2nd piston ring always stuck- what's the story ?

[RFguy]

mmm. These forged  Mahles go into L27 Buick engines.... (3.8 litre commodore from mid 90s ) 
I cant find any engine manuals for the engines. IE bore size, limits, any shape etc 

Just trying to understand these piston's normal home.
Can anyone assist or help me find a suitable forum or knows who to ask ?

thanks - Glen

[turboplanner]

2 minutes ago, RFguy said:

mmm. These forged  Mahles go into L27 Buick engines.... (3.8 litre commodore from mid 90s ) 
I cant find any engine manuals for the engines. IE bore size, limits, any shape etc 

Just trying to understand these piston's normal home.
Can anyone assist or help me find a suitable forum or knows who to ask ?

thanks - Glen

I had the same problem as you; the good 'ol boys making youtubes and comments on forums didn't go into this detail. Where you'll usually find it is in the Manufacturer's Workshop Manual in the Engine, Specification section,  but I didn't find one on the Jabiru site, probably because they don't have the funds that hundreds of thousands of engines allow. I was following the Commodore VN 3.8 litre V6 trail from its 3.8 litre Buick origin BUT:

Holden would have had an Australian company make their own pistons, so the VN Workshop Manual is likely to have the diameters and clearance settings for:

• Standard Pistons
• Oversizes available for Rebores

However, someone said Jabiru pistons were not from the 3.8 V6/not from Holden etc which makes those figures a bit shaky (or that person may just have it wrong).

 

So maybe you'll have to start with contacting Jabiru to find out if someone knows the settings and clearances or the origin of the pistons, and you can ask the piston manufacturer for the settings.

 

 

[Blueadventures]

14 minutes ago, turboplanner said:

I had the same problem as you; the good 'ol boys making youtubes and comments on forums didn't go into this detail. Where you'll usually find it is in the Manufacturer's Workshop Manual in the Engine, Specification section,  but I didn't find one on the Jabiru site, probably because they don't have the funds that hundreds of thousands of engines allow. I was following the Commodore VN 3.8 litre V6 trail from its 3.8 litre Buick origin BUT:

Holden would have had an Australian company make their own pistons, so the VN Workshop Manual is likely to have the diameters and clearance settings for:

• Standard Pistons
• Oversizes available for Rebores

However, someone said Jabiru pistons were not from the 3.8 V6/not from Holden etc which makes those figures a bit shaky (or that person may just have it wrong).

 

So maybe you'll have to start with contacting Jabiru to find out if someone knows the settings and clearances or the origin of the pistons, and you can ask the piston manufacturer for the settings.

 

 

Or maybe Ian Bent will have / share some detail?

[RFguy]

Hi Turbs

all the data is public on the Jabiru website- you just have to know where it is !

They don't specify any taper (though I suspect it would be useful give there is a hot and a cold end.)

 

The Jabiru pistons were once upon a time just 40 thou oversize  L27 engine pistons, but since, there has been some divergence in supply.

These Mahles go in the racing segment, so they'll likely to have a different bore recipe. anyway, I have asked the Australian supplier. AFAIKT they have round wire clips. 

yeah, Ian Bent might know.,

PS- manufacturer recommends no taper on the bores. But the water cooled cylinders dont get anywhere near as hot.  (but the inside of the cylinder walls - they do not  stay at water temp, despite what people think) . currently communicating with some fellows on the commodore performance  site.

 

 

 

 

 

 

 

Edited January 18, 2023 by RFguy

[facthunter]

The original Jab?Holden pistons were Max oversize .040" so you couldn't buy bigger and bore the cyls oversize. Mahle are Chinese and have been for a long time Originally from Germany.  Most Australian built engines used the one supplier which was as good a standard  as most others for the CARS they fitted. I knew the bloke who ran that department well and got a lot of info from him over the Years about "W" slot pistons. He's not with us anymore.  You would never run them in a race car.  Nev

Edited January 18, 2023 by facthunter

[RFguy]

Mahle piston has a press fit pin, will need machining  grooves for pin retention. 

