals, they have been around a long time huh.

And thruster, I think you are right. My endoscope was pretty cheap but it will go down a spark plug hole so I am going to try it. I am hoping to see a cleaned up engine from the mogas running. In the meantime, the leakdowns, done more carefully, will be the main thing for sure. Mike Busch implies that leakage past the rings is not real bad , but leakage past the valves is bad. Only a leakdown can help you pinpoint this.

 

[RFguy]

While I respect Mike Busches writings, I have his books...  "Mike Busch implies that leakage past the rings is not real bad ,  I disagree - STRENUOUSLY !  in my case- leakdown blowby (past rings) was an indication of  very serious situation of stuck rings 3/6 pots...... jammed stuck. they were never going to unstick themselves.... they would just accumulate more gunk back up behind them  because they couldnt float. ...they were jammed flush on the piston face half a circumference.

 

just measured up my ring gaps in my bores. 0.45mm +/- 0.03 mm top and bottom . Maybe a teeny bit more at the top ?
Spec is "0.40 - 1.20". geeez that is easy to meet.  Bore diameters  are also within spec. 

 

[RFguy]

rightio . bores, pistons ready to go back on.  this was easy, compared to loading the 100+ position surface mount robot correctly... yes I got the circlips in the right way around, the piston thrust markers to the front, the rings the right way up and not gap aligned, preloaded with oil, mating faces clean, flat  etc etc

 

Cleaned and assembled under  3500  lux to 6500 lux (heaps of light)  and 2x magnification on the head at all times   with 10x  binocular scope for looking at interesting features.

 

[facthunter]

Most jammed rings will break. Blow by is a leakage you don't need and it will drive oil back down from the comp rings and you get even more blowby as a bit of oil helps them seal. Scuffing marks on the rings are common when the thing is not happy. Local blow by can distort the rings. That gas blasting past is at a very high temperature. Nev

[RFguy]

Jammed rings were #2 rings. #1 rings all good except for glass hard carbon behind them.

 

fortunately , the bores are in excellent condition. the rear hot  pair 5,6 have  a couple slight discolouration the size of a thumb print on them that actually looks like it *may* have been very very slight corrosion  region- I cannot explain the marks in that mid bore region.     No damage to surface though, under 450x optical.  the front ones 2,4 had the real jammed stuck rings . near zero polishing down near the skirts, again slightly more pronounced at the hot cyls 5,6.

gudgeon pin 'bearing' aluminium  surfaces in the piston are all worn slightly more on the two  thrust sides, and they're all identical. No play, all roughly equal rotation force so  gudgeon pin fit is good. .  a layer of carbon/burned il or something in those areas of the guddgeon pin nonn thrust area (non worn area ) that got removed...  Gpin circlip grooves are not terribly well machined but typical I am informed.  

 

Edited February 25, 2021 by RFguy

[facthunter]

All circlip grooves must be the right diameter for the clip. IF the circlip moves it may eventually deform the groove and fall out. After installation check how easily they will rotate. If you know what you are doing you can expand them a little. Nev

[RFguy]

agreed.

The circlips dont move .  not at all, that are there in decent. well and truly not gonna move.  just the machining of the groove width and step could have been a bit better. it is manufacturing, not wear. The diameters are all good. the width of the groove varies a little. 

 

Edited February 25, 2021 by RFguy

[Yenn]

I have been told that adding 0.2% of mineral 2 stroke oil to the fuel is a good move for any engine. This has been worked out by people trying all sorts of additives. Supposedly used by Mc Laren in racing engines. It must be mineral oil.

The bore wears very little at top and bottom dead centre, due to lower piston speed. From my experience the greates bore wear is about quarter way down the stroke, speeds are higher and loads are higher there.

[facthunter]

The maximum bore wear is at the top near where the top ring stops. That's where the most pressure is and some of the metal is scoured away by hot gases. In a two stroke it's around the exhaust port. Nev

[RFguy]

there was quite a bit of marking and evidence of gas and ? leakage under the bottom of the head adjacent to that lowest head bolt- cylinder bore interface, the one that is under the grub screw inside the rocker housing.... 
( a few cylinders) 

--the usual hot spot and in previous gen jab engines, an area of deformation/impression by the cylinder into the head.  

Edited February 26, 2021 by RFguy

[facthunter]

Sometimes they are not retensioned correctly. The HOT ones tend to thread bind  and appear tighter than they are. Nev

[RFguy]

I dont doubt it Nev.  I'll have to look at the spec for number of cycles of those bolts, IE unload , clean, retension cycles

In other discussions in other forums we have come to the conclusion the correct way to tension thru bolts is to use lots of lube, not dry nor un prepared (like Jab manual) - IE do as manufacturer of the thru bolts and nuts suggests !!!.

 

Actual measurements show  that when not specifically lubed, the tension and stretch on the thru bolts varies up to 50% !!! leading to insufficient pre-load on the bolts and contributes tp bolt + joint failure.  Lube threads and lube friction faces on wahsers/nuts interface....

[cosmicray]

In other discussions in other forums we have come to the conclusion the correct way to tension thru bolts is to use lots of lube, not dry nor un prepared (like Jab manual) - IE do as manufacturer of the thru bolts and nuts suggests !!!.

 

I am about to replace the thru bolts on my 3300 and was intending to do it "dry" as Jabiru recommend. Can you provide a link to the alternative opinion using lube? Would like to get this right the first time!

 

[RFguy]

yeah. there's a few topics closely related- likely measuring stretch versus torque from Ta to Tb, etc , for some months, however to skip to the meat of it .... and there are numerous spreadsheets.   The manufacturer of the bolts and nuts suggests a fancy low friction lube.

I will be putting my pots on next week and there will be nice clean threads and W100 everywhere....

 

file to read here : (read all of them) 

[email protected] | Files

 

(you should read all the correspondance as it ebbed and flowed to get the full context.  there is a bit on resonances, and also the non flat washer surface  causing shear on some cylinders) ... 

Read : "through bolts" from ABOUT 14 fEB 2021 to present. 

(a message from the topic to get you started)

https://jabcamit.groups.io/g/main/message/14132

in particular 

particularly on 17 Feb,  19 Feb
and
(a message from THAT topic) 

[email protected] | Through bolt torque

Read : "Through Bolt torque " from about 28 Jan 2021 to present

 

 

 

[onetrack]

I've found Never-Seez pure nickel anti-seize is best for threads that reach high temperatures. Not only to even out tension from torquing, but to prevent thread galling when removing - particularly when there's steel threads in alloy housings. But if you utilise any lubricant on threads prior to tensioning, it is recommended that you reduce the torque settings by 10%. Never-Seez contains oil as well as powdered nickel.

 

The important thing with high tensile fasteners is to ensure you are loading the fastener to the desired tension. Too much tension stretches the fastener - too little, and it will come loose.

A stretched fastener can be easily found by miking the shank, or just holding the shank up to the light, whilst gripping it lengthwise with caliper jaws.

 

If there's more than about 5 thou (0.13mm) difference in thickness along the shank, or if you can see daylight clearly between the shank and the caliper jaws, the fastener is stretched, and it should be discarded.

 

Edited February 27, 2021 by onetrack