Billet pistons shape optimization
Billet pistons shape optimization
(OP)
Hello,
In my Company C2R Engineering we made a set of billet pistons out of 2618 Alloy. The pistons worked great, and actually won their first race. Nonetheless looking into the cylinders with a Bororscope we see some scratching on the clinder liner. Our hypothesis is that when the engine is cold, due to the clearances needed in 2618 the piston thrust is rotating the piston in the liner and scratching it we suspect this because of the noise the engine makes when cold. Once the engine gets hot, it sounds normal and works perfectly. We have barrel and cam profiles in the piston skirt. One option for reducing it would be to lengthen the skirt, but we obviously don't want to this for a racing application. Any tips on cam and barrel profiles that can be useful? We have FE capacity, any insight on the thermal loads of the piston in order to calculate expansion at different points during engine warm-up?
Thanks in advance
Mauricio Toro
In my Company C2R Engineering we made a set of billet pistons out of 2618 Alloy. The pistons worked great, and actually won their first race. Nonetheless looking into the cylinders with a Bororscope we see some scratching on the clinder liner. Our hypothesis is that when the engine is cold, due to the clearances needed in 2618 the piston thrust is rotating the piston in the liner and scratching it we suspect this because of the noise the engine makes when cold. Once the engine gets hot, it sounds normal and works perfectly. We have barrel and cam profiles in the piston skirt. One option for reducing it would be to lengthen the skirt, but we obviously don't want to this for a racing application. Any tips on cam and barrel profiles that can be useful? We have FE capacity, any insight on the thermal loads of the piston in order to calculate expansion at different points during engine warm-up?
Thanks in advance
Mauricio Toro





RE: Billet pistons shape optimization
A leak-down test of all the cylinders before engine disassembly would be a good first step.
"scratching" in the traditional sense generally "sounds" more like dirt or other contamination.
If Galling, scuffing, or scoring better describe the damage the piston skirt profile //might// be on the list of suspects.
Examination of the piston rings, the piston skirts, and the entire cylinder wall needs to be made, with bright light and magnification, and the damage quantified.
http://engineaction.com/FailureManual.pdf
https://www.ms-motorservice.com/fileadmin/media/MA...
https://www1.gardnerinc.com/dealers/kohler/kohler_...
RE: Billet pistons shape optimization
Are the scratches in line with the skirt or are they in the 2, 5, 8, 10 o'clock positions?
RE: Billet pistons shape optimization
je suis charlie
RE: Billet pistons shape optimization
RE: Billet pistons shape optimization
RE: Billet pistons shape optimization
RE: Billet pistons shape optimization
Don't forget that any aluminum piston is actually an aluminum piston coated with a micron-scale layer of aluminum oxide, which is very hard.
Aluminum pistons can and frequently do scratch/score/scuff steel liners if there are problems with the geometry that determines how they interact.
RE: Billet pistons shape optimization
RE: Billet pistons shape optimization
RE: Billet pistons shape optimization
I would still like to see pictures of what you saw, how many cylinders have "it", if a decent dyno break-in and track warm up procedure are in use, and a bunch more.
RE: Billet pistons shape optimization
And sorry to sound so gruff, if our questions aren't answered there is no reason to even post the problem on the site.
RE: Billet pistons shape optimization
I hope the boroscope image might help in the understanding of the problem. the main scratches appear perpendicular to the pin within a 15 degree range.
RE: Billet pistons shape optimization
je suis charlie
RE: Billet pistons shape optimization
RE: Billet pistons shape optimization
RE: Billet pistons shape optimization
Are you sure they are "scratches" and not aluminium deposits on the cylinder?
je suis charlie
RE: Billet pistons shape optimization
RE: Billet pistons shape optimization
Our theory is that the piston is rocking when the engine is cold in the compression stroke, due to too much clearance between the piston and the cylinder, causing the rings to scratch the cylinder. when the engine is warm enough the rocking sound disappears.
We just finish a new set of pistons with tighter tolerances at the skirt, i will let you know how those perform.
Let me know if you agree with our theory or any other suggestion.
RE: Billet pistons shape optimization
RE: Billet pistons shape optimization
RE: Billet pistons shape optimization
Is the pattern the same on the other side? (1800 around the same cylinder liner)
RE: Billet pistons shape optimization
As for the ring land clearance the cylinder is bored to 82.73 mm
RE: Billet pistons shape optimization
Heavily scuffed/scratched at the very bottom of the skirt, 180 from the top land heavily scuffed?
I'd expect the top land ( above the too ring ) to have the most clearance, and it looks like it could use more.
It is cylindrical most likely, close to a solid disk of aluminum, and the hottest part of the piston, so clearance is it's only defense mechanism to keep off the cylinder wall.
if they are not running air cleaners, they should.
If they are, the air cleaners are not very effective.
Closeups of the ring faces would be informative.
Oil analysis looking for chemistry related to the region's soil or just sand would be interesting too.
Running a single compression ring ?
Formula Renault ?
RE: Billet pistons shape optimization
RE: Billet pistons shape optimization
I think THE MOOSE is on the right track re: clearances.
RE: Billet pistons shape optimization
I have seen this kind of damage on diesel engines. I wasn't involved in the teardown or prior operation so I never really got to determine a cause. I know the engine had some bad fuel injectors. I thing jgKRI is on to something. Fuel wash is a possibility.
RE: Billet pistons shape optimization
You can't have oil film breakdown in an area where there's no oil film.
I think THE MOOSE is on the right track re: clearances.
? And what makes you think there is no oil film there? If there was none then the engine or piston ring / cylinder life would be very short lived. 100 hours maybe? And what does fuel washing do then? The ring pack is constantly carrying oil / lube to that area.
RE: Billet pistons shape optimization
RE: Billet pistons shape optimization
We need the answer to Tmoose's question.
Is the bottom edge of the opposite skirt showing signs of loading? It may not be scuffed thanks to the superior lubrication further down the bore.
Reducing the skirt clearance is definitely your best move. Next is increasing top land clearance.
Pretty sure this is not happening during warm-up.
je suis charlie
RE: Billet pistons shape optimization
I value most of their opinions.
Dan T