Worn piston rings?

I bought my 68 E type at a salvage yard so I don’t know it’s history. Also several things around the engine were missing and I have never had the engine running. From several clues, my guess is that the car may have spent around half its life unused sitting in a garage or barn.

Early on I tested cylinder compression and found only about 80 psi on 4 cylinders and even less on two. As a minimum, I was sure I had some bad valves so took the head to a good machine shop and had it rebuilt

The head is now back on but I haven’t connected the cams yet so can’t test compression. So I did a leak down test and found lots of leakage but none coming from the valves…
The obvious conclusion, I assumed, was that the piston rings are worn so I took off the head again and removed the pistons.

Everything I can see now and/or measure is in very good condition;; pistons, cyl walls, bearings and crank.

The point of this ponderous story is to ask how can I tell if the rings are bad. I put top ring in the cylinder and measured the end gap, which is between 0.80 and 0.90 mm. Is this a good thing to measure and what do these numbers tell you?
Of course I have the apparent blow by But what else, if anything, can I measure?

It seems to me you have tested for “ovaling” and other types of wear in the bores. Similarly the pistons. This done and satisfied, I would probably spring for a new, good quality, piston ring set. An XK engine factory manual would have details of acceptable dimensions.

Always an interesting question for me. Given the history and lack of it and assuming the objective is to rebuild/commission the car so its a reliable driver, and not a chequebook restoration then I would want to strip the engine and confirm the condition of all moving parts especially timing chains tensioners, oil pump , etc. then and importantly remove all core plugs and flush both water and oil ways. If the pistons etc are in good condition and the compression ratio is what you want then rings as a minimum and a hone the bores. Bearings must be replaced.
I would not be able to rest or use the car if I didn’t have confidence the basics had been attended to. If its a ‘fix and sell it’ that’s a different spec.
do it once, do it right.
JMHO
best regards

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I too would (flex) hone, flush the oil ways, crank shaft and waterways thoroughly. That is an engine out operation if you don’t want to suffer too much.
Then inspect all the bearings as a minimum, as you did, oil pump too etc. and put it back together. You will have a known healthy engine that’s not leaking or overheating and you can drive it as it was intended to. Do the basics now and you know it’s good.

The ring clearances tell me nothing but I would gap the new rings to whatever is specified and then drive it. Were the rings sliding freely?

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.031" to .035" ring gap is too much…should be a bit less than about .020"…probably bores are worn.

Thanks for all the good advice on what I should do to recondition the whole engine while it is open and I will act accordingly.

My specific question was how to measure if piston rings are worn and I believe that Lee answered that, although indirectly. My belief is that the over large ring end gap means either the bore has enlarged, as Lee suggests, or that the outer circumference of the rings has worn.
And in my case the bore has not worn per my messurements.

Does this make sense to you?

Make sure to check ring land wear: any perceptible up/down motion of the ring in the lands pretty much means new pistons.

Also, unless the block is bored, DO NOT use chrome top rings: they likely will not seat properly.

You could use an internal bore micrometer to measure the bores in various positions and compare the readings to the specifications in the workshop manual, if you do this correctly it can indicate any ovality and determine if it is just the rings that are worn or whether it needs a rebore and OS pistons and rings.

… and taper, also very important.

I have previously measured cylinder diameter at top middle and bottom and at 0 and 90 degrees. All the results are, within the accuracy of my measurements, virtually identical to original 92.07 mm.

I will try to check ring land wear but my priority is finding of rings are worn as I suspect.

One way is to use actual ring gap versus assumed new (as Lee quotes) and, assuming bore diameter has not changed, I calculate wear of ring has only been 2 thousandth of an inch. This doesn’t seem to me enough to cause the very poor leak down results I previously found . Does this make sense?

And is there another way to measure ring wear? Can I measure horizontal width of the ring (not thickness) and compare this with new ring?I

All this is to try to explain low compression pressures I previously measured plus bad leak down results. I don’t want to fix something which is not broken.

Truly quite simple: if it’s not going out the valves, or a big hole in the piston, it’s the rings.

Have you test fitted the pistons without the rings…this’ll tell you quite a bit about wear patterns and excessive clearance issues. Of course, you can also just measure the piston diameters but there is nothing like a physical “slop test” of a piston in a bore.

When i took my block to my engine machinist he checked the pistons in the bores. They were a bit loose at the bottom of the stroke. The bores were not that bad but the skirts on the pistons were worn. He advised a new set of +20 pistons and a re -bore to suit them. Each bore was machined to suit each individual new piston. New rings had to be sized to each bore (18 to 20 thou gap from memory).
Well worth doing for good peace of mind.

Generally, ring end gap is ±0.004"/inch of bore diameter.

Another explanation could be rings stuck in the piston grooves which you would have loosened when you took them out.

Another explanation could be, if the engine has been overheated a lot in the past, that the rings have lost some of their tension.

So the bores are ok top bottom right angles etc. No ridge at the top
Were the gaps correctly spaced around the diameter?
The Pistons measure ok top and bottom
How much leakage was there?

My next step would be to reassemble the engine with new rings and a hone and retest the leakage to see if there is an improvement

If you can rig up a starter and battery to the engine you could also do a compression test.

If it’s all ok then you can rebuild the rest of your engine properly.