Late Hemi, Ross pistons. 90% scoring on intake valve side, 10% on exhaust side. #1 & 3 the worst. Installed with .005 clearance recommended by Ross, .008 when removed. Ross said maybe oiling problem. I think .005 is too little for that much forged piston mass. I don't think there would have been a problem at .010, but I didn't/don't make pistons either. Thought's?
I’ve had that problem with a GMC 6 40 years ago. I heard it was called “Black Death”. I did change to the 2 extra piston I had but asked a lot of folks and the best advice stopped it. I added more side clearance to the rods and have never ever had the problem back. I did clean up the pistons with emery and finished with crocus cloth and needed to use one of them for awhile. It didn’t do any harm to the cylinder walls. I also went to C&A Zero gap rings and then to Total Seal..
I'm using Ross forged pistons. When I order a set for a 3-5/16 block bore diameter the pistons come already ground to the clearances needed. There's no block bore adjustments needed. Seems odd to me Ross would recommend a clearance dimension it should already be part of the completed size of the piston. I have experienced skirt damage like what you have with forged pistons it was caused by two things lack of oil and metal filings wearing the skirts that caused the vertical scratches. Ronnieroadster
I don't know do stock [1970] Hemi's have squinters? I know early SBF did but stopped by 67 or so. I hadn't thought about rod side clearance like Jimmy Six said, pretty sure rebuild of a stock engine, I will check.
When you say late Hemi,,,,how late ? 2nd gen,,,3rd gen,,,,,? When you ask about 70 Squinters,,,,,do you mean nozzles inside,,,,or oil grooves at the rod cap centerline ? And Ronnie is right,,,,,,,I have mine bored to size,,,,,,the clearance is supposed to be made into the piston . Many times I have seen engines start to slow down before they seize,,,,,,too tight bores . Tommy
I just ventured downstairs and looked at a spare set of 426 Hemi factory rods,,,,,,no groove for a spit hole . I looked at 3 sets of 440 6Pack factory rods,,,,no spit groove . They must have relied on the oil slinging off the rod journals ,,,,,stronger rods I guess without the groove ? All the pistons I buy now are dead on the money,,,,they all mic the same in a set . Clearance is always right on too . I haven’t used any Ross,,,,,,top quality from their reputation though . If the bores are supposed to be.030 over,,,,or whatever size,,,,they should mic that dimension . At least that’s my opinion,,,,,,,I’ve never seen an engine lock up from a bore that is 1 over the spec .,,,,,I’ve seen several get tight from too close tolerance. Tommy
When SBC’s eliminated the groove in the rod cap the bearings still had them and the cam wear started but I never saw the piston damage. You’ve got the same lines I had and the only way my cylinder walls got oil was from the rod to crank throw side clearance. The original rods had a spit hole for the cam but the long rods I had made don’t so I rely on the extra clearance for the oil to the walls and cam. . I’m sure stock at a governed 3200 rpm it didn’t matter but my engines see close to 6000 for 3 miles at Bonneville. The guy I trusted ran Blown Chryslers so I felt it wouldn’t hurt to do it. I quadrupled the clearance but each crank throw has 1 rod. Yours share a throw so you will need to and possibly use a scraper to control the amount. My pan has scrappers and when I ran roller lifters I made one at the block/pan seal with .015”gap to slow the oil on the lifters but not restrict the cylinder walls. Good luck.
Just for the halibut... I would give the skirts a dimensional check with a good micrometer. Crocus cloth should clean those up nicely.
Go back to the basics. Measure the bore and piston. Grab a set of of something like this and see what ya got. https://www.williams-industrial.com/us_en/piston-feeler-gauge-set-pb_gs-4.html
Everything looks pretty clean, so how long did it run before the problem showed itself? Were all 8 cylinders like this, or just the two pictured? What type of oil was used? Do the headers for those cylinders look any different from the rest? Do the tops of the pistons all look the same? There are a couple different alloys for forged pistons, and clearances are different. Which alloy are these?
>>>Everything looks pretty clean, so how long did it run before the problem showed itself?>>> My thought/question too. So why was it taken apart in the first place? Symptoms?
