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

[dmech]

6 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.

Ihabe informed glen  rf guy of company to get pistons and part numbers. He has since posted pics which have part number on them, check his earlier post

[turboplanner]

35 minutes ago, dmech said:

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

Good, you got the message. Mahle produce a massive range of pistons.

[dmech]

3 hours ago, facthunter said:

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

Have seen a few sl and ci piston failures

Breaking etc over my 47 Yr  in engine reconditioning usually this was addressed by the manufactura or after market.

ACL made pistons oem for local vehicle  makers.

I to have made hyperutectic  pistons with slots for race engine that took out to state titles over 2 yrs  this was running up 7000 rpm 32 laps, absolutely  no probls with pistons. Also another s 6 sedan 2 state titles same engine all of shelf ordinary  Rd parts used ACL duralite 060" no speed shop parts used in this engine mod cam ground to my specs by camshaft engineering as that was the only maching that I farmed out.

Also a lot ofst stock and modified sedan engines for others  all very competitive.

I had a datsun 1600 st stock that took out over all points at our club for 3 consecutive yrs this engine was running ACL slotted pistons and often ran up to 9000rpm, yes I did have some valve failures but was able to eliminate them. This car also took out the Brian rose memorial for 2 consecutive yrs ,this car was the smallest  engine , competing  against 3300cc holden toranas. So I to have had plenty of experience with pistons and I do know what causes the cast piston to break in the jab I have also seen solid slots break and seize in the jab as well, and even one early gen 1 ACL piston which was the only one I had to discard  the crank case and 2 con rods also camshaft and some followers. 

You say you have made pistons,please tell us here on the forum how you would design a piston for the jab ie; material heat treatment cam on skirt skirt type that would fit in jab engine clearance periphery  , skirt surface Finnish for the steel n

Barrels , taper on skirt, tell us what you know!!

Adrian

[dmech]

21 minutes ago, turboplanner said:

Good, you got the message. Mahle produce a massive range of pistons.

Mate, I been engine reconditioning  for 47 yrs I've used a lot of mahle pistons, been to many seminars  conducted by them per invite from my suppliers.

I think you just being the ostrich  here

[RFguy]

Golly what has been going on while I have been underground all day.

 

Dmech, if you are going to tout ,  on the Jab-Camit forum, just how great these pistons are in an experimental Jabiru and only offer or disclose an extra 20% of the information that is required to really make the conversion work , only providing tid bits of information,   then people will be left to figure out the rest by themselves, which is what I am doing. 

 

Left to understand and figure out my myself : a) exactly what bore taper / shape you ended up with and b) why you decided to use your own rings instead of the piston suggested rings (and I have  determined there is more clearance in the jabiru bore than the Holden bore) . 

 

Dmech is would be a asset and opportunity for the Jab community to know exactly what some of those missing pieces of the jigsaw are.  You wont even give them to me in private, (since you do not want to modify any one's engine parts it would seem) , hence I am now full broadcast getting enough knowledge to figure it out for myself. 

 

I am thankful dmech  for the information you provided, But of course, its not the whole story.

ANyway, I will leave it at that ...

Edited January 19, 2023 by RFguy

[facthunter]

I've already said the forged teflon coated skirt was very nice, and Ive added a bit more information for all& sundry. How many of those engines you hotted up were aircooled?  THAT makes a big difference and race engines are not on continuous full throttle A full skirt piston with reduced cam will disperse more heat. Lots of cam is only used with "W" slots and oil ring groove slots. I'm certainly not interested in a pi$$ing contest with you but what I put here is from many years of experience and development at a higher level by far than you with specialists who don't react so defensively as you do. I'm not proposing anything radical in that circle. You are the one out of step with CAR type in aircraft engines and you're not the principle person I'm responding to. Lots of people read these forums. Nev

[RFguy]

I have posted already- the parts are the homoglated holden saloon racing pistons

They're specific to Precision Intl, and AFAIKT , the only suitable candidates. MAHLE MOTORSPORTSSKU: PHO38006040MMS is the part number at Precision Intl, and there is a ringset for it on the site. 

Yes Jabiru have a new HD piston for Gen3 , but I cannot seem to get any info from them . maybe busy with the new year.  I wil call them tomorrow see what they say, I  know they're $200 a pop

 I did post pictures of the ACTUAL MAHLE PISTON in question

https://www.recreationalflying.com/topic/38710-2nd-piston-ring-always-stuck-whats-the-story/?do=findComment&comment=540314&_rid=11148

 

Edited January 19, 2023 by RFguy

[facthunter]

Mahle certainly have a good reputation but may not be able to tailor the article  to exactly what you want as some others Might. Try SPS in Victoria.   Special Piston Services. Let me know how you go Glen.  For my purposes also I don't need to make stuff if it can be purchased.  I'll be getting in touch with them, shortly. Nev

Edited January 19, 2023 by facthunter

[RFguy]

Hi Nev , yes, I am already in conversation with SPS, thanks for the confirmation they're on your list, also. 

