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

[turboplanner]

23 hours ago, RFguy said:

#5 piston. step crown thermal application shows same distribution as that colouring !

 

There is negligible scuffing /marking of the bores at the back cylinders  , the unhappy bores are the pair at the front- the air blast cooled bores...

 #1 piston. (has ben through the MEK) 

BORE 1 - LOWER

 

BORE 1 UPPER

 

 

What was the pressure on the cylinder the black piston came from. I can’t remembers seeing one so black so far down.

[RFguy]

Turbs the rear leakdowns were 75/80. The front ones < 50 . mid ones, ~ 65.
That's according to the book.... (not mine) 

CYL1, CYL 2 mid ring were jammed stuck. However ! I did find stuckish mid  rings and also top  rings on all cylinders.

---Bear in mind this engine had been sitting around under a bench in a workshop for a year before I got it, so plenty of ring lands and rings had a chance to go hard and stick even if they were not at last flight.

[Blueadventures]

5 minutes ago, turboplanner said:

What was the pressure on the cylinder the black piston came from. I can’t remembers seeing one so black so far down.

What was the aircraft rego?  VH-BBQ🙃

[facthunter]

VH POT? 

[RFguy]

5,3,1. bear in mind #1 had been sitting in petrol for a week and has got a bit of a scrub.

 

[RFguy]

6 4 2

 

[facthunter]

Those pistons have been "cooked". The only engines I've seen that looked like that were an AgCat (probably Jacobs) running on mogas and that was in the 90's.  Nev

[turboplanner]

1 hour ago, RFguy said:

Turbs the rear leakdowns were 75/80. The front ones < 50 . mid ones, ~ 65.
That's according to the book.... (not mine) 

CYL1, CYL 2 mid ring were jammed stuck. However ! I did find stuckish mid  rings and also top  rings on all cylinders.

---Bear in mind this engine had been sitting around under a bench in a workshop for a year before I got it, so plenty of ring lands and rings had a chance to go hard and stick even if they were not at last flight.

Jabiru were using Holden pistons which had offset gugeons, but they were fitting them back to front of the Holden piston direction. I'm not sure which engine they came out of, but think it was the VN3.8 V6.   The V6 was a Buick engine which ws never any good in the USA, and Holden bought it, spun it in the opposite direction and it was a great engine.

 

Compression was 165 psi with a minimum 99 psi.

 

If the pistons you have there are VN pistons, you have a little design history there - reversed engine + back to front pistons.

 

If they weren't, you may be able to track down the GM part number of your pistons and find out what compression the pistons were designed for.

 

At 75, 80, 50, 65 I'd say the engine has given good service and needs a rebuild, so nothing strikes me as odd there.

 

Those figures indicate the fuel charged air has been escaping past the rings and combusting down the side of the piston, leaving tell-tale carbon deposits and that's probably what the gummed up the second ring slot rather than escape from the chamber.

 

I would think the cylinder marks are just from the piston bumping around.

 

In comparing the rings that came out of the engine to a new ring, I would expect to see the tension had gone and rings worn allowing the blow by.

 

You may be lucky enough to be be able to hone the bores and still stay in spec, but I would fit new pistons mainly because of the constant blow-by cooking suffered by the old pistons.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[RFguy]

Thanks for the input, Turbns, always useful. Yes I have read the 'reversed offset piston discussion '  from 2010-2013 here...  many posts discussion but no real conclusion obvious. 

I am very interested in your comment : "Those figures indicate the fuel charged air has been escaping past the rings and combusting down the side of the piston, leaving tell-tale carbon deposits and that's probably what the gummed up the second ring slot rather than escape from the chamber. "

right. worn rings.

I will compare the tension and springyness also of new and old rings. I have a set of new rings but it's likely to get new pistons.

Yeah an Mahle make a racing car series piston  (and companion rings) for these Holden engines, its a forged 4032 and is close. Dish CC of top is very similar 

It has a wire loop pin retainer  IIRC, and the slightly compression height means 8.6:1 instead of 8:1.  so its pretty close. 
Gap between top of piston and (closest) head surface  would reduce from 2mm to about 1mm  when hot.....
$800 set of 6...

As NEV points out, these pistons are designed for mom and pop to drive to the supermarket and tow a trailer and 4 kids  on the highway between sydney and melbourne.  My take on the thermal attributes is that there is very much a max ( time x heat)  limit with these pistons as the heat will soak into the skirt across the skirt gap thru the pin boss region cause it to become.. a little big.

 

[turboplanner]

1 hour ago, RFguy said:

I will compare the tension and springyness also of new and old rings.

If you sit an old ring on top of a new one, that should show up a difference allowing blow-by.

