Light Aircraft down Near Bundaberg

[facthunter]

Mahle (originally German) have been made in China for years. ACL made most of the OEM pistons in Australia. The said Holden (V6) piston was designed for a pressed in the conrod assembly for the Gudgeon pin and the bosses are therefore too short for the use of circlips which to compound the problem they changed the circlip type from round section to square (seeger) where they then had some problem with dimensional accuracy and lack of spring causing the clip to wear out the side of the piston in use when it came loose in the groove.. You couldn't write this stuff. Just setting up for a problem. The choice of a +040" size was to prevent owners overboring the cylinders, as it's the maximum size already. With all the overheating issues the temp of the piston has only been assumed to be OK. The design of that piston is not optimum for heat transfer in an aircooled motor. Nev

 

 

[jetjr]

CAE kept with round circlip, which never came out but were a difficult to fit in situ.

 

Not only was there square circlip quality problems, they bent, but they were/are punched so have round and square edges, wrong way round can come out.

 

There are several people looking for replacement pistons if someone knows of source and skills to machine in round pin groove

 

 

[facthunter]

Round is best. Have a chamfer on the end of each pin, and no bends on the end of the clip. Needs a hole drilled in the piston located so the end of the clip can be removed with circlip pliers getting under it. You only have to install one end I when not on the bench. Have a slotted plasticised cloth to prevent any circlip entering the motor if it flicks out of place during assembly. The small hole aids slipping the clip in place. put it in the 9 or 3 o'clock position.

 

To machine the groove use an arbor with a slight taper set up accurately in a 4 jaw chuck and have your boring bar with the correct profile for the clip wire diameter and plunge it outwards half the diameter plus a few thou.. Get that figure right. The chamfer makes the pin push the clip in further if it contacts it with any real force and must be concentric with the OD of the pin to work effectively. If you want to be assured it's seated make a bar of the right size that just goes in past the clip when it's fully home to test it . It will be original dia of the pin less wire diameter. Nev

 

 

[coinz]

CAE kept with round circlip, which never came out but were a difficult to fit in situ.Not only was there square circlip quality problems, they bent, but they were/are punched so have round and square edges, wrong way round can come out.

There are several people looking for replacement pistons if someone knows of source and skills to machine in round pin groove

Having pistons made up these days is simplicity itself,i've done it for motorcycle pistons.Usually a minimum of 4 are supplied.And you can go back in 6 months and have that piston exactly replicated,just quote the build number and they punch that into there computer and that's that,identical pistons to the originals.Is there any reason owners shouldn't be fitting forged pistons in these 4 cylinder motors,and are the 6 's susceptible also. Also +040" pistons suggest heavy piston,not ideal.

cheers

 

colin

 

 

[jetjr]

Older type, appeared cast, were 430g

 

Newer all machined are 412g

 

Jabiru are lighter with significant crown material removed and have left and right versions

 

They also fit them reverse to many's thinking but thats a debate already well worn

 

Who?

 

Any LSA aircraft or Jabiru engines who need to operate without CASA restrictions MUST fit Jabiru supplied parts, whatever they are selling at the time.

 

 

[onetrack]

... and apparantly there has been issues with machining out of round ...

Are you sure someone didn't find the piston was oval, and then complained about it, thinking it was faulty? There is such a thing as "cam-ground" pistons.

Cam ground piston definition | Engineering Dictionary

 

 

[bexrbetter]

Having pistons made up these days is simplicity itself,i've done it for motorcycle pistons.

Any contact info please? I can't get any sense out of Mahle in China.

 

any reason owners shouldn't be fitting forged pistons in these 4 cylinder motors,

74mm stroke at 3000 rpm offers piston speed/forces at 1500 feet per minute that are not high enough to worry about.

 

... and 20hp per liter, I can't believe the Jab has any issues at all with those easy stresses.

 

 

[jetjr]

Circlip grooves made out of round, then repaired and installed

 

Not out of round pistons...

 

No doubt correct about low stresses on piston but it seems still too mich for some reason. Could of course be a QA issue.

