Given some of the dumbsh!t things I’ve seen people do, I have no doubt it could happen.
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Hello Paul,
I’m often asked about ways to “Fool Proof” CNC machining applications and programming. My reply is fairly consistent in that, “you can try the best you can, but there are some very talented fools walking amongst us”.
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…just stop drill the crack and forget about it…
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Now there’s real talent. 
Regards,
Bill
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One picture of #6 would show a crack in the dome as well. Some pics show a discoloration between the inside / outside of the presumed crack. I tend towards a peeled cracked lacquer, with black residue along the line. Also for a circular crack would there even be enough room for the crack to be of so even width all around? If you can get a micrometer down there you might could measure the thickness of the lacquer, or no height difference at all.
Ha Ha Eric-
…just stop drill the crack and forget about it…
Only the old school mechanics/ welders may know what you are talking about. It is very effective but a little difficult to do in this case.
The more I look at one of the photos the more I think it is a coating that has an edge showing as a “crack” - Lets see tomorrow.
Negative deck height (piston proud of block) and overhanging head gasket?
-Ken
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Both possibilities, if the block was decked.
Here is a video from Youtube of a guy putting pistons in a '69 E-type. Has several close-up shots of the tops of the pistons. Something to compare yours with.
Would you learn anything (??) by taking a wire coat-hanger. Cutting it to maybe 8-9 inches in length, then sharpening one end to a point. Inserting in the plug hole and dragging the point end lightly across the “presumed” crack. Both directions. Feeling for elevation changes in either direction. Or an actual crack if the point penetrates. SD Faircloth
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If it was a crack there wouldn’t be any measurable difference in height, and if it was a deposit, good luck measuring either.
It’s a deposit! What’s the opinion of the guy who assembled it? Do what he says…
From experience it’s really hard to crack a piston. Pistons proud of the deck is the only theory that would make sense to me, but without trace on the crown?
It’s easy for me to make suggestions from halfway around the world, but with a newly rebuilt engine that is mounted on an engine stand you could have the head off in a half-hour. Turn the crank to TDC, take off the cam covers and remove the accessible sprocket bolts. Rotate the engine another turn, take off the other bolts and knock the sprockets off the cams. Removing the oil feed bolts should only take a minute since the firewall is not in the way. Unscrew the dome nuts and the hex nuts up front of the head. Loop a sling around the camshafts and lift off the head assembly complete with manifolds and carbs. Then you can have a good look and do all the suggested tests easily. Last winter I took the head off my MK2 just so I could paint the valley. It’s not that big a deal.
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The more I look at these, the less I’m convinced they’re cracks, and the more convinced I am of them just being odd carbon marks.
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Seriously?
Turn the engine to tdc, then another 360° turn, loosen camshafts bolts, another 360° turn so the cams are at tdc as they are removed. It’s not a big deal but still a last resort.
Compression test done? Be one of the first things to check. 9:1 pistons s/b 180 psi. Do you know the manufacturer of the “piston set”. Are you able to get the dyno print out. That will give you a very good idea how it was run especially rpm’s run. If everything else shows quality work on the engine talk to the builder. From what you say it sounds like the builder knows what he was doing. Talk to them.
Myself I highly doubt you have cause for concern.
mmm. I once had Hepolite pistons hitting the head at 7200 and above. Just needed about .010 more clearance. Didn’t crack the pistons. No, not on the Jag.
I like the carb cleaner/solvent/stick/rag suggestions, do that, take a second picture and see if the “crack” has disappeared.
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Back when I was a novice (butcher) I managed to crack a piston skirt on a brand new piston. Went back to the engine builder who provided the parts and he drilled the end of the crack, as suggested. I drove that car for a year or so, and then sold the engine to a stockcar racer; that engine won the championship 2 years running still with that cracked piston.
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On my GT5 Datsun1200, which regularly saw 9800+ RPM, I had to figure in rod stretch: fortunately, Carillo specs that out, so I was able to avoid any problem!
As it was, I was running 13:1 Cosworth pistons, and about 0.040" V/P clearance… no room for error.
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Thinking about this, before I went so far as to take off the head, I’d position #6 midway in the stroke then use a dental spatula through the spark plug hole to scrape several times across what appears to be a crack at the front of the piston where the arrow is, then take another look through the borescope. It should then be plainly evident one way or the other.
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Exactly what I was suggesting with the pointed coat hanger wire. SD
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A engine that someone else has rebuilt is just that; an unknown quantity.
As above if someone has forced the pistons in on rebuild, because for instance they haven’t gapped the rings properly, it’s easy to crack the piston land. Similarly, if they’ve broken a ring on assembly. Or just not used the ring compressor properly.
In this whole thread no one is convinced one way or the other and there’s a lot of opinions. I don’t think the pictures are clear one way or the other.
I don’t think that you have any option but to take the head off and have a damn good look at those pistons.
If they are cracked they’ll fail, bits of piston will get jammed in your valves, the bores will be damaged and you’re up for a total engine and head rebuild.
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