1972 r107 350SL Inlet valve

[Isdyldan]

Hi, please could I get some advise before I start taking things apart? I have had this car about a year, and it had been stood for many years - maybe 15. It starts and runs, but I have a miss fire on cylinder 5. A compression test showed zero compression. I hooked up an air line to the spark plug hole and with both valved supposedly closed (removed valve cover to check lobes) there is air rushing out the inlet manifold. So it seems like the inlet valve is with stuck open. The valve clearance measurement is fine, and the spring looks just like all the others. So I wondered, is there a possibility it could be stuck open or is it more likely to be broken/damaged please? If there is a chance it is stuck open, is there a way of unsticking it without removing the head please?
Thanks, Dylan

 

[Uncle Benz]

Could be worth finding someone with a borescope to have a look inside via the spark plug hole. You’ll see right away if there is a piece missing from the valve

 

[Isdyldan]

Thanks - yes that would be a good idea. I have a cheap one somewhere but not sure if I'd be able to aim it up to view the valves after going down the plug hole. If I can find it I'll have a go. If the valve is knackered it will obviously have to be head off. Is it at all possible though that it could be stuck open, or would that automatically show up as a very large rocker gap and compressed spring please?

 

[star]

If it was stuck open the valve clearance would be too big. Early V8 needed a tool to set shim to get the hydraulic tappet unit with range.

 

[Isdyldan]

Yes I thought that would be the case. This isn't hydraulic actually though - manual adjustment. I guess its going to be head off then. Or just run it as a V7.....!

 

[Uncle Benz]

Thanks - yes that would be a good idea. I have a cheap one somewhere but not sure if I'd be able to aim it up to view the valves after going down the plug hole. If I can find it I'll have a go. If the valve is knackered it will obviously have to be head off. Is it at all possible though that it could be stuck open, or would that automatically show up as a very large rocker gap and compressed spring please?

my borescope has a tiny mirror attachment at 45 degrees to look up at the valves

 

[Isdyldan]

Well I found my borescope and it fits down the hole, but not with the mirror attached - its less than a mm too big! Does anyone have any other ideas please, or is there possibly a procedure printed or video'd anywhere for head removal?
Thanks

 

[LostKiwi]

Looking along the line of valve springs a stuck valve will show as lower than the rest.
You could try using a drift to operate the valve to see if it returns when allowed to snap back, then try air in the cylinder again. Do not hit the valve spring cap - only the valve itself - if you do you may release the collets which could allow the valve to drop into the cylinder.

 

[Isdyldan]

Looking along the line of valve springs a stuck valve will show as lower than the rest.
You could try using a drift to operate the valve to see if it returns when allowed to snap back, then try air in the cylinder again. Do not hit the valve spring cap - only the valve itself - if you do you may release the collets which could allow the valve to drop into the cylinder.

Thanks - I'll try that. It certainly could be a disaster if the valve fell out! I'll study some diagrams and photos before I go hitting anything.

 

[LostKiwi]

Thanks - I'll try that. It certainly could be a disaster if the valve fell out! I'll study some diagrams and photos before I go hitting anything.

Don't hit directly. Use a drift sat in the valve and tap that. You could use the rocker perhaps.
Looking at pics online it doesn't look easy although if you can remove the cam follower it should be possible.
If the valve is sticking it will be quiet on its rebound. If it's not sticking there should be an audible 'thock' as it returns.
For it to have zero compression I'd expect the valve to be open by a mm or more. That should be visible by looking along the head at the spring caps. Perhaps try laying a ruler along the spring caps. They should all be almost the same height - at the very least inlets same heights as each other and exhausts same height as each other. One significantly lower says stuck valve. One significantly higher says burnt valve (though this is not definitive as a valve can be burnt and normal height).

 

[Isdyldan]

Thank you - I will try all you have suggested this afternoon. My concern is that there is a valve clearance on that inlet valve, but slightly less than it should be - 0.05mm instead of 0.1mm, which could therefore indicate a burnt valve. I thought it was exhaust valves that normally get burnt.

 

[LostKiwi]

Thank you - I will try all you have suggested this afternoon. My concern is that there is a valve clearance on that inlet valve, but slightly less than it should be - 0.05mm instead of 0.1mm, which could therefore indicate a burnt valve. I thought it was exhaust valves that normally get burnt.

Yes normally it is exhaust valves but any valve can burn if it leaks (for example due to incorrect clearance)

 

[Isdyldan]

Oh dear. I tried adjusting the clearance but there's a handy tool needed. I have managed it in the past on another car with a 17mm spanner, but this one is really tight and I don't want to round the nut.

 

[Isdyldan]

I got another endoscope with a mirror but the mirror looks sideways and not up, so no good. I managed to get it looking up by taping a bit of string to the end, and am fairly sure I can see a spring or a bit of wire jammed in the valve. The image here is not good as I needed 3 hands! You can just make out the blurred wire in the top right corner - it was clearer honestly! So I just wonder how to get this out. If I can get it to fall down I could remove with a magnet.

 

[Uncle Benz]

I can see what you mean. Could well be something stuck in there. I can't recall how much of a view of the back of the valve you can get with the intake manifold removed. If you rotate the engine to get the valve open you might be able to dislodge it

 

[LostKiwi]

If I didn't know better I'd say that was the spark plug electrode but it can't be as you used the spark plug hole to get the bore scope down!

I'd be inclined to remove the inlet manifold and look down the port. You may be able to remove what ever it is using a long pair of pliers or magnet. I doubt it will go down the bore (and you don't want it to really as it may be a devil to remove if it's anything but straight).
It's clearly pretty well wedged at present as compression testing didn't dislodge it.

I'd also be inclined to put the bore scope down the inlet to see if anything is visible before trying to remove it just to get a better idea of what you're dealing with.

 

[Isdyldan]

Good idea - I'll try access down the inlet manifold. Definitely something in the valve - here's a better photo:
Dylan

 

[LostKiwi]

When you do, try shining s torch down the spark plug hole too. See if it reveals more detail.

 

[Isdyldan]

I wonder if it may be a bit of the piston broken off and stuck up there - this photo looks like the edge is damaged. I'll have a go at removing the bit, but I think ultimately I'm going to have to prepare myself for removing the head. This car is a long term project, and although not planned, I'm quite looking forward to taking it apart. I've only taken heads off an old ford and an MGB before - not an OHC. Dylan

 

[LostKiwi]

That does not look good.
I'd be removing the head to check it properly.