 

 

[coinz]

Bex,i went through Ross racing pistons in the US,others had been using them and they were recommended.They phoned me back and discussed the drawings i had sent them,so i ordered a set.The machine work ,has to be seen,lightening done on inside crown too.

 

I have to agree these engines appear to be unstressed,2 weeks ago i stood not 20 meters from a jabiru leaving the ground on takeoff,(Palmira) delightfully quite engine,so unstressed.Pistons shouldn't be breaking up in this day and age,i take workers out to mines,gensets (Perkins) run 50,000 hours + with no more than oil changes,because they're designed correctly.

 

cheers

 

colin

 

 

[facthunter]

With the power density that the jab has it doesn't need forged pistons. Forged is often heavier and often has a higher expansion rate. The usual way to get lower expansion coefficients is to include silicon in the alloy mix. Piston loads are related to piston speed and conrod length V/s stroke (angularity side loads) and to a lesser extent to pin offset which tilts the piston but can tend to equalise side loads from power stroke phase to the compression stroke phase..

 

My concern with the Holden piston is the gudgeon boss lack of length and the poor heat conduction through from the crown to the skirt and lack of skirt contact area with the bore surface. In the steel cylinders I also have concerns about the 3 piece oil rings, which I think cause a lot of the scratching of the steel bores which when honed too finely don't hold oil. Nev

 

 

[onetrack]

Heat is the destroyer of pistons, not piston speed - along with detonation, of course, which is highly undesirable, very destructive, and which must be avoided at all costs.

 

A forged piston has a condensed and aligned grain structure in the metal, as compared to a cast piston, which has no grain structure - and in many cases, cast pistons also have casting flaws that are not detected.

 

A forged piston has consistent material properties, whereas a cast piston has inconsistencies in material properties.

 

Cast pistons are used in mass production because they're cheap and occasional failures are unimportant - in fact, occasional failures are good for companies, they sell parts and make more money from the parts, than they do from the new manufactured item.

 

A forged piston has greater ductility and strength over cast - when a forged piston breaks, it breaks into a few large pieces - whereas a cast piston will fracture into many small pieces, and effectively totally destroy itself.

 

As you can imagine, a failed piston that still stays partly intact and still supports the little end of the conrod to a large degree, is far better than a totally destroyed piston which then allows the little end of the conrod to flail and cause catastrophic engine destruction.

 

High silicon content in the aluminium is important, the silicon adds substantially to tensile strength, reduces heat expansion, and improves wear resistance.

 

The best forged pistons are manufactured using the isothermal forging process, whereby the die is heated to the optimum temperature for forging - 370-430 degrees C - which ensures the piston metal stays at that temperature, as it is forged, resulting in optimum metal flow characteristics and grain structure.

 

 

[facthunter]

The alloy used in the making of forged pistons has to be more plastic at the right temperature to suit the process. This often means the thermal expansion will be more so running clearances are larger .The Forging Dies are more compromed in detail than cast pistons can be. You just can't forge things as detailed as you can cast them nor can they be CNC'd to such detail. Vaccuum die cast pistons are very consistent in quality and dimension. The best aircooled aircraft heads are forged and then finished by precision machining. It's the ultimate for that application. Most are castings. High performance diesels and F1 car engines need exotic high strength materials in things like pistons, but the low power for capacity (displacement) engines do not. The right design will produce a very adequate product that is cast. A lot of piston problems relate to having cast in steel struts to control expansion. These are never used in high performance applications by anyone who knows what they are doing. A "W" slot piston is a low performance item. Never used in racing. Nev

 

 

[onetrack]

And then we also have the greatest advance in engine technology in recent decades - ceramic coated pistons.

 

Ceramic is a refractory material, it doesn't absorb heat, it insulates, and it stops the transfer of heat to the piston, resulting in considerably less piston failures.

 

Piston Domes | Swain Tech Coatings | Industrial Coatings | High Performance Racing Coatings

 

 

[Yenn]

Wasn't the problem with circlips due to Jab not stipulating that they had to go in with the gap in the circlip being either top or bottom of the piston. With the gap at right angles to direction of travel, inertia closes the gap allowing the circlip to come out.