1 hour ago, RFguy said:

I have a set of new rings but it's likely to get new pistons.

Yeah an Mahle make a racing car series piston  (and companion rings) for these Holden engines, its a forged 4032 and is close. Dish CC of top is very similar 

It has a wire loop pin retainer  IIRC, and the slightly compression height means 8.6:1 instead of 8:1.  so its pretty close.

There's a formula around somewhere that allows you to calculate the theoretical compression pressure between those two and might confirm the difference between your book and the 165 psi Commodore pressure.

1 hour ago, RFguy said:

Gap between top of piston and (closest) head surface  would reduce from 2mm to about 1mm  when hot.....

Phil Irving recommended a clearance from the valve heads of at least 1.6 mm, so it depends where the valves are. This can be checked by a trial assembly with plasticene on top of the piston, bearing in mind full extension of valves are: Exhaust; before TDC and Inlet; after TDC.

 

If you feel contact as you hand turn the crank softly, you can cut a valve pocket in the top of the piston (Phil doesn't recommend cutting deeper than 4.7625 mm).

 

At one stage I decided that my piston problems could be solved by going to a forged piston like the ones you mention, and can say that I blew the first one just as quickly as the others.

 

1 hour ago, RFguy said:

As NEV points out, these pistons are designed for mom and pop to drive to the supermarket and tow a trailer and 4 kids  on the highway between sydney and melbourne.  My take on the thermal attributes is that there is very much a max ( time x heat)  limit with these pistons as the heat will soak into the skirt across the skirt gap thru the pin boss region cause it to become.. a little big.

The difference in application is that a car engine is designed for intermittent power demand up and down the rev range ( so it gets hot climbing up the hill but cools down on the other side) whereas an aircraft engine, is designed for constant power demand at cruise.

 

However GM designed these cars or towing caravana, horse floats and big boats of up to 2 tonnes, so that aspect doesn't worry me.

 

You are possibly quite right about the second ring sticking through sitting for a long time with the second ring containing most of the liquid, but I thnk that's just a byproduct.

 

Do you know how many hours the engine had done?

Did you check the diameter of the main jet? 

 

 

[facthunter]

The gen 3 doesn't have the Holden  Pistons. They had to have the circlip  grooves done to adapt the different gudgeon and also to confuse things further, Changed to seeger type circlips and gudgeon in offsets. The offset can even out thrust face  load differences but it's main purpose is to ease piston slap around TDC. . The pistons were already max available oversize from OEM as they didn't want you to bore the motor oversize.   Nev

[RFguy]

320 hours TSO.  JET was , if you beleive what is written scrawled on the carb, a 2.52 or 2.55  IIRC. but I will pull it apart and find out. 
The plugs dont look overly rich run  though. It has all the usuall 320 hours of AVGAS cakeup. 

 

There is a guy who has done a few engines with the Mahle so far - iterative process (there were bangs) . shaped hone/bore is part of the answer.  and shims to get compression back down and get a bit more clearance there.
 

OK on the valve clearance and technique to check it.  These Gen3  pistons I have have valve cutouts in them - designed to be a no contact even if valve hangs out........(so that will give some clues)  But the previous 3300 Gen 3 I overhauled did not- just a dish. If a hydra lifter gets itself all pumped up for some bad reason and one gets too much lift then I;'d say I'll want that extra 2mm of clearance. 

[facthunter]

If they pump up they will be off the seat continuously. The lift is still dependant on the cam lift. Plus the amount it starts off seat.  It's going to be running pretty ordinarily. The inlet could be pressured enough to charge other cylinders. This is engine blow up stuff.   Hydraulic lifters belong in fat V8's that people don't want to put a spanner on. I nearly went to the next world when One 0-300 lifter pumped up with me and a 912 failed to make A Natfly with another lifter failure..  Nev

[RFguy]

6 hours ago, turboplanner said:

In comparing the rings that came out of the engine to a new ring, I would expect to see the tension had gone and rings worn allowing the blow by.

 

Well Turbs, are are bl00dy right....

The new rings are stiff and resist squeezing. they feel strong.
The old rings squeeze readily and without much force between the fingers at all. rather boing boing. 

New rings are 10 to 11 thou wider than old ring  (IE inner diameter to outer diameter, not 'thickness' in the land.) . That was just one sample.

Wow well, learned something new.

[turboplanner]

4 minutes ago, RFguy said:

Well Turbs, are are bl00dy right....

The new rings are stiff and resist squeezing. they feel strong.
The old rings squeeze readily and without much force between the fingers at all. rather boing boing. 

New rings are 10 to 11 thou wider than old ring  (IE inner diameter to outer diameter, not 'thickness' in the land.) . That was just one sample.

