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

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)

Mark- depends if you are pricing a new core, or pricing a core with accessories...

By the way Kiwi, the 914 'lookalike' was 30k aussie. 912uls lookalike was 16k aussie. I wonder if they followed suite of rotax when rotax recently in a service bull uprated all the exhaust valves in the 914 ?

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.

In general, but not always, ---in China quality at least on high volume parts, is reduced until the customer complains. I'm not quite sure how that works with airplane engines. Most of it is junk but they do make some good stuff. I buy my miniature coax cables from a single supplier on the HongKong border who has not made a bad nor failed cable in 10 years... The high end printed electronic circuit board business is interesting. it's labour intensive , and hard. lots and lots of precision. 12 stacked layers with blind internal interconnects over 300x300mm all stacked up in X and Y within 0.02mm.... and the best complex circuit boards ( IE before parts are soldered to it) come from China, no doubt about it worldwide. Working on low margins with extremely complex requirements and every circuit board is 100% tested for its literally thousands of connections internally .....I guess that hones their skills when they will go out of business if its not right and every single article is 100% tested for quality. But plenty of other stuff - like some of the cheap tools they sell, wel they really are first grade junk. and look at half the stuff in bunnings. junk. I did talk to the ZOngshen gentleman about getting a single engine at low cost to demonstrate to local market (if it was any good). that was 2021 but in the end I bought a used 1900 hour 912ULS.

But I agree with you Nev, I would consider a Rotax a much more difficult, sensitive and critical engine to copy well. compared to say, a Jabiru.

Try the Zongshen. for you, from your point of view, cant be worse...

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

I have had some contact with them a year or two ago. They were good to deal with. Price is by negotiation.... I made the point that pricing their 912ULS equiv engine at approx AUD20k ,--- that people would not pay 20k for a unknown, non tried and tested product, when they could pay 28k for a genuine rotax...... That is if people are going to spend 20k, they will spend 28k. So I think pricing came down to about 15k from memory. I am unsure to whether they are building under license, or buying some critical parts from rotax, or simply doing their best copy. Anyway, what are you doing to the poor Rotax to hurt them ? They DO need cylinder bore airflow, and you need to ensure the oil system doesnt end up with too much vaccuum, and keep them cool but nothing else.....

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.

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.

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.

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.

6 4 2

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

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.

It is I think, for training purposes, as excellent real genuine clear easy to see video sequence of ... what happens when airspeed drops too far for the maneuver (S&L or banked) We all do it up at 5000' . and ab-initio , dont think there is internal from the cockpit appreciation of what the plane is doing.

I thought it was a airport re development. Either way, I think we can say not an aiport fault... and are we sure that video is legit and authentic and is the aircraft ? Did the engines not respond to command for thrust (RPM, pitch) and the pilots try and 'extend the glide ' ???? Pilots who fly these dont fall for the downwind landing and ground speed illusion error. The airplane is doing half the checks for them . (and screamign)

well that's a wing drop if I ever saw one.

that's an engine I got used, 320h TSO, latest Gen3. I assume it went through the usual process. I have simply pulled it apart to think about it.

Well that is an interesting point Nev- the stress with power down between top and skirt. The two instances I have personally been told about, not 2nd hand, the pilot and plane at WOT was at top of circuit climb, had done or was about to reduce power , and high RPM.. (exact timing not known or remember) ......... Other people have reported go around breakages, which is a hard one on the engine if power applied 'instantly' to chilled bores. I am used to watching the local school' Pipers engine stop and prop rock at shutdown. My previous 3300 did rock at shutdown, but then after 1 or 2 minutes became tighter... The older engines tend to rock and not get so tight I am told.

Jabs rarely bounce off compression. when stopped (after run) , they're usualy tight. There is conjecture about this whether it is pistons tight in the cylinders or crankcases goign out of alignment. Jab bores are plain cro-mo. Onetrack- certainly the cast Jab pistons are shaped . In research Ive come across about 6 different 'webbing's under the piston crown each requiring their own ovality type and characteristics so that they are round when hot..... It's quite a science and likely an equal amount of testing that results in cylinders that are worse for wear and unhappy pistons.

well, these bores wont be refurbished by me with my long hone stone tool at home , it will be a shop most likely that does this stuff. We'll see how the measurements with the new micrometer come out when it arrives. comes back to my mantra of that jab engine should not fly if leakdowns are < 65/80 because the engines cannot tolerate the ills..... ( unless you really know what you are doing). Flying with low leakdowns in a Jab is a bad idea IMO because the engine cannot tolerate the ills that occur because of all the many reasons bad leakdowns are occurring. . Lycos seems to have enough in hand with design and materials to put up with poor condition without breaking. If I put the pistons back in, its likely they'd be OK since this engine didnt yet break and I am not hamfisted with power application. And jabs dont go bang very often. Still plenty to learn. Set of Mahles (x6) and rings (x6) is only $800 aussie....one could argue that would be a better use of my time but until I understand everyhting, that's too much of a experiment to run.

HI Nev. it's not uncommon either..... The Mahle 4032 forged piston is a option. the dish CC is about the same, compression height is about 0.8mm higher than the jab piston which puts compression up from 8:! to 8.6 ish. exactly how tight it gets between head and top of piston at full smoke I dunno yet. (check) . uses round wire clip I think. This is not a split skirt of course, and my guess is that the skirt will be narrower because it cops more crown heat But before I go swapping pistons I want to know ... everything. (These pistons are used by others in jabs with varying mods...) The bore region mid depth (the four points) if they show more than 0.01mm out that'll be no good for ring seal. (Jab limit in book is 0.02mm) (I have ordered a really good bore micrometer to improve on my crappy one) . The lowest regions of wear where the skirt is with piston at BDC - I can feel the wear depth with my finger. I'll likely get a honing shop to overbore them out to get back to round at the 4 pointsd in the middle IE only about half a thou (IF HALF A thou is what is required to get round) . I will need to consult gurus. IE I would consider half a thou mid way, then maybe 7 tenths of a thou at the bottom. Dont think I need to touch the top, ALTHOUGH going to Liquid heads from air cooled heads takes 50 deg C off the top inch, so perhaps take the half though down to 2/3 depth, then 7tenths thou from there. Still all miiles and miles within the (quite wide) Jab tols. (or lyco for that matter) . Different to the tols on track racing vehicles I observe. I think there IS a problem with the lack of taper in the cylinder i think . There are many with hourglass shapes. . hot in the middle, colder at the ends, and so narrowerin the middle , wider at the ends- is fairly common for air cooled cylinder bores it seems.