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You are correct those are Sealed Power. Look just like Tom's pistons. A picture of the underside of the piston can tell you if excessive heat was in the chamber while running.
The underside should remain pretty clean in normal running engines. If excessive cylinder temps are happening the underside of the piston will discolor. First they turn a golden color then darker brown and then black depending on how much heat they see.
A lean burn condition can run the temps up pretty high, same as advanced timing.
When you see this type damage it is also important to look at all 8 pistons and compare them to see if the issue is in all 8 cylinders or just one.
How many miles were on this engine, the second piston ring is 50% worn already which points to high miles or a rough cylinder finish.
My 383 had 195 psi compression in every hole and 30% leakdown in every hole. Second rings were gapped at close to .040" and shot due to improper cylinder finish. No broken pistons, though.... mine are forged Sealed power units from long ago.
I never liked the hypereutectic pistons. Too prone to rattle and I saw many of them fail just like the 2 examples. You could gall or lift the top ring off of a cast piston but I never saw one break the second and third ring groove. If you look at the pic of the gorged piston Jeff posted the top ring is way closer to the top. Those and 400 sbc pistons had the top ringconfigured that way. They didn't come with instructions to increase the end gap. And they never gave any trouble.
I never liked the hypereutectic pistons. Too prone to rattle and I saw many of them fail just like the 2 examples. You could gall or lift the top ring off of a cast piston but I never saw one break the second and third ring groove. If you look at the pic of the gorged piston Jeff posted the top ring is way closer to the top. Those and 400 sbc pistons had the top ringconfigured that way. They didn't come with instructions to increase the end gap. And they never gave any trouble.
I agree, Robert. I never liked the hypereutectic pistons or pistons with Teflon on them either. I prefer forged, even the old fashioned heavy ones, which I've never broken in any engine. The piston I pictured will be re-used with new rings if the bores are okay....these pistons are close to 30 years old, IMO.
Most new cars run Hypereutectic pistons today.
The problems most remember with hyper pistons happened when the technology was new. Early 90's. That is a long time ago now.
You are correct those are Sealed Power. Look just like Tom's pistons. A picture of the underside of the piston can tell you if excessive heat was in the chamber while running.
The underside should remain pretty clean in normal running engines. If excessive cylinder temps are happening the underside of the piston will discolor. First they turn a golden color then darker brown and then black depending on how much heat they see.
A lean burn condition can run the temps up pretty high, same as advanced timing.
When you see this type damage it is also important to look at all 8 pistons and compare them to see if the issue is in all 8 cylinders or just one.
How many miles were on this engine, the second piston ring is 50% worn already which points to high miles or a rough cylinder finish.
All pistons looked the same as did the undersides with not much coloration at all.. My first thought was high temp and/or detonation and I was looking hard for any evidence of that but found none. It was someone on this forum that suggested I look at the ends of the rings and that's when I discovered the peened ends and knew why the piston broke. Timing is 10* initial with stock distributor and vac can. Mechanical is all in around 24-2600 rpm. Engine had about 5,000 miles or less and here's picture taken during the assembly.. Machining was done by a well regarded shop that's been doing this for 50 years that I know of and maybe longer. I wish I still had the rings to look at the top and second one for the wear you're commenting on. It was my impression that everything looked fine except for the peened ends. I did a light hone before putting in new piston and rings but did not take pictures that time.
At the time I did not have an air/fuel gauge to confirm mixtures and was running a Demon 650 that was on my 327. Plug color was always a nice tan but it's possible it got lean at higher rpm but I rarely ran it there and then only briefly. I believe this piston broke after a good run up in 3rd gear, maybe between 5 & 6,000 rpm. I came up to a stoplight and had smoke pouring out from under the hood as a result of crankcase pressure blowing out the PCV valve and spewing oil all over the engine and hot headers. But from the looks of the ends of the rings, they had gotten hot enough times to peen them over so it may have been lean at high rpms and I never shut it down immediately after to get a plug reading.
I'm running a QF 750 now with a A/F gauge. Other than that and opening up the end gap on the top rings, I don't know what else to do to prevent a replay but open to all ideas.
