Valve recession?

[Katman]

Ive been told it can happen due to aggressive cam profiles where the cam follower comes away from the cam as the profile drops. This causes the valve seating speed to be excessively high.

Not sure if this is true or BS but another way of looking at it I guess.

Yeah, nah.

I've seen it in an array of motors - all with standard factory cams.

[Motu]

Valve problems before unleaded fuel was of two types, and depended on seat width. Wide valve seats...eg BMC, would burn exhaust valves and seats, and sometimes on 30 degree inlets (GM) we would see burnt inlets. Narrow seats (American) would have big recession on inlet valves - very much so on Holden engines. Nothing to do with fuel, very much to do with seat width, and possibly spring pressure. Calling Max Headroom, I know he was working in engine reconditioning shops in this era.

That recessed inlet valve is exactly what you would see on any red Holden engine...and Falcon, Chev etc.

[Katman]

.....and possibly spring pressure.

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?

[nzspokes]

Yeah, nah.

I've seen it in an array of motors - all with standard factory cams.

I had been told it was common on kawasaki gpz motors. But that could also been someone looking for a head job on my old bike. It ran fine.

[mossy1200]

I dont think it will be long before we get a double dip recession and you will see even exaust valves will be effected.

[Katman]

I've seen it often in the 4 cylinder 250 motors and always put it down to the sort of revs they pull but today pulled the head off a TL1000S that shows the same problem.

[mossy1200]

I've seen it often in the 4 cylinder 250 motors and always put it down to the sort of revs they pull but today pulled the head off a TL1000S that shows the same problem.

Would it make sence that a high reving 250 will strike twice as often but a large valve will strike less often but with a larger mass and heavier spring causing damage also.
It would also make sence that the seat is reinforced by the surrounding casting but the valve isnt so the impact shock is spread over a huge area. idd also suspect that due to the valve being a cheaper repair than a demolished seat that the modern design would be to make the cheaper to replace part the weakest link for warrenty reasons. Therefore aftermarket valve maybe stronger but when failure does happen later on the repair becomes more costly. This most likely wont happen untill the motor is shot though because your already on a second set of valves at this stage having replaced the original set.

[bogan]

Is it possibly something to do with the temperature gradient on the intake valve? Top is cold, bottom is hot, conflicting expansion from conflicting temperatures pre-stressing the valve and meaning it'll take less stress to deform it. Coupled with the other theories put forward it might explain why it happens to the intake and not so much the exhaust.

[Katman]

Is it possibly something to do with the temperature gradient on the intake valve? Top is cold, bottom is hot, conflicting expansion from conflicting temperatures pre-stressing the valve and meaning it'll take less stress to deform it.

Nice theory.

[Motu]

The engines prone to valve recession were of American design, with hydraulic lifters and so fairly slow on lift and drop, and low max revs, so spring pressures were pretty mild. The English who preferred wide seats had higher tuned engines with more aggressive cams and double springs as standard. Rather than spring pressure, it's loading on the seat itself... a narrower seat means more pressure on the seat, and less area for heat transfer.

In a modern engine the same things apply, but cams are much more aggressive in profile, lifting and dropping the valve at a much faster rate....the timings are much milder, but the valve opening and closing are very much faster - the valve is at max opening for a longer time.

[Ocean1]

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?

If the spring pressure is low the valve won't follow the cam when it closes, so there won't be any nice gentle touchdown, it'll slam. I hear that's a problem with the CRF intake, the fix there seems to be to use the OE valve and upgrade the spring.

It would also make sence that the seat is reinforced by the surrounding casting but the valve isnt so the impact shock is spread over a huge area. idd also suspect that due to the valve being a cheaper repair than a demolished seat that the modern design would be to make the cheaper to replace part the weakest link for warrenty reasons. Therefore aftermarket valve maybe stronger but when failure does happen later on the repair becomes more costly. This most likely wont happen untill the motor is shot though because your already on a second set of valves at this stage having replaced the original set.

In the old days you'd get guys cutting seats for well oversized valves. They'd often grind right through the hardened cast iron surface, resulting in rapid seat wear. Now almost everything is alloy with hard seats, and yes it makes more sense to sacrifice a valve than a seat. Actually with modern materials I don't think either should need looking at for at least 100k. Competition engine's a different kettle of fish.

[mossy1200]

Maybe the damage on the valve is happening in the BOOM not the impact so big thin wide valves and high compression are a factor or light thin small motor valves.

[pete376403]

Is it possibly something to do with the temperature gradient on the intake valve? Top is cold, bottom is hot, conflicting expansion from conflicting temperatures pre-stressing the valve and meaning it'll take less stress to deform it. Coupled with the other theories put forward it might explain why it happens to the intake and not so much the exhaust.

I'm wondering this as well, the inlet valve head gets cooled by the incoming air/fuel them exposed to combustion temperature thousands of times a minute, where the exhaust valve gets to stay at a more constant (although elevated) temperature .

[pete376403]

Maybe the damage on the valve is happening in the BOOM not the impact so big thin wide valves and high compression are a factor or light thin small motor valves.

But when the "boom" takes place the valve is on its seat, so supported physically and heat can transfer through the seat into the head.

[Katman]

Is it possibly something to do with the temperature gradient on the intake valve? Top is cold, bottom is hot, conflicting expansion from conflicting temperatures pre-stressing the valve and meaning it'll take less stress to deform it. Coupled with the other theories put forward it might explain why it happens to the intake and not so much the exhaust.

Actually.......


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



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