[RFguy]

Turbs, how does the 2nd ring get  'pressurized' ?

I'm seeing how rings really have to match well to land depth, height  , bore size etc, given the ring land height is critical enough (to getting pressure built up behind the ring via the gap),  how the hell does the 2ND ring do anything ? (IE if the first ring is doing its job how does the 2nd ring get any pressure behind it ) ?   Does ring gap width influence the time to pressurize the ring ? IE infintesimally narrow and you cant get the gas in there fast enough ?  What does the ring do about the time the exhuast valve closes ? just push out on spring only ? does the top ring get sucked in  on intake suction stroke at high vaccum ?

[turboplanner]

2 hours ago, RFguy said:

Turbs, how does the 2nd ring get  'pressurized' ?

I'm seeing how rings really have to match well to land depth, height  , bore size etc, given the ring land height is critical enough (to getting pressure built up behind the ring via the gap),  how the hell does the 2ND ring do anything ? (IE if the first ring is doing its job how does the 2nd ring get any pressure behind it ) ?   Does ring gap width influence the time to pressurize the ring ? IE infintesimally narrow and you cant get the gas in there fast enough ?  What does the ring do about the time the exhuast valve closes ? just push out on spring only ? does the top ring get sucked in  on intake suction stroke at high vaccum ?

In the initial take up of the compression stroke there's some gas leakage past all rings.

Top ring: gas is pushed down the gap between piston and cylinder, ring is pushed down so gas pressure fills between the groove and top of ring and around inside of ring; pressure forces the ring against the cylinder. Think of it this way; if the top ring makes a perfect seal the second ring doesn't have to do anything, but if there is leakage through the ring gap, and the takeup pressurisation the end result is some pressure against the cylinder, and so on down, with the oil ring stopping oil coming up from the sump and the splashing. Some pistons are designed for all the rings to take compression load rather than just the top ring, by tapering the piston up to the top. If you're designing your own piston, what counts is the compression pressure reading.

Q2: Yes, at the narrow end you don't want the ring to bind under heat; in a new design you experiment to get the highest cylinder pressure.

Q3: Yes, but binding is the main issue.

Q4: It can just sit there spring-only until the compression stroke starts, then the faster the take up the more gas you can squeeze into the combustion chamber, so the more power.

Q5: No. the intake stroke is just pulling max volume, not pressure. There's no pressure in the sump either because of the breather. 

 

The reason for being particular about all the component sizes, tolerance, clearances from the one manufacturer is that he has done the laborious work of tweaking all these things for optimum, so you don't have to do it.

[cosmicray]

13 hours ago, RFguy said:

 MAHLE MOTORSPORTSSKU: PHO38006040MMS

 

I have the same pistons in a 3300 and with 6 hours running so far they are working okay. What are you doing about compression? I have used a 2.3 mm shim to keep the compression close to standard.

[RFguy]

Cosmic, Gather with 2.3mm head shim you milled out the head recession  IE replaced material with shim ?

 

I calculated 8.64 : 1 compression with no shim.  (IE up from ~ 8). 

 

....need ~0.85mm shim to bring it back to  stock.  (at least from the Jab pistons that I have) 

I'd be quite happy with 8.6 compression with my liquid cooled heads.  Dunno about valve clearance though. assemble and check.

 

 

Edited January 19, 2023 by RFguy

[RFguy]

Talked to Jabiru this morning, as usual spares and engines very helpful.
No, the Gen3 spare pistons are the same split skirt, its the Gen4 that has the new non split skirt pistons. The Gen3 engine manual refers to the 'new'  non split skirt pistons but that's not the current supply.  confusing . I can understand the manufacturer sticking with something that works a fair percentage of the time without trouble.

So, Mahle or "other" it is....

[facthunter]

Doing the circlip groove post manufacture is not too satisfactory and the dimension and shape has to be pretty precise. Jabiru used wire and later Seeger (sguare section ) clips. Preferable to be done in the original CNC programme. Tapered bore gudgeons are best also  Nev

Edited January 19, 2023 by facthunter

[RFguy]

FWIW In the previous engine I rebuilt- the circlip grooves were 'all over the place'  - looked (under microscope) like inaccurate machining rather than wear.  the groove wasnt centred on the rot axis.

I'll measure these pins to see if there is any taper on the originals. 