My 94 vette has always had 1 piston slap. Doesn't hurt anything but who wants the noise. I just redid my LS1 because of piston slap. I can't count the number of complaints I've seen from LS owners with new engines, as in new cars not freshly rebuilt. with piston slap. The newest latest greater hypers I just got for my LS1 come with coated skirts. They have windows in the coating so you can measure the base piston size. The coating increases the diameter of the piston where it's "supposed" to be sacrifical so it wears to the proper fit. 45 years of engine building and I would say that is bs.
65 Air...that looks like a very clean engine. I don't see any pistons near TDC.....I am wondering what your quench is....how far down 'in the hole' are you? If a lot, that could account for the detonation/damage.
65 Air...that looks like a very clean engine. I don't see any pistons near TDC.....I am wondering what your quench is....how far down 'in the hole' are you? If a lot, that could account for the detonation/damage.
0.025" down. Heads are Trick Flow Twisted Wedge with 63 cc volume. If you still have 195 psi with that much leakdown, you must have a cam similar to mine in that it creates a lot of cranking pressure. With my original gap on the top rings, I got 230 psi with throttles closed!
Robert, you are correct the pistons are built to size and coated after the build. Most are applied with a silk screen process so the thickness will vary.
The Skirt coating was first used in mass production by Mercedes in the late 80's and has been applied by nearly all OEM's since around 2005.
The reason has nothing to do with fit. It is all about avoiding piston scuff.
Most pistons scuff when exposed to some really odd short term situation. Maybe working really hard pulling a hill causing a short term overheat in that cylinder.
With coatings the piston lives and the owner never knows they were in danger. Without coating sometimes the piston scuffs.
Overheat can raise the cyl temp till oil breaks down leaving the piston exposed to little lubrication.
Most scuff's you see are carbon black, the heat cooked oil into the piston scuff and also the cylinder wall.
You know right away if heat was involved. If the scuff is aluminum color it points to fuel wash normally and the scuff happens due to a lack of oil not from burned overheated oil.
It is considered normal for some coating to scrape off on installation because the silk screen process is not that exact and the coating thickness may be more in places than the piston clearance in the bore.
For the small cost coating does offer insurance against that one time overheat.
Even Diesel engines have now adopted Skirt Coating at the factory.
For the past 10 years some of the Diesel pistons utilize many piston coatings. Moly Coat on the Skirts, Manganese Phosphate coating on the Piston Pin and Ring Lands, a heat reflecting coating on the piston heads.
They also run with more than 40 pounds of turbo boost and are expected to live for 1,000,000 miles. Manganese Phosphate has been used for years to coat valve guides so the valves can live while working in a environment with very little lubrication, today it is working in more areas.
We all know new engines routinely last 300,000 miles, it is not all related to fuel injection. Lots of little things are helping. Some of these are available when you freshen up your Vette.
I just checked the 2015 Speed Pro Piston Ring Chart, It does say .004 x bore size for passenger car or moderate performance engines.
For the 400 with 11:1 compression I would think that is on the high side of moderate performance.
They recommend .018-.020 top ring for moderate performance or drag racing.
I was going to post an excerpt from a JE piston blog that entails the coating conforms to the cylinder wall shape for the best clearance. There was similar info on the Silvolites somewhere I don't remember if it was in the box or online. I spent a good while studying which piston I was going to use in my LS1. I would have used cast but they are only available from some offshore outfit and at a price so low I wouldn't use them. I deleted the reference as I don't need the Internet police telling me I have posted copyrighted material.
Took the 51 for a test drive around the block this afternoon. Ran great! After 6yrs in hibernation, the air cond even blows cold. BUUUUUUUUUUUUUUUT, sitting 6yrs has NOT been kind to the brakes-----------------kinda scary when I needed to stop. Only had brakes on 2 wheels-------------and one of them locked up. Although I did get it back into the garage safely.
I just checked the 2015 Speed Pro Piston Ring Chart, It does say .004 x bore size for passenger car or moderate performance engines.
