I have a 401 nailhead out of a Invicta that I've been told has been rebuilt within 10K miles It has .030 over pistons and the crosshatch is still plainly visible with no ring ridge I've got scoring on all pistons to a varying degree and all scoring is on the thrust wall. (worst would be about a inch across on the thrust face) Cylinders are marked a little on the worst one but nothing that catches a fingernail either on the pistons or walls I checked clearances on the worst piston and cylinder (pictured) and I can insert a .003 feeler blade between the skirt and the cylinder wall (.004 won't go) Is this common with the bigger skirted pistons? If it was a smallblock I'd be tossing them for sure but I'm not so sure on the nailhead Thanks Jeff
Are the pins offset on the nailhead? some engines have offset pins to lesen the load on the walls and pistons, it ***embled backwards then, ouch. Also is there any signs of overheating, that causes some extra squeeze.....also do the pistons have the cast in steel straps and cam ground (oval) or are they round...if round they need more than .003" Just some thoughts.
Do you know if they're cast or forged pistons? Stuck back in a corner of my giant brain I seem to recall that .003" might be a bit loose for some cast pistons but maybe a skosh tight for some forged pistons. Is there any piston noise when the engine is cold started?
3 thos. is a little tight. I would check ring end gap. A tight ring will cause all kinds of strange stuff. Because you don't know what happened in the first 10,000 miles, you're kind of at a disadvantage as to what happened. It may have overheated, it may have been fired for the first time and just left to idle where no oil was thrown on the cylinder walls. Keep checking all the clearances and see if the con rods are in the right way. Happy hunting.
.003 is fine for a cast piston. I've bored many blocks. The clearance will be on the piston. Example, a .030 piston for a 4" bore will measure 4.028. A feeler guage will get you a rough idea of the clearance, but you really need to have it measured with a micrometer. Scored pistons could be from: 1)Scored piston pins making the pistons rock. 2)Not enough clearance. 3)Overheating. 4)Dirty ***embly. 5)Rods installed wrong. If they have oil holes in the rods they need to be on the cam side. 6)Bore could be tapered. Some blocks are thin in the cylinder walls. You need to let the cylinder cool down before you measure it. They shrink in the middle if the machinist uses to much pressure, not enough coolant, or to fine of a stone. Good reason to have the cylinders measured with a mic. 7)Sugar in the fuel(make any enemys?) It may help if you can post a larger picture. Hard to tell. Jeff
I fixed the picture so you can see that piston better I did get my bore gauge and the taper on the worst cylinder is less than a thou I don't have a micrometer here that goes up past 4 inches so I have to wait to check that The rods are installed correctly and I don't have this damage on all pistons I'm wondering if it wasn't a poor breakin/dirty ***embly combo of the 2 Now for the $500.00 question I scuffed this piston with scotchbrite and 90% of the mark came off with just a few scratches left over Would you use these again after running a ball hone through cylinder? Keep in mind that this engine spun over as smooth as silk wwhile rotating it on the engine stand Thanks for the help Jeff
Hell yes I would use them again. If they are smooth as you say and you honed the cylinders. Most likely it was from a diry ***embly process.
I would sand the piston skirt with fine maybe 400 grit on a peice of wood. You want to make sure it didn't start transfering metal. You will know right away when you sand on it if you had cast or steel embed into the aluminum. If you did, take a knife and s****e it away. Mic the piston to make sure the skirt is still in good shape and not colapsed. It may help to knurl all 8 pistons. It helps hold oil between the piston and the cylinder wall. Just have a light knurl put on and file the knurl away untill the piston is a slip fit. Good Luck!! Jeff
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