Hot rear cylinder example Jabiru 3300A-2575 and plenums

[RFguy]

In the Brumby 912ULS)  in hot weather, I make the most of the cool engine  (70C) and climb has hard as I can until the yellow mark is hit  (110 oil) . then its back to cruise climb or step climb. There is plenty of engine and coolant to warm up, and that takes time, so you might as well use it to get intot he cool air fast.

 

The Rotax takes a while to warm up  from Taxi- IE there is quite alot of  thermal storage reserve to use up.

 

Hence makes  best progress into the cooler air if I get up there as quick as I can.

Whether the engine is 100% or 75%  the time it takes to heat from  takeoff  to 7  minutes in is about the same...

 

The Jab being air cooled and  low weight doesnt have the reserve heat capacity and the cruise climb is started quickly.

 

[Yenn]

In the photos above showing the area where the hot air exits the cowl, it may look good and it may also work, but what do you think the passing air is doing behind the lip on the lower cowl. I reckon there would be very turbulent air there. It could probably work better with a smaller exhaust area and better shaping to supply smooth airflow.

The whole aim is to get the air into the cowl, duct it around so that it goes between the fins on the head and cylinders and then get it out with the least drag.

[RFguy]

this might help explain things...
 

ex1.pdf inlet1.pdf

[RFguy]

or in JPGs

 

[RFguy]

back from 

 

 

Onetrack, thanks for the thorough explanation.

"so that it allows just the right amount of oil through, between stem and guide, to allow lubrication"

wow, we are talking super super tightly kept clearances here. 

 

I have a good vaccum pump , maybe I should check sealing with a jig after some lapping.  they're out now , I wll need ot take a good look at em after  

[facthunter]

There's no need for anything fancy. Blue the seats and bounce the valve off it as you turn it  slightly and visually inspect  OR put a little  CRC on the seated valve and wave an air hose near the appropriate port and look for bubbles. IF you have good valve facing  and seat cutting gear, there's no need to lap the valve in.

   I seem to have lost a post on guides and materials. Some bronzes can be quite tough but they conduct heat away better so are often used in aluminium heads but tend to come loose more frequently. Chrome works very well on cast iron for  compatibility. I would NOT use any "STAINLESS"  valve unless nitrided,  in any sort of aeroplane, period. Nev

Edited January 31, 2021 by facthunter
clarity

[RFguy]

Good advice, thanks Nev. I reckon the CRC  and waving the air hose into a port is a good one. 

 

I really never had to do too much on heads in the Datsun days (so I am learning a bit here) .... never had guide or valve problems. mostly conrod  and crankshaft issues. 

 

[onetrack]

Glen, don't forget that valve guides can get worn oval, and measuring them for ovality, as well as for high spots and low spots, is important to ensure you end up with guides that are within specification.

[RFguy]

yeah , I need a shaft gauge that can measure that. the valves certianly dont wobble...

 

under the binocular microscope  the ex valves have a few high spots on them , which are easily pushed off 

oven cleaner will do wonders.

 

Not much on the seats of the heads pretty well spanking clean.  got to find some MEK without the extra gel stuff them put in it.

LOTS of buildup around the seats and exhaust port . behind the seats  in the head there is about 1mm of buildup ! goly golly.

 

this lead and crap buildup is a serious problem. 

 

 

 

[facthunter]

You can't really leave it there either.   I've seen "big inch" aero barrels looking a lot worse. They only look good when you  first take them out of the box they come in. Nev

[RFguy]

might even end up with some improved gas velocity with the gunk cleared from the ports.

Is soaking in detergent overnight going to assist decaking the heads and piston crowns with MEK ? 

 

 

[facthunter]

Your solvent will tend to evaporate on it's own. Mix it with some used oil and brush it on.  Repeat a few times. Nev

[RFguy]

OK. yeah vapour pressure is 9.5kPa (20C).  evap pretty quickly. 

[onetrack]

Soaking in plain cold water is one the best and simplest ways to soften carbon deposits.

 

Then there's "Redex", too. Brush it on and leave it for a day - then the deposits will come away easily with a brush and some petrol, just like magic.

[jackc]

15 minutes ago, onetrack said:

Soaking in plain cold water is one the best and simplest ways to soften carbon deposits.

 

Then there's "Redex", too. Brush it on and leave it for a day - then the deposits will come away easily with a brush and some petrol, just like magic.

Redex......there is a name from a bygone era 🙂. Firezone from Golden Fleece, too!

[onetrack]

Redex is still available - but not from the Redex Company, which was bought out long ago. It's now owned by Holts Lloyd and they produce a range of Redex fuel treatments.

I still have some of the original Redex, in an original Redex gallon tin!

 

Golden Fleece products were exceptionally good. Their oils were top-notch, I used GF oils and greases and fuels in bulk for many years, in the '60's, 70's and early '80's

[RFguy]

Pulled of cylinders and pistons yesterday
No scuff, no marks, great looking pistons and bores. (no hone pattern visible 370h, AVGAS)

Found THREE STUCK #2 rings. absolutely jammed in . top ring always OK.
And they were the three cylinders that had high blow by. That wouldnt have resolved itself.