I also asked Jab today about their current replacement piston (cast/forged, alloy type, any failures ). has no split skirt.  

 

[RFguy]

Supplier (Precision Intl)  is most obliging with photos. These are std saloon car series homoglated racing parts...

 

Edited January 18, 2023 by RFguy

[facthunter]

That's  forged solid skirt  teflon coated. It's close to  what  you d want in a jabiru.  Better to be flat top (Less heat).  Nev

[RFguy]

nice looken eh ?
need piston retention pin clip grooves machined in.

I am told people take a 0.5mm off the top rim  to tweak compression down / increase clearance very slightly.  Not sure I d want to touch the very important end of the piston. .
Nev, evidently, the overbore in the  GMH engine is 97.52 my bores are 97.6

Edited January 18, 2023 by RFguy

[facthunter]

You might end up with the top ring too close to the top surface.  Slipper pistons might need a oil jet for supplimentary cooling.  Nev

[RFguy]

slipper.... depends what it is made of. most graphines are smooth and have high thermal conductivity. but. 

I always wondered if the sump could be modified to have oil jets spraying  up the guts, it can a pretty good view of things.

[Thruster88]

Stuck valve relief? 

[RFguy]

eh?. 

[turboplanner]

5 hours ago, RFguy said:

Hi Turbs

all the data is public on the Jabiru website- you just have to know where it is !

They don't specify any taper (though I suspect it would be useful give there is a hot and a cold end.)

 

The Jabiru pistons were once upon a time just 40 thou oversize  L27 engine pistons, but since, there has been some divergence in supply.

These Mahles go in the racing segment, so they'll likely to have a different bore recipe. anyway, I have asked the Australian supplier. AFAIKT they have round wire clips. 

yeah, Ian Bent might know.,

PS- manufacturer recommends no taper on the bores. But the water cooled cylinders dont get anywhere near as hot.  (but the inside of the cylinder walls - they do not  stay at water temp, despite what people think) . currently communicating with some fellows on the commodore performance  site.

Just following on from what we saw before and the comparison of the old and new rings, the next step is to sit the new ring on top of one of the bores and gently squeeze it in then push it down the barrel with a piston head to the compression chanber area. After sitting the ring at 90 degrees to the bore with the piston head check the ring gap against the correct piston specification to make sure there is enough gap to prevent binding, then measure the ID of the ring while it's in place against the bore against the ID specification of the correct piston to ensure there is a gap between the ring ID and the piston groove base. This space is where gas is expanded to push the ring against the bore. Then pull the ring out and holding the ring between finger and thumb check the full circle of each group to make sure there is no binding in the groove. There may also be specifications for groove height and ring height. I think we previously we discussed the bore and piston dimensions, so now they are ready for assembly. I just want tp stress again that I'm talking about the correct Jabiru piston for your Jabiru engine and the correct rings for the same engine.

 

What will go in various watercooled cars are usually what you can dig up from underneath one of the benches, find some rings that look right to the eye, scrape the crap off and stick them down the bores and the car will work for years, but that's an intermittant power application; not here which is constant power.

 

In looking at the old piston colours and worn+weak rings at 320 hours, we could think about what caused that because plenty of Jabs make 1,000 hrs. That's why it's important to check the main jets (idle jet and slide needle operate below max power). And it's also worth thinking about  what started this. I don't think you've mentioned burnt exhaust valves, so it may not have been mixture. It is possible that there was an overheat on a 35+ deg. day with a WOT takeoff and climb long enough to weaken the rings, or it may have occurred over time.

 

I would concentrate of Jabiru pistons in the 3 gen Jab engine, if that's what the piston has come out of. Looking at other opinions on other engines is like getting opinions from Fottbal, tennis, golf, squash players; they're all ball games, but you want the 3 gen Jab game.

