RFguy

My bet Turbs that the melting the piston crown you were looking for more than 36 hp/litre........

150C was used in the oven looking at expansion from 25C >> 150C. Temps can get up to 350C in the crown, 250C in the skirt with sustained high output (IE heat soak) - in engines maintaining stoich mixture . . there is a huge difference as you know between short powers bursts and sustained high output. The models are very good now., and sophisticated. Lower specific output (kW/litre) helps alot here...

I'll need to consider a test program, also. fortunately, with LCH, I can run at WOT indefinetly on the ground... but that's not really the problem- its the cyclic nature of the flying of course, not just sustained WOT. Some spring clamp thermocouples on the bores will be interesting . I do beleive that there is significant room for problems between the bore and the piston half way down... With the likely 60 deg C difference in cylinder temps between a water jacket and the cro-mo air cooled bores, the piston will get much hotter , and despite the bores having a 0.15mm head start (std setup) the pistons with their 1.4x higher expansion coefficient(of cromo) will zoom past the bores when things get hot. I think the commodores get away with the very small clearances because they never really get very (very) hot. With the air cooled bores, while they can get hot(ter) and expand, there is significant extra heat in the higher expansion body (piston) to cause trouble. IE you add another 50 deg C to the pistons and bores and now that 1.4x expansion difference of the piston is a real problem. So the bore temperatures are what needs to be monitored to understand where the clearance might be at . I've been reading up on research journal papers on piston temperature modelling, measurement etc.

SPS have come back and said yeah on putting the grooves in no worries.. DO I read it correct that Jabiru went to circlips (weegan) because people couldnt seem to fit their wire clips reliably ? Car racing community seems big on wire clips. Best thing to do now is to buy a set of pistons. I will be interested to know how they measure up. They're obviously fine in a water cooled untapered bore of 97.52mm. Jabiru bore at 150C (125C rise) in the middle (half way down) on top (measured) will go to 97.77mm (from 97.6) . Jab pistons at 97.45mm and 200C rise on the pin boss region would go to 97.82,.(based on last round of measurements) .... I will do another round of thermal expansion measurements in the oven at 150C now I have my fancy external micrometers. Blast air cooling over the top of the jabiru bores, with no air underneath would certainly distort the shape. Danger occurs at top of WOT climb, nose over- airflow improves, speed goes up 25% = 56% more cooling available = Sudden bore cooling. OR backoff from WOT and full rich = cool to cruise throttle and much leaner off the main jet - now gets hot.........AND / OR backoff then skirt much hotter than crown now briefly and crack.. Or go around from glide approach with cool bores and suddenly hot pistons . but it is full rich so fairly cool. lots of guesses there.

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.

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

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. ?

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 ?

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....

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.

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 ?

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

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

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 ...

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.

eh?.

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.

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

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

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.

yeah they have all the sizes 80,100,115,145..

11,000' DA is it ? . so ATR72-200 reference landing speed ~ 110kts. TAS = 134... looks fast over the ground. did they think it looks too fast over the ground ? But hangon these are ATP , not kids flying kites. severe wing icing ? (costs 10-15 kts according to the book) runaway trim ? passengers and lead bars loading in cargo slid to the back of the aircraft ?????

Even as a 19- you would need to be the original builder. But it wouldnt be quite as blatently flouting the rules. Or as a 24-, you could get an AP ( approved person ) to sign it off and the aircraft goes Exp24. However, I dunno whether an AP would sign it off .... Nev- can't go GA experimental - there is no paperwork path for a 24- airplane to go GA experimental. I have enquired.... so- consider you may have no insurance if it gets sticky. Rotaxes are fine I suggest the root of the problem be found. It is possible that you are just good at winning the lottery. (chance)

Or swap the rocker covers over - as long as any prospective future buyer is aware....

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.