Crack in Piston on Rebuilt Engine -How Could this happen?

Our Series 2 Engine No. 7R 6436-9 developed an oil leak from the back seal and was stripped and overhauled by a professional engine builder 4 years ago and has not been in the car since as we decided to tidy up the engine bay while the engine was out and this led to a full bare metal restoration!!
The engine overhaul involved among other things:

  • Crack tested major components. Crankshaft found to be cracked – obtained replacement second hand unit and crack tested: OK.
  • Rebored block to +.020”
  • Surfaced block and cylinder head
  • Ground replacement crankshaft to .010
  • Renewed exhaust valve guides, inlet guides found to be OK.
  • Fitted unleaded seats to cylinder head
  • Machined all valve seats
    Included in the invoice was * Piston set 9:1 +020
    The engine was run for 10 minutes after the rebuild before being shipped with the stripped down car from NZ to Australia.
    It has since been in storage and in the care of a Jag expert here in Australia and was tested and “run in” on a dyno about 12 month ago.
    We picked it up and brought it back to our garage a few weeks ago.
    I did an external clean and manual turn over of the engine yesterday and decided remove the plugs and check the cylinders with an endoscope.
    To my great surprise and concern I did not like what I found on the top of No6 piston -see Photos
    No 5 Piston also has what looks like a small crack??
    Are these really cracks and how could this happen with only minutes of run time on the engine?

Where to now???

SWAG, fouled valves. If so could happen in a number of ways. Possibly turning one camshaft independent of the other while shimming?

No. Scratch that. Too tight a ring fit? @Wiggles ???

Might occur trying to force the piston into the bore with the top ring on that side grabbing the top of the block. What does the full cylinder wall adjacent to the crack look like?

Thanks for the quick response Nick,
So you think they are definitely cracks in the piston as I suspected?

I did not look so closely at the cylinder wall Terry when I saw the cracks but the honing marks are definitely visible and unscored in the other cylinders - I will have another look Thanks

Grahame, you were a little hasty hitting post on the initial post here, three photos weren’t finished uploading. Just in case you’d like to go back and fix this, you can select to edit using the three dots under the post.

It does look very much like cracks but spray it down with brake cleaner if you haven’t done that yet and check again, if they still look the same you can worry. It could be stains, certainly on #5.
If #6 is cracked I‘d call the guy who assembled it and maybe he can work something out.

By the way you might have needed new valve seats anyway but the originals were good for unleaded. All Jaguar engines have hardened valve seats that work fine with unleaded, no additives or anything.

There is no question that you can get thin grey deposits on the surface of pistons, that are hard to get off and that look like cracks under certain lighting conditions, and some of these photos, even of #6 look like that. It’s also difficult to envision how you could get a crack where this one seems to be. Odds are that it’s not a crack but the last photo sure looks like one I must say. Can you get the light on the borescope directly over it?

The more I look at it the more I think it’s just a deposit.

Yea: Id hit it with some Scotchbrite, to ensure if or if not they are cracks.

What I noticed is that the lines conform to the surface and even follow the little arrow. A real crack wouldn’t follow the machining grooves I suppose.
I can see why it would crack there and it’s a good time for manufacturing errors to show, but I can’t believe that it is actually cracks.

Could you try Magnaflux ? Its a two part system for finding cracks.

Magnaflux only works on ferrous metals…


It’d have to be Zyglo.

My first thought was that there seem to be an lot of deposits for a rebuilt engine with new pistons. I have no direct experience with what a new engine that has been only run for a short time should look like so perhaps someone with experience can comment. My gut reaction would be off with the head.

That is not an unusual amount of carbon for a short run engine: that said, for peace of mind, I would be pulling the cylinder head and very closely inspecting those “cracks.”

looks to me like there was some raw gas that landed on the piston and caused a stain???

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You fill me with a bit more confidence and I shall try to my best “key hole surgery skills” to clean it with some brake cleaner as suggested. I am not a doctor so the spark plug hole operating skills will be just another thing to learn being careful not to leave any instruments in the patient.

Very reluctant to pull the head on a ready to go tuned engine Wiggles but your sage advice is noted and if all else fails in cleaning and dye testing through the plug hole then off with the head.
I have lots of time to worry over it as the body is still a long way off needing an engine.