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Update on Spa Engine Failure

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Old 10-07-2017, 02:27 AM
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Now that the dust has settled, I thought an update on my problems at Spa would be in order. I've now had Ian's (AKA Fatbloke) old engine in my car for several weeks, and 700 miles in all seems OK (fingers firmly crossed).

It seems that one of the inlet valves on no. 1 cylinder dropped, causing carnage with the piston and head. When the engine was dismantled pieces of metal were found in the inlet manifold and in the other three combustion chambers, hence my assumption that it was the inlet side that let go. The forces involved were such that the underside of the piston head also has damage, where the valve head has cut a deep groove in the upper face. (see second photo.) The bore of no. 1 is badly scored, and the scattering of debris has meant that the other cylinders are also scored. Something also went through the side of the cam cover - I assume part of the valve stem. If anyone has wondered about the forces involved in an engine that revs to 9K, they only have to look at these photos!

What I don't know is the cause of this. It happened when I changed from 2nd to 3rd gear, so the obvious assumption would be a mis-selection to 1st. As it happened I had my Go Pro running, and looking at the recording there is no sudden spike in the revs. Also there was not a sudden jolt as you would expect. I think I can therefore rule out that cause, leaving a mystery as to why this happened.

Andy at Veysport did the engine swap for me, and that turned out to be a bigger job than expected. My car is DBW, but Ian's engine was pre-DBW. Several of the sensors needed changing, and my damaged cam cover had to be reused, with a patch welded over the hole. Even then the engine wouldn't run properly, until Andy discovered that the sender on the front of the crank was also different, and had to be swapped over.

As an aside, I wonder if anyone has a decent DBW cam cover they would like to swap for the pre-DBW unit from Ian's car?
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Old 10-07-2017, 03:05 AM
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Metal fatigue caused it. You're not the first to have a valve head fall off, for no apparent reason. It's too 'clean' a break for you to have bashed it into the piston by mis-shifting. It's gone all crystalline inside the top of the valve head.

The greater the time the valve spends slamming shut against its seat, with two strong springs yanking on the stem at 9K, the greater the fatigue at that spot. IIRC, the valves on these don't bounce until about 11K, but above 10,250 the stem fatigue starts getting a bit exponential and not good.

I thought that was what was happening to my engine whilst it was letting go, but all the stems and cotters were correctly-seated. So the rings probably picked up and snapped the con-rod instead.
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Old 10-07-2017, 06:01 AM
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I suspect the valve stem retainer failed. The valve then drops and the head gets smashed off.

the engine probably had an overrev earlier in its life. Check the other valve stem retainers for deformation or cracking and you'll probably find they're in a bad way too.
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Old 10-07-2017, 08:33 AM
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I agree /\ /\ strong chance of cracked retainer letting go. it is for this reason I changed all my inlet retainers to the later stronger ones, good insurance policy.
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Old 10-07-2017, 02:28 PM
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Originally Posted by tozerman View Post
I agree /\ /\ strong chance of cracked retainer letting go. it is for this reason I changed all my inlet retainers to the later stronger ones, good insurance policy.
What year were the later stronger inlet retainers introduced.....thanks.
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Old 10-08-2017, 12:48 AM
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You need them from MY 2004 onwards.
there is a very informative info on this in the UTH forum. Gives you part numbers and everything you need to know.

it is purely an insurance policy against a past over rev, or future one for that matter..
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