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Over the last year I’ve been having issues with my diamond pistons with an LSX performance block. About 6 years ago I built this engine with wiseco pistons and ran it at about 900rwhp for years. Then a skirt collapsed on the piston when I put a 45r turbo on it. So fully rebuilt it and bought diamonds latest and greatest 2618 pistons that they rate for 2k horsepower. Everything looked great and we torque plate honed the block of course and the bore was .0001 in bore straightness from top to bottom. Got the engine all built and I did an initial break in at 20 mins at 2k rpm and did an oil change. Then proceeded to do 1k miles before doing any big power pulls on the new engine but when I got to putting new plugs in it I found oil on them and scoped the engine and it looked awful and showed signs of the pistons galling to the bores. The engine never made any big power and only had a few pounds of boost. So got the engine apart and sent the pistons back and diamond warrantied them and said they don’t recommend a 4in stroke with the standard deck LSX block. So I sold my dragon slayer crank and got a 3.622 forged crank for the next engine. I just got done doing all the same and double checking everything and well the engine did the same exact thing and same amount of mileage. The first engine we set the piston to cylinder wall clearance to .006 on the first engine and .007 on the last and made no difference. Any help would be great thanks in advance.
Whoever said that a 4.00 inch crank isn't good to use in an LSX block doesn't know what they're talking about. Looks to me like P-W clearance is the issue which allows the piston skirts to make too much contact in the cylinder.
I did everything you did and mine sure doesn't look like that after 1000 miles. I have JE Ultra pistons with a 4" stroke crank. Mine had .005" total piston to wall clearance, which is what JE called out.
Also, I am no expert engine builder, but your cross hatch looks odd unless I am just seeing it wrong. It looks like it changes patterns from the top of the bore to the bottom and the angle varies widely.
I did everything you did and mine sure doesn't look like that after 1000 miles. I have JE Ultra pistons with a 4" stroke crank. Mine had .005" total piston to wall clearance, which is what JE called out.
This sounds right. My JE package PTW spec was .004x. i can't remember the fourth digit. Roughly .001 per inch of bore.
FWIW, OP, your cylinders look like mine in the 428 did on tear down. I had .023" piston rock when I was measuring for head gaskets. I should have stopped then...
Diamond themselves told me about the 4in stroke... saying it was pulling the pistons too far out of the bottom of the cylinder which I did notice but didn’t have the problem with my wiseco pistons. My machinist measured the piston to cylinder wall and concluded it was set to what they advised. But there’s definitely something wrong with the setup. Didn’t know if anyone else ever had this issue with an LSX block or anything. I’ve tried diamonds and I’m not convinced it’s the pistons but I’m forced to try je or someone else next time. And weird that diamond warrantied them and didn’t say anything?
Diamond themselves told me about the 4in stroke... saying it was pulling the pistons too far out of the bottom of the cylinder which I did notice but didn’t have the problem with my wiseco pistons. My machinist measured the piston to cylinder wall and concluded it was set to what they advised. But there’s definitely something wrong with the setup. Didn’t know if anyone else ever had this issue with an LSX block or anything. I’ve tried diamonds and I’m not convinced it’s the pistons but I’m forced to try je or someone else next time. And weird that diamond warrantied them and didn’t say anything?
I have never seen a set of Diamond pistons from anything but a picture, but that piston looks really long compared to my JE Ultra piston. Did anything else go bad like a bearing?
I have never seen a set of Diamond pistons from anything but a picture, but that piston looks really long compared to my JE Ultra piston. Did anything else go bad like a bearing?
no, the bearings show aluminum in them from the pistons coming apart but that’s it. Here’s a photo of the 2k diamond pistons.
We set my 434 up at .0035” ...Wiseco’s and a 4.155 bore ductile iron Darton sleeve. Nice and quiet. You have too much PTW clearance. Piston rocked harder than a Metallica concert and ate itself up. I’m building a engine right now with JE slugs set at .0044” per JE because of coatings applied to piston skirt, or else it would be .0038” uncoated.