Pop the crankcase core plugs and look for casting flash or other restrictions in the water jacket of the cylinder in question. that sort of looks like a failure caused by overheating. If it’s a port injected engine, look for a plugged injector leaking intake gasket, or something else causing a lean condition that would overtemp the piston and cause skirt expansion severe enough to take up the piston to wall clearance. We also saw random failures like that when our lube oil supplier messed up the additive formulation on a 5000 gallon shipment of engine oil. The oil got dumped in the lab bulk tanks and engines started scoring before the incoming oil quality tests were completed. That was a long time ago, though.
Stock engine from a 70/71 Hemi Cuda restoration project as far as I know, friend's car. He is not responding so will catch up posting when I know more.
I think retarded timing could cause that as well. Too little timing heats the skirt too much timing heat the top of the piston. I think. Having said that, looks like a lack of piston to cylinder wall clearance to me.
Doubtful this is a piston clearance issue. Measurements will confirm that. Timing? Ring condition? So many other factors/possibilities. Ross is dang good. I wouldn't be surprised if they would analyze them and come up with a failure analysis.
I like to measure things, easy, usually involves no cost and rules out a dimensional issue. Case closed. Then I like to line all the parts up and look at them. Then see where they live, look at that too. Go upstream, look at the cylinder heads, even the intake runners.
I had a 218 bare block machined. I used .080 pistons I bought, and assumed part A would fit in part B just fine. I didn’t have the tools at the time, nor the knowledge to use them, to pre-check everything thing. I was just a plastigauge guy at the time. My buddy asked it the pistons “were fit”. Huh? Of course I couldn’t admit to not knowing what he was asking. So, in my mind, put the piston in the hole, (without the rings, going too down) and “feel” if there was a difference. What did I know? Nothing. But one piston was tighter, I could feel it sliding it in the bore. I pulled it up and checked a few other bores, they were fine. Took the block back and got it taken care of. I mention all the above just to ask a simple question Did the owner try and save $$ and assemble it himself, or was it a take in and pickup when done? If it was assembled by himself, he may have missed a tolerance or two. If fairly recent and a shop did the assembly, it would be on them to correct. Just my thoughts.
That where I was going as well. Get the basic info and then measure across the cylinder walls 90 degrees from where the scoring occurred. That should still be the original dimension of the bore.
In the 50's I was told you never finish hone a bored oversize engine unless you have the pistons on hand. Just to check, never know whos measuring tools are going to be off in what direction. I still check with feeler gauge before the piston goes in. Personally I would have questioned the .005 that Ross specked. If read whole post pistons came out with .008 clearance. I maintain if they went in at .010 there would not have been an issue. Still waiting for friend to tell me what the shop, supplier consensous is.
I think the bigger problem is, who measured it? A machinist? What machinist? Was it someone that works at the machine shop? Was it double and triple checked? If I owned a machine shop and had to delegate work to others I would be a nervous wreck! I need to see measurements myself. Not that I am in any way perfect it’s just the only way to know the whole story.
From my exp. 5 thou should have been adequate. My blown alky engine was 8 t0 10 and rattled profusely on start up. But that was a race engine. The rule of a thou per inch of bore size is still good. 4.250 bore at 5 thou should work well. You have so many unknown's, it will take further research to find out what actually happened. If in fact, it was assembled with 5 thou, I would start looking else where for a solution.
I agree that 0.005” is plenty, I personally won’t finish hone a block without the pistons. I would never bore and hone a block over size and assume the oversize pistons are right, I measure each piston and finish the cylinder for its piston size.
I was 17 and working in my HS shop class, we were rebuilding a 289 Ford. We had all the parts laid out on a bench top covered with brown butcher paper. I got the task of honing the block and fitting the pistons, used diesel fuel as the cutting oil. We used a long feeler gauge with a pull-scale on it to check the final piston fitting. Everything was marked as to where it went, the instructor was fussy as it got, but everyone on the task remembered something.
Not being an engine builder, would you explain ring gap effect on the skirts? I’d always assumed (if too tight) the lands could be damaged.
That’s pretty cool. I’d guess a set of feeler gauges that were round, not flat? Do you recall the go-no go reading on the pull gauge? I assume it measured in a weight scale?
I’m lucky that we have all the toys at work, dial bore guages for measuring bore and micrometers for the pistons, hone to final size with a Sunnen CK10. Maybe I’ll try to remember to do a thread on the next one I do.
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