The poor leakdowns on the other cylinders was due to buildup on the stems stopping the valves closing fully. (plus stem gunk) Valve faces were all good. no damage.

This rich AVGAS environment is a bit of a pain. .

glen

[facthunter]

The original hone pattern is very fine and won't last long where the rings travel. Nev

[RFguy]

So far, its  big cleaning job . 1 to 2 mm of  beige buildup on everything. I have been trying different chemicals, cleaning methods.

 

Surprisingly, the pistons in the pressure cooker for a few hours with a little surfectant  was the most effective.

 

Xylene. nup. Acetone. nupe. ultrasonic cleaner at 65C. nup.

 

I await the arrival of my MEK !.    have cleaned one head but was a fiddly job.

 

Valves are done in caustic soda.

 

cylinders top and bottom soaked in Xylene for a few hours, then the carbonized gunk  rim at the top, and the hardened gunk around the o ring fit at the bottom is scrapped clean with a polycarbonate knife. 

 

ring gaps cleaned out with the end of a rag back and forth with kero then acetone.. 

 

[Bruce Tuncks]

Yep, I have always had that beige build-up, what with 18 years of avgas running. It has flaked off and so not continued to increase , and it has not badly effected the leakdown results.

But right now, I am trying an experiment running on mogas. So far, I can detect no difference in the way the engine runs, but I am hoping it will clean itself up inside.

Thanks Glen for the info about how different cleaners worked. What do you think about just hoping that mogas will do the job?

And thanks Ken for the info about that gen 4 engine on mogas at Gawler.

 

[RFguy]

Hi Bruce

I would like to start with a clean engine before MOGAS. then, at least when its not too hot, I will run mogas when available. 

 

The beige building is caked on a high temps and pressures. So I figure the pressure cooker had a chance . Needs a few hours. 

 

The heads can be done with careful soft wire rotary brush etc, watching out for the seats etc, progressively moving to softer brushes as one gets closer to the metal. Inside of the head in an doutlets seems best tone with a dremel and soft brass brush. 

 

*There is an area just under the exhuast seats that is an issue, beige buildup there has causes the valves not to go all the way closed. The seats are perfect on head and valves...*

 

(remember 3 #2 rings out of 6 pits were jammed) . maybe this thing is running a little too rich.  I read good things about "the Mystery OIl "additive. 

 

The MEK appartenly is to fix all. Just soak the lower side of the head in a bath of it overnight and stick then in a bath of bubbling hot water and it just falls off I am told. We shall see. Been trying to avoid the MEK...

 

Edited February 13, 2021 by RFguy

[facthunter]

There  are "Proprietary" cold bath dissolving solutions that don't harm aluminium. They probably don't like selling them to just anyone.. Get an order from an engine shop. Most ring carboning is related to the top one(hottest) Your ring grooves may be a tad tight when made or might be from some deposit. When the piston cools is may trap the ring as the Alu piston expands /contracts much more than the ring material. Sometimes the piston material may drag and trap the ring in the groove. A small chamfer of the edges will prevent that. Nev

[RFguy]

Thanks Nev. And they were really, really jammed hard. - half the ring from the middle around the one end. the other end was free to move.

So far, have been cleaning out the ring grooves with the end of a cloth. IS there some sort of emery cloth cord ? bruxhes dont effectively get in there. ..although gunk in the corners is probably not so much the problem as just buildup top and bottom so it can grab.

I will measure and check the clearances. You may be right that these were a tad undersize. I got the rings out by heating up the piston  to about 100 C in some degreaser and it pulled clear. perhaps if theyu got really hot in service, they would un grap. But there was no clearance at 100 deg C at all, firrmly stuck.

am a bit dissapointed with myself that I didnt  remember the orientation in the cylinder of the jammed ring -ie like bottom or top side  etc. 

 

Edited February 14, 2021 by RFguy

[facthunter]

 I usually break a  used ring and use it to clear the carbon as it fits the groove perfectly. It's pretty easy to damage the ring lands and you have to clear them right to the bottom for good practice as the  groove should only be just deep enough to allow the outer surface of the ring to be just below flush. The "Cold Bath " solutions should soften this carbon. It's usually much worse in 2 strokes and they are "Pinned' as well to locate the ring gaps.  Some comp rings have a top and bottom and must go in the right way . Stagger the gaps on the comp. rings.  Nev

[onetrack]

If you can find a good engine reconditioner (or performance modification shop), they usually have a hot caustic bath machine that cleans components to like they just came from the factory.

They probably use those proprietary cleaners that Nev mentions. "Robowash" is one of the machine builders.

My local performance modification shop still does a fair bit of engine reconditioning, although a lot of his work is vintage car engines and race engines.

But he'll hot bath a block for about $60 and a head for about $45 - and the stuff he uses leaves a chemical on the surface, when it comes out of the bath, that prevents flash rusting or corrosion/oxidation.

 

 

Edited February 14, 2021 by onetrack