Valve recession?

[pete376403]

Actually.......


.....nice theory, but.......



.....if it were that simple why wouldn't the problem be more widespread?

Maybe it is? How many car motors ever have the head off before they finally make it to Pick-a-Part? When I was working as a mechanic (Mitsi/Chrysler dealership) a valve grind was par for the course at around 50,000 miles, especially on Hunters and Avengers.

High performance bike motors, such as motocross singles are expected to be rebuilt every ?? hours, so maybe the valves get replaced before recession is obvious

[Katman]

Maybe it is? How many car motors ever have the head off before they finally make it to Pick-a-Part? When I was working as a mechanic (Mitsi/Chrysler dealership) a valve grind was par for the course at around 50,000 miles, especially on Hunters and Avengers.

High performance bike motors, such as motocross singles are expected to be rebuilt every ?? hours, so maybe the valves get replaced before recession is obvious

Thing is, it's not something that occurs unannounced.

As the valve recesses the valve clearance reduces to the point of affecting the running of the motor.

[bogan]

Actually.......


.....nice theory, but.......



.....if it were that simple why wouldn't the problem be more widespread?

Yeh, that was just the theory of why it happens more often on the intake side. Other theories like valve bounce or whatever would still need to cause a large stress to begin with.

[gammaguy]

Does anyone have any interesting theories as to what causes valve recession?

Interestingly, I've only ever come across it occuring on inlet valves.

times are tough,the recession affects everyone,even valves.

inlet valve seats are often softer,so recess more readily

exhaust valves protrude into the hot gas stream so it is more likely the valves themselves which erode

in my time I have seen both types,inlet and exhaust,valve and seat.

the causes are many and varied,but poor maintenance is the root cause of most of it.

[Crasherfromwayback]

I find myself wondering whether inlet valves are made with the same carbon content that exhaust valves are.

Is it happening with titanium valves at all?

[Katman]

Is it happening with titanium valves at all?

Not that I'm aware of.

[Crasherfromwayback]

Could the lead in the old fuel be softening the impact on the inlet valve? And if they're built with the same material now as they were before not coping?

[Paul in NZ]

Now that's interesting.

That's the theory that I heard today that prompted this thread.

If spring pressure had dropped though, wouldn't it result in less hammering effect?

Not really - the spring pressure drops and then the valve can bounce when it closes resulting in multiple closures. In (say) a triumph twin the valve timing is such that most of the noise you hear at idle is reverse pressure on the timing gears as the cam goes over centre before TDC so there is a lot of spring pressure. It was always the thing back in the day, put in a hot cam and stronger springs etc???

There is a fair bit of metal crashing into stuff....

I agree with Motu. Old BMC and english stuff you were forever grinding / replacing valves. I don't see it as much on my Triumph now. Maybe due to the fuel? However the 98 leaves a horrible carbon mess in the head which might help coat everything (I dunno)

[ducatilover]

Could it also be a clearance issue? When I pull my valves (if I can rig a spring comp.) I'll be able to tell you, one of the exhaust valves was too tight when I got it, the other three valves too loose, so one valve has been running hotter.
With the 250cc motors, most people tend to avoid doing clearances because most n00bs are not willing to pull the cams and measure shims, where-as with a mototr like my 600, you can tell it has been maintained correctly, 73ks and the cearances were still spot on, same with my CB400 super four, both of these not exactly low revving motors 5-7krpm at 100k, so doing lots of revs everywhere, like a 250 doing 8-9krpm at 100.

Should I pull my valves out to see if the clearances has made one wear more/abnormal?

[Katman]

I discovered the problem on this motor during the valve clearance inspection.

Seven valves were within spec while one was too tight.

After going down three shim sizes and finding the clearance still too tight I realised that there was something amiss.

After pulling off the head with the tight valve it turns out that the other inlet valve has also started to show signs of doing the same thing (even though it still measured within spec).

One would assume that the inlets on the other head will be likewise.

[ducatilover]

I discovered the problem on this motor during the valve clearance inspection.

Seven valves were within spec while one was too tight.

After going down three shim sizes and finding the clearance still too tight I realised that there was something amiss.

After pulling off the head with the tight valve it turns out that the other inlet valve has also started to show signs of doing the same thing (even though it still measured within spec).

One would assume that the inlets on the other head will be likewise.

So each could be a possible causation for the other? Valve wear obviously = clearances get tighter and if one had set the clearances wron = valve wear.
This orcourse doesn't mean much as that shouldn't really be limited to the intake... unless bad mechanics don't believe in exhaust cams?

I reckon Bogan's theory is pretty on to it. Mine are just ramblings of an idiot on meds

[bsasuper]

Having worked in the engine reconditioning industry, its really the quality of materials used, and accurate machining.If the seat/valve isnt machined correct it will wear faster, subaru was a great one for having out of round seats from the factory.Of course when unleaded fuel was cut, there was no lubrication and cusion effect of the lead, and the lead substitute which was usually potassium, did not work well.One bright spark kiwi came up with the fuelstar tin treatment canister, which did work but was priced a bit high.Ferrari was using hollow valves filled with sodium decades back to keep the heat down, correct spring pressure is vital to keep valve rotation to a minium as well.Interestingly diesel inlet valves suffer even worse than petro as the fuel in injected direct to the cylinder so the valves dont get the cooling/lubrication effect of the fuel,I wonder how the direct injection petrol engines will last?Camhaft lobe wear is more of a problem these days.(who measures lobe wear when doing a shim check).

[Subike]

after reading this thread, and understanding most of it
I wish to hear opinions about using different grades of petrol .
IE , Regular verses Higher octane.
I understand lower compression engines do not need the high octane fuel, to operate efficiently.
that a low compression motor will only need 91 octane to do its job properly.
If a person was to use 98 octane in the same motor,
would there over time be a detrimental effect to the valve gear and combustion chamber of the low compression motor.
My thinking is that unburnt fuel could build up as carbon, or sooty deposits, eventually lowering the efficiency of the engine.
Thus increasing engine wear, by "washing" the cylinders of lubricant and or clogging the exhaust with unburnt fuel deposits.
Decrease engine power, and lower fuel economy being the result of using the wrong fuel for the engine.
Opinions please?

[Katman]

My thinking is that unburnt fuel could build up as carbon, or sooty deposits, eventually lowering the efficiency of the engine.
Thus increasing engine wear, by "washing" the cylinders of lubricant and or clogging the exhaust with unburnt fuel deposits.
Decrease engine power, and lower fuel economy being the result of using the wrong fuel for the engine.
Opinions please?

Interestingly, the TL motor that started this thread is showing signs of dirty (but not at all recessed) exhaust valve faces.

[bsasuper]

Carbon buildup in the combustion chamber and on the back of the valve, is two things, poor quality fuel and oil from the engine.If you want to see less/no carbon buildup , use caltex fuel.