Is there by design more clearance between pin and conrod, or pin and piston ?

[facthunter]

The taper is in the bore of the gudgeon pin. Small internal size in the middlewhere the bending load is .  The OD of the pin itself is parallel

     Fit on piston and small end bush?. Hand push fit in piston and about .008" in a diamond bored bush in the rod. IF the bush metal is crap It will crush in when heated a few times and risk seizure on the pin. Best is Copper lead on a steel backing  often rolled from a sheet and has a split consequently and hard to ream to finished size. Some hone them to size but watch for grit embedded in the metal afterwards . Nev

[facthunter]

A good diamond small end borer will make it absolutely true to the Big end. No twist or bend. You need to check all that plus no perceptible OFFSET either. I noticed at least one of your six were bent by the contact at one side at the top of a piston.  (above the gudgeon).. The diameter of each ring land should get smaller as you go towards the top. .THAT Part of the piston should NOT contact the cylinder wall Nev

[RFguy]

Before I knew even a little bit about pistons I used to think the crown got the heat out by oil film between the crown region  (top to oil ring) and the bore walls. But it is clear to me now , at least with these split skirt gap pistons in a non-taper bore , that the crown region even at max temp never approaches the cylinder diameter .. it's the region below the oil ring that runs in the oil film and the walls  . and the oil rings themselves conduct heat. .  The piston pin has bugger all thermal conductivity so not much goes down there. 

So from that point of view, one way to look at it is that  the big slot with the skirted piston is a lost opportunity to remove heat (putting aside there are different cold ovality shapes for different designs) , compared to a solid skirt piston.  Sure then the skirt doesnt get as much heat and doesnt change dimension much so probably should head a quiet life (maintain reasonably tight clearances)  in driving down to the shops to get some groceries lifestyle. 

?

Edited January 20, 2023 by RFguy

[facthunter]

Using the distortion applied by the top part being hotter than  the skirt imposes big stresses to it and the  concept gets very complex with air cooled motors where cylinder wall temps are far higher than with liquid cooled motors. The structure is also obviously much weaker  also. A piston is a cross head and a seal but also a way of  cooling nearly 1/2 of the combustion chamber area. Oil rings are a high friction component and on steel CAN remove too much oil film which will be further reduced in a LEAN mixture (oxidizing) situation. Lean mixtures are also more prone to detonation.  Avgas has an octane range provided in the spec and the avgas octane is worth about 5 more than Mogas because it's done to a different test process.  Nev

[turboplanner]

44 minutes ago, RFguy said:

But it is clear to me now , at least with these split skirt gap pistons in a non-taper bore , that the crown region even at max temp never approaches the cylinder diameter.

The 1500 deg C Combustion Chamber zone is from the head, down the bore to the piston top/top ring.  On the compression stroke BDC it is technically full of ambient temp air; which is increased in temperature by the compression to the spark point/ignition of mix and on the downward strock the chamber is like a refrigerator valve; as the pressure drops cooling occurs.

 

You'll notice on this site that people talking about CHT measured OUTSIDE the combustion chamber and EGT measured OUTSIDE the combustion chamber and in the COOLING cycle.

 

The colours on the piston will also give you an idea where the greatest activity has occured, and for people like me a few dozen pistons with no crowns, or melted tops with the piston melted down to the first ring which has flown upwards and is helpfully embedded and locked into the crown like a sailor on the USS Philadelphia, or a jar of exhaust valves eroded by heat, or the side punched out of the crankcase where the piston locked up and the piston rod bent and snapped in two, or where the piston locked up and the con rod was strong enough just to push the head off by shearing or tearing out the through bolts; both these latter examples give you a feel for where that 1500 deg C lives

[RFguy]

Actually, I note the term for these gapped crown-skirt pistons is better referred as a "thermally blocked skirt" .

 

[facthunter]

"W" and "T" slot are industry descriptions. Gone into at great length by Harley Davidson in the 30's. The close fits were done to  the original pistons and often replacement ones that were not that style copied the tight fit  erroneously and some motors got a bad reputation for seizing  when they should not have. These were mostly sidevalve or "FLATHEAD" motors that run very hot. on the exhaust valve side of the cylinders..  Causing cylinder distortion. Nev

[RFguy]

Now the thing with this rebuild that was different to the last :

 

The previous wrist pins I could push them out of the pistons with my strong thumbs  during diassembly.

This rebuild I had to press out  the pins  from the pistons.  IE felt interference fit in the piston.

FWIW  : this new rebuild had been sitting around and not run for 18 months.

 

[facthunter]

Tight there  can cause distortion. Ok if it's only one side and press into it Last.  Nev