 

The skirts on racing engines are cut away to reduce friction and so increase power, which is not something you need to do. The downside is more engine noise which you don't want. I cut the side of the skirts off in engines which will require a constant 10,000 to 12,000 rpm, so the influence is greater in that application, but you only have to worry about 3300 rpm and less. There are forged pistons with full skirts if forged is what you want, but remember that race suppliers only have to cater for a very short life - around 800 km for endurance racing, 30 seconds for drag racing.

 

 

 

 

 

 

 

 

 

 

 

 

[RFguy]

HMM ok, so this is assessing ring suitability. I will do this when back on the engine in a day or so. I've asked Jabiru for their latest offering, also. 
Exhaust valves look OK.  moderate (average) head recession.

[facthunter]

A Slipper Piston is when the sides go nowhere near the cylinder walls. It reduces the opportunity to dissipate heat by direct contact.

 The OEM piston make for most Au cars was ACL.   Nev

[dmech]

3 hours ago, turboplanner said:

Just following on from what we saw before and the comparison of the old and new rings, the next step is to sit the new ring on top of one of the bores and gently squeeze it in then push it down the barrel with a piston head to the compression chanber area. After sitting the ring at 90 degrees to the bore with the piston head check the ring gap against the correct piston specification to make sure there is enough gap to prevent binding, then measure the ID of the ring while it's in place against the bore against the ID specification of the correct piston to ensure there is a gap between the ring ID and the piston groove base. This space is where gas is expanded to push the ring against the bore. Then pull the ring out and holding the ring between finger and thumb check the full circle of each group to make sure there is no binding in the groove. There may also be specifications for groove height and ring height. I think we previously we discussed the bore and piston dimensions, so now they are ready for assembly. I just want tp stress again that I'm talking about the correct Jabiru piston for your Jabiru engine and the correct rings for the same engine.

 

What will go in various watercooled cars are usually what you can dig up from underneath one of the benches, find some rings that look right to the eye, scrape the crap off and stick them down the bores and the car will work for years, but that's an intermittant power application; not here which is constant power.

 

In looking at the old piston colours and worn+weak rings at 320 hours, we could think about what caused that because plenty of Jabs make 1,000 hrs. That's why it's important to check the main jets (idle jet and slide needle operate below max power). And it's also worth thinking about  what started this. I don't think you've mentioned burnt exhaust valves, so it may not have been mixture. It is possible that there was an overheat on a 35+ deg. day with a WOT takeoff and climb long enough to weaken the rings, or it may have occurred over time.

 

I would concentrate of Jabiru pistons in the 3 gen Jab engine, if that's what the piston has come out of. Looking at other opinions on other engines is like getting opinions from Fottbal, tennis, golf, squash players; they're all ball games, but you want the 3 gen Jab game.

 

The skirts on racing engines are cut away to reduce friction and so increase power, which is not something you need to do. The downside is more engine noise which you don't want. I cut the side of the skirts off in engines which will require a constant 10,000 to 12,000 rpm, so the influence is greater in that application, but you only have to worry about 3300 rpm and less. There are forged pistons with full skirts if forged is what you want, but remember that race suppliers only have to cater for a very short life - around 800 km for endurance racing, 30 seconds for drag racing.

 

 

 

 

 

 

 

 

 

 

 

 

Forged pistons, so your saying that these won't work in jab engine.

I can tell you that the marles that I have fitted  have done more than2 thousand hrs in a flying school a craft , this is the same set and there are two other planes getting close to the same hrs, without any problems. We have approx 15 a craft flying successfully  with mahles fitted including mine. I have done the hard work and expense to do the necessary engineering to make these work in the jab engine.

The trouble with forums there is a lot of hypothetical   theories,  and arm chair engineers.i have done the engineering and trials and testing with the help of a flying school , about the hardiest  test  available and this has been on going since 2018.

 

I don't want to hear of people throwing mahle pistons in there planes and then getting on these forums bagging them because they have experienced  expensive problems .

