akuma-z Posted January 26, 2011 Share Posted January 26, 2011 I tore one of my motors down to check the internals and found some damage on the side skirts of piston 1. The motor is an L28 with what appear to be Ross forged pistons. The motor is stock bore. I did a compression test and all cylinders were somewhere between 180-190 psi. The damage appears to be isolated to piston 1. The bore visually appears un-damaged. All of the bearings/crank journals were in good shape so it does not appear that any metal fragments made it through the oil system. Prior to tearing down the motor it ran strong and did not show any signs that there was a problem. The CR was a little high to be running on pump gas (E31 head) and I know there were occasions where I could hear pinging intermittently when it was hot out. What has me confused is that the damage is all isolated to the side skirt. If the damage was caused by detonation I assume I should also see damage on the tops of the piston. My best guess is that there was not enough oil getting to the cylinder or improper installation but I am open to any suggestions. I am almost tempted to put the motor back together and push my luck with it since it still had good compression and I don't want to put any more money into this motor. Quote Link to comment Share on other sites More sharing options...
Derek Posted January 26, 2011 Share Posted January 26, 2011 Did you assemble the motor yourself originally? Maybe it was that way when you got it. Derek Quote Link to comment Share on other sites More sharing options...
akuma-z Posted January 26, 2011 Author Share Posted January 26, 2011 Did you assemble the motor yourself originally? Maybe it was that way when you got it. Derek I did not assemble the motor so that is very possible. I originally bought the motor for the head and decided the tear the bottom half open to see what was in there. Quote Link to comment Share on other sites More sharing options...
rayaapp2 Posted January 26, 2011 Share Posted January 26, 2011 (edited) Ive seen that with ring gap issues. Im sure there are lots of other things that can cause it though. Ive also seen it with forms of pre-ignition where the flame fronts collide and amplify and causes damages like that when parts of the pistons melts to the cylinder wall and takes out the sides of the piston. But usually related cylinders(ones that share carbs) will follow suite. Ive always had a hard time diagnosing engine tare downs with that sort of damage. On the bright side I think you only have order a single piston and rings through Ross once youve determined the shape of the block cylinder wall. The last one I had required a ridge reamer to remove bits of aluminum from the top where it had piled up, and then I was able to hone it out and stay within the wear limit to use a std piston. EDIT: There appears to be a pattern there where one side was worn at a specific spot in its rotation(top picture) and the other side appears to be worn all the way down the skirt which could mean rings dont have much to do with it.. Edited January 27, 2011 by rayaapp2 Quote Link to comment Share on other sites More sharing options...
Warren Posted January 26, 2011 Share Posted January 26, 2011 (edited) Have to admit, I've never seen that damage except in one place and that was a larger 2 stroke model airplane motor with a bent rod. Check that particular rod, top and bottom, especially the bronze bushing at the top and I'd be almost willing to bet that it's either severely worn or bent or the rod itself is bent or twisted...usually caused by detonation. I could be wrong, but I'd put my money on the rod before anything else. Edited January 26, 2011 by Warren Quote Link to comment Share on other sites More sharing options...
Tony D Posted January 27, 2011 Share Posted January 27, 2011 (edited) Download This PDF! Page 13 should explain it... On the following, "80" should be noted. It does not look like it was from crown down, and does not include the rings, so a combination source should be considered. Anybody wonder why running pig rich (note crown/ring area) is bad? I think you were wall-washing. I like the GRAPHICS on this one slightly better... Plus I have always liked TRW/Koblenschmidt! Nice Ubermenchen with the cape as well... Between these two OEM Piston Damage Books, you should see the common patterns that emerge. Edited January 27, 2011 by Tony D Quote Link to comment Share on other sites More sharing options...
akuma-z Posted January 28, 2011 Author Share Posted January 28, 2011 Thanks for the PDF. That is exactly the kind of information I was looking for. Quote Link to comment Share on other sites More sharing options...
1 fast z Posted January 30, 2011 Share Posted January 30, 2011 Either the piston to bore clearence was too tight from whoever built the motor, or the motor got hot at one time and the piston to cylinder clearence got too tight that way. Alot of people try and set forged pistons up to hyper or cast piston clearences, but they have to be about twice as loose at a minimum. Quote Link to comment Share on other sites More sharing options...
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