For the 400 with 11:1 compression I would think that is on the high side of moderate performance.
They recommend .018-.020 top ring for moderate performance or drag racing.
Here's a good one I haven't seen before. I've seen a zillion blown head gaskets but never 3 cylinders in a row on a normally aspirated engine. It's an old Blueprint 383 but less than 5000 miles. They said it's missing bad on 2 cylinders. I told him to do a leak down but he didn't have a tester so I said just put air in the cylinders and listen to where it's escaping. It was going out the ex and intake.i said you've got a blown head gasket. Didn't expect this. Fortunately cylinder walls look good with no scuffing so I'm going to do a quick valve job and he'll be back together. Luckily it didn't warp the heads or block.
That gasket is a Fel-Pro 500 series gasket, at least it appears to be. Those have a stronger fire ring than stock gaskets, nearly as strong as the Performance gaskets.
They resist detonation 3x better than stock gaskets and are a good choice for performance engines like the stock Vette engines.
Severe Detonation can do this, in severe detonation the head bolts will stretch and cylinder pressure can blow between cylinders. In severe detonation the fire ring will get pounded.
It looks like someone takes a ball pen hammer and beats the fire ring down till it splits. (fire ring is the steel that circles each cylinder and holds in your combustion)
The head jumps up and down and beats the fire ring till it fails. Detonation could be lean burn, excessive timing, low grade fuel for the application. The pistons look OK so I don't expect this to be detonation.
On tear down did you notice any head bolts that did not have full torque on them? If using air tools you would not feel it but if using wrenches you can feel a loss of torque.
Any loose head bolts are a sign of overheat. Overheat on the head gasket can be from high cylinder temps, this can be in only one cylinder or in one bank. Can be localized.
Measure the gasket thickness all around the failure points. It should measure .040 compressed. If you find it measures .004 less in areas around the failure you know it was caused by overheat.
When a head overheats it grows and crushes the gasket. After it cools the head relaxes back to its normal size but the gasket has been crushed and is now smaller than before.
These crushed areas will fail, usually within a short time of the over heat situation. It might be a few weeks later and you forget it got hot or you had a problem.
Causes can be a Lean Cylinder running very hot, excessive timing or retarded timing, plugged exhaust, see this on cat equipped cars, Air Pockets trapped in the engine, not common on small blocks.
It can also be a standard overheat while driving in traffic.
Cylinder temps can overheat without ever causing your temperature gauge to climb if you have lean burn, a leaking intake gasket can do this. It is localized heat in the combustion area.
The head chamber will get very hot and grow crushing the gasket.
If the gasket is crushed it had overheat, nothing else will do this. The burn through areas happen after the combustion starts blowing past the crushed areas.
The hottest areas in the small block are usually dead center between the middle cylinders where the exhaust valves are side by side.
The sad part is sometimes people repair the damage and then the same engine has the same failure again because the cause was not fixed.
If you look at the underside of the piston you can read the temperatures the piston was exposed to. All 8 should look nearly the same on the piston underside.
If you see gold color on nearly new pistons that is excessive combustion heat, dark brown or black show the underside is hot enough that oil is burning to the piston underhead showing excessive heat for extended periods.
Fel-Pro has a diagram showing how to measure a gasket to check for overheats on their website. All you need is a pair of calipers to verify what happened.
Then you get to figure out why. You can also take the diagram to your customer and show him proof of what happened and why you are repairing it.
Comes in handy in warranty disputes.
PS: The new MLS head gaskets (multi layer steel) will not crush, they will bounce back after the overheat. They cost about triple what a standard gasket costs but will take more abuse.
The negative is the gasket is traditionally the fuse, it blows before you break other parts. With an MLS in this engine you might have damaged a piston first and not the head gasket.
Mark
Last edited by Westlotorn; 05-19-2019 at 03:37 PM.
My 383 was blown like that when I got it, well, half as bad. Between 5 and 7. $38 in gaskets and an afternoon and back on the road it went.....for another 4 years, anyhow.
05-17-2019, 09:55 AM