I was advising glen of our successful  use of these pistons by email and pm,s.

And don't appreciate it being put on forums

At this stage all info I gave to him was for his own use on his a craft.

Adrian

[turboplanner]

5 hours ago, dmech said:

Forged pistons, so your saying that these won't work in jab engine.

I can tell you that the marles that I have fitted  have done more than2 thousand hrs in a flying school a craft , this is the same set and there are two other planes getting close to the same hrs, without any problems. We have approx 15 a craft flying successfully  with mahles fitted including mine. I have done the hard work and expense to do the necessary engineering to make these work in the jab engine.

The trouble with forums there is a lot of hypothetical   theories,  and arm chair engineers.i have done the engineering and trials and testing with the help of a flying school , about the hardiest  test  available and this has been on going since 2018.

 

I don't want to hear of people throwing mahle pistons in there planes and then getting on these forums bagging them because they have experienced  expensive problems .

I was advising glen of our successful  use of these pistons by email and pm,s.

And don't appreciate it being put on forums

At this stage all info I gave to him was for his own use on his a craft.

Adrian

Well then, I would have thought you could quote a model and series for the engine, part number for the correct Mahl piston, setting and tolerance, because without that information these 2,000 hour pistons could be anything.

[dmech]

5 hours ago, turboplanner said:

Well then, I would have thought you could quote a model and series for the engine, part number for the correct Mahl piston, setting and tolerance, because without that information these 2,000 hour pistons could be anything.

I did inform glen where to get pistons from, and he has progressed from there.

These pistons have been available in australia for years.

[facthunter]

Forged is not a required . Good cast ones can be lighter and use a higher silicon which has a lower coefficient of thermal expansion and can run "tighter" clearances. The big improve is NO slots.  Nev

[turboplanner]

2 hours ago, dmech said:

I did inform glen where to get pistons from, and he has progressed from there.

These pistons have been available in australia for years.

So still unable to provide part numbers.

[dmech]

2 hours ago, facthunter said:

Forged is not a required . Good cast ones can be lighter and use a higher silicon which has a lower coefficient of thermal expansion and can run "tighter" clearances. The big improve is NO slots.  Nev

OK nev I agree cast is ok and slotted piston of right design more than adequate. 

The jab one can be modified To work.

If you know pistons , you should be able to work out the problem, it stocks out like dogs balls. You say cast ok, I say to you stop talking about it and find one, or show us a design that WILL Work!

I have a modified  jab slotted skirt piston 4 of, any one care to try it , do the test under  the prolonged worst conditions..

At the Monment we have the mahles straight off the shelf  and if fitted correctly with the right rings work well, we have 2 engines that reached 2000 hrs under about the worst conditions and still going good, they more than likely go on for another1000 hrs , however the engine is mandated  for complete rebuild at 2000 hrs

 

Adrian

 

 

 

 

 

[turboplanner]

This is a Mahle piston

[facthunter]

They make many different types. That one is not what is required, Looks like something out of a 4 valve diesel or Heron Head.

 D mech I've  made hundreds of pistons built race engines. Knew the chief ACL person etc I don't have to supply Pistons for a Jabiru to be permitted to comment. IF you think slotted pistons are suitable I'll disagree with you on that point. Their failures are all over automotive experience. Piston tops coming off and skirts collapsing  cracking off one side.. These types will crack in service as their very design principle will impart great stress in the piston as well as reduce heat travel from crown to skirt. They are invariably fitted to medium performance liquid cooled motors with a low BHP/displacement design to keep the motor quieter.. Nev

[dmech]

3 hours ago, turboplanner said:

This is a Mahle piston

Can't  see mahle written on it.

Can you provide more detailed pics similar to how glen posted  earlier.

How about part number may be it will fit the jab , not sure where I will find an injector to suit.

BTW it is for Ci engine.

But could just throw it in and see, at least that will be more than what happens on forums

Adrian