 

With ceramic coated pistons, where does the heat go if it is insulated from the aluminium piston. Some engines spray oil into the piston to transfer heat to the oil and therefore to the oil cooler.

 

 

[facthunter]

IF it's loose enough to move at all it will wear the area around the groove out in a short while and escape. The piston speeds are only about 30 MPH and the piston starts and stops relatively gently. The location of the opening should not matter if it's got enough spring tension. The problem was the spring tension above all although as it's punched out of a sheet it's more flat oneside and that should be to the outside. If the groove is too deep it won't have the right tension or the material could be too soft an deform when being installed. IF a square section clip is installed in a 1/2 round groove it isn't satisfactory either.

 

Oil cooling to the underside of pistons is common on aircraft engines that are supercharged, but other beefing up is done as well even for relatively small boost figures. Nev

 

 

[jetjr]

Dont think anyone really knows what caused the circlip problems but a series of pretty poor decisions and QA on suppliers was the root problem Id guess

 

A long series of explanations and excuses were floated at the time

 

CAE added under piston cooling jets

 

Interesting the engine in the original post is actually a Jabiru fitted with CAE heads, I believe the heads are still intact as are the valves so no fault can be levelled there.

 

It will be interesting to see how badly damaged is the case.

 

 

[facthunter]

I do a lot of piston grooving for circlips. The quality of the clips are pretty suss sometimes, and they are hard to get. (The wire Ones). Half the time I use end pads of brass or aluminium. They have to be designed carefully too. Whatever you do it must be done properly. Nev

 

 

[johnm]

We are drifting Fh ........?. But im being nosy ....... why are you a groover (im 60) ........... of pistons ?

 

 

[facthunter]

Because most auto pistons have no grooves and I make some of them (less now) which have to have grooves (sometimes). Most people don't know how to install a circlip without distorting it. ALL the Rotax 2 stroke ones I've seen as replacements are too soft. Not good.. I've described in detail how to do grooving back a little way. Nev

 

 

[dmech]

Any contact info please? I can't get any sense out of Mahle in China.

 

 

74mm stroke at 3000 rpm offers piston speed/forces at 1500 feet per minute that are not high enough to worry about.

 

... and 20hp per liter, I can't believe the Jab has any issues at all with those easy stresses.

mahle .040 forged available from precision international in aust all states nice piston, can be circliped easily no off set pin

 

 

[bexrbetter]

mahle .040 forged available from precision international in aust all states nice piston, can be circliped easily no off set pin

Thank you but I was referring to custom pistons.

 

 

[onetrack]

Bex, the only people I know making custom pistons, is JP Pistons in S.A. They specialise in obsolete pistons for vintage, veteran, and classic cars, as well as motorcycles, trucks and stationary engines.

 

They can also manufacture custom pistons to a clients request.

 

JP Pistons Home Page

 

 

[Downunder]

Bex,i went through Ross racing pistons in the US,others had been using them and they were recommended.They phoned me back and discussed the drawings i had sent them,so i ordered a set.The machine work ,has to be seen,lightening done on inside crown too.

[jetjr]

mahle .040 forged available from precision international in aust all states nice piston, can be circliped easily no off set pin

Great info

 

Need more knowledge on no offset pin?

 

Doesnt this get into the whole debate on which way to orient piston?

 

Wouldnt know a part number by any chance?

 

 

[facthunter]

There can't be an absolute answer to that, as there's now too many variables, but since a factory rebuild was always good value compared with doing it yourself, many went down that path. That was used against the engine as people read into the situation the engine previous engine must have failed.... Anyhow I digress..

 

If something looks to be of looming concern with your motor now or soon ,start getting prices, as to what your present engine is worth. Don't expect much.. Engines will be rebuilt using used parts perhaps for a while, but I wouldn't do it personally. It's not likely to make economic sense, with a relatively low priced new replacement,( If it's any good), available. Check it out with owners. The longer the time these motors fly, the more the experience of them gets, the better based the choice becomes for you. Nev