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Thanks for all the help thus far.
If I remove the piston, and find broken or damaged rings,
what type of surface prep will I need to perform on the cylinder wall prior to
installing the piston and (replaced?) rings?
This is absolutely not true greg.. there are books you can read to understand the concepts here. Lean of stoich operation has nothing to do with burning holes in pistsons. you can safely operate ANY engine lean, with not only very low CHT temps, but low EGTs as well. anyone that operates a piston engine airplane knows this. you are confusing lean of stoich, with near stoich, and of course, detonation. this is where CHTs and EGTs are at their highest levels its not lean or rich, its STOICH! near stoich, you get into risk areas for operation.
again, coorelation doesnt constitute causation. just because your "friend " has burned pistons, dosesnt mean its from running "too lean". its because he has run too much tempurature in the combustion chamber, and that is usually caused by running too close to Stoich ratios.
there are all sorts of charts available to see the relation ship of lean runing to engine combustion temps. the leaner you go, the faster all the temps fall off, just to the point where the engine will start to mis-fire... just before that is the most fuel efficient range to be in as well as coolest temps of the CHTs and EGTs. (cylinder head temps / Exhaust gas temps)
sure, is the problem you have seen with 928s , caused from running leaner than the target fuel mixture?? yes..... 14.7 :1 is leaner than 12:5.. but, if you are in the 17 to 18:1 range, you can run cooler there.
airplanes (and cars) can run at near 75% of their power output in this range very efficiently. dont take my word for it... go read a Lycoming engine operational manual as i have and many airplane guys have!
Originally Posted by GregBBRD
I listed 4 different potential causes....if you have another one, please add it.
Lean will not only expand the piston in one of these 928 engines, but will eventually burn a hole in a piston. These two valve high compression Euro engines running on crap octane fuel are famous for this kind of damage.
I don't know anything about small low stressed airplane engines. But I'm guessing that they are different than a street engine. Don't generally they run 100 octane fuel, with less compression, have twin plugs, run less ignition advance, and turn lower rpms?
I do have a really good friend with a bit of a higher performance airplane engine in his P-51. He's on his 4th set of pistons.....and the damage in the bores and the pistons look a lot like this....in the ones that are not melted.
Hello, finally had a break from work to get some small steps forward on the 928 !
Please take a look at these photographs and give me your thoughts?
This is piston/cylinder # 8 , the yellow and red tape are provided for orientation.
These 'scratches' cannot be felt with the fingertip. You can barely feel the deepest with your fingernail..
The piston rings are intact, and rotate smoothly.
Any help and advice is very much appreciated.
I hope to turn this 928S Euro into a fun "daily" driver. I am not looking for Concours d'Elegance, my primary goal is to put this car together "right" and drive the flat spots off the tires ! LOL ! !
Thanks ! ! !
thats probably as bad as it gets before you start boring and recoating pistons. however, if you dont feel the scratches , it might be ok for a re-ring project only. the pistons are missing a lot of the iron coating, but thats not that uncommon for an old motor. I just like to use pistons with a little more of the coating remaining. you see the scratches on the cylinder matching those on the pistons. probably some carbon got in the engine passed the rings and scratched things up. not bad on the other cylinders though.
if its not a prized racer, re-ring it and slap it back together. (if you can do the paste process for the alusil, do that too. it takes any microscopic edges down on the scratches and makes things look brand new.
I have kind of a wild idea about these scratches. Based on the suggestion that the engine had been taken apart before and upon the looks of the cylinder walls below where the rings sweep, it looks to me like these cylinders were honed with an abrasive hone process and then put back together. This idea is also based in part on the very pronounced ridge that can be seen at the top of the cylinder wall where the top ring stops. That, I think, should not be there.
If these cylinders were abrasively honed perhaps some abrasive particles were left stuck in the softer aluminum walls and they have come loose and caused these scratches.
Edit: or, if I am right, the scratches might also be caused by silicon carbide particles left hanging and coming loose after it was put back together. And if I am right that does not suggest what should be done with this engine now. I think it will need to be miked to see just what the clearances are now (and I am guessing that they are going to be pretty far out) then possibly re-boring, refinishing the cylinders and oversized pistons???
I have kind of a wild idea about these scratches. Based on the suggestion that the engine had been taken apart before and upon the looks of the cylinder walls below where the rings sweep, it looks to me like these cylinders were honed with an abrasive hone process and then put back together. This idea is also based in part on the very pronounced ridge that can be seen at the top of the cylinder wall where the top ring stops. That, I think, should not be there.
If these cylinders were abrasively honed perhaps some abrasive particles were left stuck in the softer aluminum walls and they have come loose and caused these scratches.
Edit: or, if I am right, the scratches might also be caused by silicon carbide particles left hanging and coming loose after it was put back together. And if I am right that does not suggest what should be done with this engine now. I think it will need to be miked to see just what the clearances are now (and I am guessing that they are going to be pretty far out) then possibly re-boring, refinishing the cylinders and oversized pistons???
Kind of far out, I know, but what if?
nah, my shop inadvertantly did a light hone on scots engine (the 4.7 euro) and it ran fine and had no scratches when we disassembled. those scratchs are stuff in the oil or carbon. small pieces that get caught in the rings and wear things down. notice, the other cylinders are not near as bad as #8
also notice where the most of the wear is... the carbon pieces are near the top, the get brought down an inch or so, and then the piston comes up and pushes them back up.. over and over again, and thats why the majority of scratches are near the top 1". my theory.
I don't think this was carbon or stuff in the oil. Dirty air. Maybe the intake manifold was sandblasted or the filter wasn't installed properly or ripped.
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
If it was a customer car, in my shop, I'd bore it and go oversize. Too much risk/effort to put it all back together and not have it work properly, only to have to do it again.
Took me almost 20 years to "learn" this:
"Never make a customer's problem your own problem. Once you touch it, you are responsible for it, regardless of what the customer says at the time.....because they never remember having that discussion."
However, if it was my own car and I was down to my last couple thousand dollars.....and didn't want to spend it on boring and oversize pistons (or perhaps more importantly..... didn't think I could find a local machine shop that could do the boring and honing properly) I'd find a replacement piston, minimum.
Thanks for all the help thus far.
This kind of detailed advice is priceless to a guy trying to be careful, learn, and have fun at the same time.
My next step is to borrow or invest in a good bore gauge.
I'm in no hurry. I want to do this to the best of my ability, and try to keep balanced between doing "the right thing" and staying inside a reasonable family budget.
Thanks for the heads-up on the piston replacement.. wonder how hard it is to find a decent 4.7 piston ??
Thanks for all the help thus far.
This kind of detailed advice is priceless to a guy trying to be careful, learn, and have fun at the same time.
My next step is to borrow or invest in a good bore gauge.
I'm in no hurry. I want to do this to the best of my ability, and try to keep balanced between doing "the right thing" and staying inside a reasonable family budget.
Thanks for the heads-up on the piston replacement.. wonder how hard it is to find a decent 4.7 piston ??
Dan
I think i have access to 6 of them... ill have to check.
12-21-2015, 03:13 PM
