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Holey Christmas present


PeteStupps

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Merry Christmas all,

Just on my way back to London from the ancestral home in my 1300fwd, with 1800 troublesome miles since I rebuilt the engine. Before leaving I reset the valve clearances, which were all slightly slack (because it's colder than when I last did them?), and I had a slightly worrying rumble at about 1800 rpm. Assumed that was something resonating cos it was loud inside the car but couldn't really hear anything leaning over the engine. Plugs all looked ok, though 1 and 4 were just a bit on the pale side. 

Then cruising along at about 60 suddenly lost power and a mad racket was coming from the engine. Pulled over and engine stopped, popped bonnet up, smoke was pouring out of air intake, which turned out to be from rocker cover breather. Whipped plugs out, no.4 was very stiff and when I got at out it looked like pic below. Oh dear.

Looking through plug hole I could see exhaust valve was battered, and then disaster; massive hole in the piston. 

Any suggestions as to why this has happened?? I've never holed a piston before. 

DSC_5642.JPG

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Has it dropped  valve or are all the stems/collets still in place?

My Spitfire 1500 did this to us back in 2015...

 

DSC_0251.thumb.JPG.55dfad755426026db36093bf0dc3e847.JPGDSC_0243.thumb.JPG.31f2e6cb564a3f3538f1b48f73c527ba.JPG

 

No hole in the piston top, but it smashed the valve guide out of the head and broke the end off the rocker before squashing the valve and wrecking the end of the plug (similar to yours)

Good luck!

Sam

Edited by yorkshire_spam
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Thanks!

Valves are all still apparently where they should be, so not sure what's happened. I can just see the inlet valve head through the spark plug hole but not exhaust. Not sure if I should be able to see either of them. Turned crank over and rockers and valves are all moving as you'd expect. 

I am wondering if the rumble at low revs was any sort of clue, but can't think what. Oil pressure was very high, over 75 while cruising. Had only done 15 miles so oil wasn't hot. Wonder if exhaust valve seat dropped off and broke up or something? It'll be a while before I can take head off and investigate.

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It's not an interference engine though is it?? It's bog standard deck and head height. 

Slight update: there's no gap at all between cylinder 4 exhaust valve and the rocker. Suggesting valve head or seat is not in same position as it was 15 miles earlier. Inlet on same cylinder is normal, ten thou gap.

I can see a massive hole in the piston crown, can't get decent photo of it through the spark plug hole though.

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Ouch! Not good...... :pinch:

Sounds like it’s dropped the exhaust valve or seat seat - unless there’s a stray something gone down the intake........

You can maybe check by backing off the adjuster on no.4 exhaust and seeing whether the valve follows it up. Somewhat academic now......

Did you limp it home with no 4 plug disconnected?

 

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45 minutes ago, thebrookster said:

Did you have new valve seats installed when you rebuilt the engine?

Phil

Yes indeed, hardened exhausts. I am wondering if that is the culprit, especially as I was going to get same bloke to do my Spitfire head. 

Nick - no I didn't fancy 130 miles down the A1 like that, plus she wouldn't start when I tried anyway. I called the AA and waited in the pub. Halfway back now. 

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12 minutes ago, Nick Jones said:

You can maybe check by backing off the adjuster on no.4 exhaust and seeing whether the valve follows it up. Somewhat academic now......

I thought about doing that, but it was raining so decided it could wait! I'll be looking after the kids for a few days now so proper investigation will have to wait.

Having had some time to think about it I'm a bit depressed now. Initially it was an interesting short-term inconvenience but now the prospect of fixing it has sunk in. This was my only road-worthy car. Bah humbug

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Just now, PeterC said:

Plug earth strap detached> very retarded spark >preignition> piston crown under ev melts.

Just guessing

Peter

Not with mechanical damage like that to the plug..........

Only roadworthy car....... not any more :oNo dropping of the sump for a quick piston swap on a 1300 FWD.........

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Seems like quite a big gap for a simple missing seat...... but I suppose that the valve head is likely to be distressed as well...... interesting the the push-on cap stayed in place - collets often let go in similar circumstances, adding to the mayhem. 

Curious to see how things look with the head off....... Have some small-crank parts (Head,block, rods etc) here if needed.

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Thanks Nick, I'll post some pics as and when I get the head off (won't be for a few days) and we'll see what the damage is. If the block is scrap then I'll probably put a dust sheet over the car and ignore it for a year or two! 

I wondered why they used those push-on valve caps on these instead of the collet type. These are a lot easier to assemble. It's only got the standard soft springs, which probably helped keep things together. 

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I have an update on this sad tale, after getting a few cold hours in the garage. Got the head off, and there is most definitely a hole in the piston! Exhaust valve seat is still securely in position, well battered though. The exhaust valve itself is not entirely present and correct.

Looks like the valve head separated from the stem... I've heard of this happening but not seen it before. Is it likely to be anything other than a manufacturing flaw?

Here's some pics of the piston and bore:

 

IMG_8358.jpg

IMG_8359.jpg

IMG_8361.jpg

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.. and now some pictures of the offending valve, or what's left of it. 

I don't know if it's possible to tell what caused the failure - presumably not, as the broken end will have been peppered by debris after it broke. 

DSC_5711.thumb.jpg.ce4b671d89ba3bef16ec95a90eda822d.jpg

I did a side-by-side comparison with an old exhaust valve from the same engine, then against a new inlet valve just to see if it broke somewhere obvious. The old valve is too coked up to see but against the new one you can see the break lines up with the weld (?) line. Are the stem and head welded together? 

IMG_8366.thumb.jpg.51bf4dfc4d5385c5bc9c6d1a41c446f1.jpgDSC_5720.thumb.jpg.7b6a6b8c857c1870deacc4b0ce1cc5dc.jpg

 

Lastly here is a close-up of the stem end where it broke. 

IMG_8386.thumb.jpg.9b473e16eecc53a9efd7a08b2f4d811a.jpg

Edited by PeteStupps
Edit: one of the images
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Ouch......  The piston is clearly very dead.  The valve likewise.  That's a very clean break on the stem...... presumably showing that the head was welded to the stem and has separated.  Pretty clearly a manufacturing defect IMO and I'd worry about the other three......

The head might actually go again.  It's ugly, but provided the basic shape remains, and any sharp edges that might glow and cause pre-ignition are dressed out, no reason why it shouldn't function.  The only part that gives me pause is the seat insert....... something to discuss with your man perhaps?

Your 1500 head would need a very big skim to get the CR up on a 1300.  I have a 13/60 head here but it's very well used......

Nick

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