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Exhaust valve/piston interference--2.5 NAD

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Anyone ever seen this before or know what caused/cures it?

BACKGROUND--One very cold morning this past winter I jumped into my rover and (without much warm-up) began driving home. After a quarter mile the 2.5 NA diesel began tapping pretty loudly (taps in phase with engine RPM). No change in performance, just a loud tapping. I stopped to investigate, couldn't find anything obvious and limped 2 miles home. Noise cleared up after another 1/4 mile. Timing belt was changed 10,000 miles ago.

I figure it's time for an engine overhaul anyway, so that's what I did. This engine had been overhauled at some point by the MoD and had all cylinders sleeved. I just got the head back from the machine shop. New valves, guides, seals and the mating surface machined flat. Waiting for the cylinders to be bored +.020" and new pistons & cam bearings to be fitted.

While waiting for the block to get back from the shop, I've finally had time to closely examine the old pistons and I've discovered that the exhaust valves have interfered with the pistons (or the other way round), as shown in the photos. Other than limping home I hadn't driven the truck since. Looking at the second photo of the uncleaned piston with all the soot under the valve strike area, as well as the circumference of the valve in the first photo, it would seem that this was not an ongoing issue, but rather something very recent, so I suspect that was the cause of the noise.

So... back to my question...Anyone ever seen this before or know what caused/cures it? Having discovered this, I'm now worried that machining the head mating surface flat will exacerbate this problem.





[EDIT:] Just remembered that when I replaced the timing belt 5,000 miles ago that somehow I ended up 1 tooth off on the cam wheel. Cranked it--wouldn't start, disassembled and corrected. Ran as good as it did prior after that. Could this camage have been caused 5,000 miles ago by being only 7.5 degrees off on the cam? [END EDIT]

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Several things can cause that - incorrect tappet gap (too tight), slack timing belt, excessive head/deck skim. Worse things - crank or cam sprocket loose/worn woodruff key. Could be sticking valve, but for more than one to do it at the same time is a bit unlikely. Valve caps is another possibility (jumpimg and the edge suddenly greatly reducing the tappet gap to a minus figure). Loads of possibilities really.


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I now suspect that being off (very briefly) by 1 tooth on the cam wheel when I replaced the timing belt 5000 miles ago may have caused this. I can't find a chart showing 2.5 valve opening/cam curves to confirm this though. Is it possible that the cam lagging by 7.5 degrees for a couple of cranking revs caused this? The damages to the piston tops are only a few thousandths of an inch deep.

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