Wow well, learned something new.

Well, a nice set of straight bore walls and a gap check with each ring in the bore should see the end of the black sided pistons and dirty ring groove.

 

The main jet is still worthwhile checking out in case someone thought the jet was too small and made a mistake drilling a hole that was too big flooding the chamber with fuel which started leaking down, combusting and causing the rapid wear. I wouldn't be in a hurry to go back to the original Jab size because that was introduced for better fuel economy, and I wuld prefer longer engine life, so if the min jet in your 320 hour engine IS bigger than the standard engine, I would only be going back to somewhere in between that wouldn't flood the chamber at cruise rpm.

[RFguy]

I guess same thing with springs, when a spring is cooked , the tension is lost. Just like fuseholders with springs holding the thing in compression. fuse gets hot. springs gets hot. no more spring. 

on the jet. and I also have a wideband oxygen sensor to check what the average mixture is doing, IE an extra readout comnpared to just EGTs and plug colors. yeah  I see wher eyou are goign - high wear perhaps caused by excessively rich conditions for too long. 

[facthunter]

The depth and material used makes the ring hard to compress or otherwise. A deeper ring may require a deeper groove  The ring should not project beyond the piston dia. itself when in the bottom of the groove, or it's likely  to break.  Nev.

[RFguy]

using my trusty Moore and Wright internal micrometer, with grats of 0.5 thou

#1 bore. 97.6mm +/- 0.5 thou  generally., and in the bottom skirt centreline shiny worn bits ( BDC, bottom of the centreline skirt) is +1.5 thou over minimum.

This bore is at the narrow end of the permissible bore width range, FWIW

Edited January 17, 2023 by RFguy

[facthunter]

It's unlikely to be a uniform figure. There's ovality, warping and taper to consider in limits. and often the base will be bolted to a thick plate when honed to simulate assembly.    Nev

[turboplanner]

6 minutes ago, facthunter said:

It's unlikely to be a uniform figure. There's ovality, warping and taper to consider in limits. and often the base will be bolted to a thick plate when honed to simulate assembly.    Nev

 

Based on that, what would you do with the bores next, given this is an application that needs a long and reliable life, vs my action to hone the bores with 3 honing stones and kerosene on a 1/4" drill to get to the next race meeting.

[facthunter]

Turbs,I don't know the thickness or the nature of the coating. If there's any significant marking the top race bikes replace the item no question and it the fault's theirs for free.. It's considered desirable to deglaze a normal cylinder to assist ring bed in and oil film retention..  The Best ne is a sunnen which is quite rigid and can true the surface but not right up to the top of a blind barrel. but usually close enough if it's new. (expensive)  Nev.

Edited January 17, 2023 by facthunter

[RFguy]

given how they are measuring, they'll get a 3 stone hone in the bench drill with kero... (Nev- no coating)

but what I do with the piston question is next. I'm suspect that Jab with their latest pistons for Gen3 have got it right yet (still cast my understanding but the split skirt is gone. ) 

 

I'm told on the first set of 4032 forged Mahles that someone used, one grabbed , but I have no knowledge of where and how, but subsequently  a shaped hone (no other details) was used. I dont know maybe installed pin offset was wrong ...... I'd need to get a set and see what the expansion does. Since they are designed to go in standard water cooled L27 bores, the expansion cant be too different from the stock casts. (although in this service they may run alot hotter than standard pistons service) ..... I will need to find out exactly what size the standard bores are... all I know is they are designed for "97.5mm bores" (40 thou over)  but the exact details could be found out. 

 

Edited January 17, 2023 by RFguy

[RFguy]

Running the expansion numbers, the diameter measurement numbers on the skirts both adjacent to the pins on the pin boss (which could  very close to  crown temperature on sustained high output heat soak) , and down the centreline of the skirts  - in conjunction with the (measured) bore diameters indicate that interference WOULD  occur under likely conditions.

My guess is as the interference/ sqeeze on the oil film  begins to occur, (minimum clearance)  the region transfers  more heat  (narrower oil film) from that region into the bore, and keeps the "situation"  at brinkmanship. (IE pistons cool down just enough to maintain the status quo) 

If the oil film ceases to exist, it's likely the heat transfer becomes much worse, since there will be a lower actual contact surface area. (its why in electronics we use a film of grease between two flat parts to aid heat conduction) 

[facthunter]

The hottest parts of the cylinders are near the exhaust valves at the top.  The lube oil film is only microns and temperature is the enemy.  Nev

[facthunter]

Have an oxidisng situation (Lean) and you'll burn the oil film off.   The oil rings don't leave much oil and a bit helps to stop or reduce blow by.    Nev