We set my 434 up at .0035” ...Wiseco’s and a 4.155 bore ductile iron Darton sleeve. Nice and quiet. You have too much PTW clearance. Piston rocked harder than a Metallica concert and ate itself up. I’m building a engine right now with JE slugs set at .0044” per JE because of coatings applied to piston skirt, or else it would be .0038” uncoated.
the clearance is different for different applications. I stole this diamond clearance card from online but they recommend minimum .004 and then for street strip power adder .001-.003 added clearance which puts them in the range of what we set it at. Also ide think if too much clearance was it then why did the pistons look not as bad with the looser setting vs the slightly tighter piston to cylinder wall setting?
the clearance is different for different applications. I stole this diamond clearance card from online but they recommend minimum .004 and then for street strip power adder .001-.003 added clearance which puts them in the range of what we set it at. Also ide think if too much clearance was it then why did the pistons look not as bad with the looser setting vs the slightly tighter piston to cylinder wall setting?
The wider clearance allowed the piston to rock more which allowed the skirts of the pistons to make contact with the cylinder walls.
The wider clearance allowed the piston to rock more which allowed the skirts of the pistons to make contact with the cylinder walls.
but it never had piston slap? And .007 piston to cylinder wall is within diamonds recommendations. In fact that’s what they told us to set it to. We were originally gonna go for .005 but called them and they said that’s too tight for that power level and application
I have Wiseco's in both of my engines and they are within those same spec's as Scott mentioned.
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07 P-W is too much I mean that's 0.0035 per side when you should be more like 0.0015.
there’s plenty of guys with diamonds running .006 or around there with no problems though? Here’s a few examples on this forum. You guys could be right just don’t see why diamond would say run it that loose. I emailed diamond my photos and everything and waiting to hear back.
the clearance is different for different applications. I stole this diamond clearance card from online but they recommend minimum .004 and then for street strip power adder .001-.003 added clearance which puts them in the range of what we set it at. Also ide think if too much clearance was it then why did the pistons look not as bad with the looser setting vs the slightly tighter piston to cylinder wall setting?
I agree with you regarding the app specific setup. Gets even more specific with other piston alloys. But if I’m reading the chart right that you’ve posted, it says to set up .001 to .003 less for an aluminum block application, which negates the power adder numbers above that, and gets you back to .003 to .004....I could be wrong as I’m not familiar with Diamond specifically. But 2618 material is all the same. I could understand you growing ring gap, but I’d never grow PTW out as far as you tried it. Piston slap is hard to hear in a modified vehicle. Easy to hear in a bone stock unmodded deal, but once we add headers, intakes, camshafts, etc, the odd noises grow fainter.
I don’t think your seeing this because your too tight. Just the opposite. That’s my observation from these pics and numbers you’ve listed in this thread. I’d consult a builder with a good rep, such as TSP, Thompson, Mamo, etc, and see what they say.
With you being boosted .006-.007 is what I've found too on the Diamond pistons. It sure looks to be excessive P-W clearance though.
well I’m gonna re measure myself and see what I get vs my machinist. I would also think a looser piston would take much longer than 1k easy miles to cause issues? And ide think the skirt would be cracked or collapsed showing noticeable piston slap?
well I’m gonna re measure myself and see what I get vs my machinist. I would also think a looser piston would take much longer than 1k easy miles to cause issues? And ide think the skirt would be cracked or collapsed showing noticeable piston slap?
Piston skirt is designed to ride squarely along the cylinder wall. If the piston rocks, it isn’t square and wears uneven, causing galling and scratching such as you have there. Also costs you power because the rings aren’t square in the bore either. Good idea on you measuring this yourself. I always double check my machinist, and he works for Joe Gibbs. Trust nobody.
I do not know Diamond pistons, but on the JE pistons, they have a very specific area marked where this is no coating on the piston skirt to measure the clearance. If you measure with the coating, it will be about .003" larger on the diameter. You are to use the number without the coating for sizing.
Is it possible when somebody set up your pistons they measured across the coating and did not take the coating into consideration?
I do not know Diamond pistons, but on the JE pistons, they have a very specific area marked where this is no coating on the piston skirt to measure the clearance. If you measure with the coating, it will be about .003" larger on the diameter. You are to use the number without the coating for sizing.
Is it possible when somebody set up your pistons they measured across the coating and did not take the coating into consideration?
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LSX piston problems 
