Exhaust valves - latest

[david.g.chapman]

I have extracted the exhaust valves on my twin cam engine, and have noticed pitting on the valve face when grinding them in. The pitting shows up as 0.5 to 1mm diameter dark patches on the ground valve face.

One of the four valves was leaking.

These valves were new 3000 miles ago, and have been running with new bronze guides and 3 angle valve seats (suitable for unleaded).

Is this usual, or an indication that the valves are getting too hot?

Valve clearances were OK.

Any help would be appreciated.


Dave Chapman.

[rgh0]

The pitting sounds excessive for just 3000 miles. Sound like your mixture or ignition timing is out and leading to buring of the valve sealing face. If you are seeing it on all 4 exhaust valves then probably not related to assembly ausing leakage problems.

If not geting any pinking and have bronze guides then less likely its ignition timing. The bronze guides conduct heat better so even if running hotter due to to advanced timing less likely to burn the valve faces.

I would guess your mixture is too lean exposing the exhaust valve seating surfaces to a hot oxidising exhaust mixture that is buring them rapidly. What colour is your exhaust pipe is it black or a rusty brown colour ?

Rohan



Rohan

[david.g.chapman]

The exhaust is black, or very dark grey.

The exhaust valve heads (the side that faces the piston) were a sandy colour in the centre, with a darker rim. As I cleaned them, the deposit looked like fine sand as it came away.

The valve face was black. The pitting showed up as I ground them.

The valve seats were not pitted at all.

Dave Chapman

[rgh0]

Dave

None of the colours decribed seem to indicate overheating or excessively lean mixture. If the mixture was excessively lean I would have expected a brown rusty exhaust pipe on an unleaded car. But diagnosis by email is always an uncertain proposition.

I guess I would just rebuild te head making sure I got the valves seated properly and with the right spring pressures and cam clearances. I would then ensure I got the timing and mixture right once I had it running including a test on a rolling road dyno to ensure its right under full load.

If I was not total confident to the quality of the exhaust valves I would also potentially replace them with new ones of known good quality. Always possible to get a set of dodgy valves.

Rohan

[type26owner]

I would be suspicious that the contact area of the valve head to the seat was too narrow. That is the pathway by which the bulk of the heat is extracted from the valve head. The three angle seat shape is way overblown from the actual performance to be gained but has a greater risk of burning up the valves. I always go for maximum contact area to the valve head and then if there is any extra seat area that sticks out it gets chamfered only. The difference is in the 1-2% range in power output at high rpms and can only be detected on a dyno. Not a practical choice if you're mostly tooling around on the public roads IMHO. Don't let some 'expert' lead you down the wrong path to burnt valves.

Is there any difference in the carbon deposits on the tops of the pistons? That's where you get the best indication something is amiss or not.

[steveww]

I second Keith on the whole 3 angle thing. I did not bother with 3 angles when I rebuild the head on my S4. The seat area was pretty thin anyway with out making the situation worse. As Keith says there is so little to be gained with 3 angle seats for road use it is just not worth the effort. Leave it to the racer boys who rebuild the engine every season.

[type26owner]

Hey, I forgot to add this bit too. If you make full contact then the normal recession of the valve into the seat is slowed down to the point you'll never have to reshim the valve clearances. Just set them all to the high side of the tolerance range to begin with. The waterpump will fail first.

[david.g.chapman]

Thanks a lot for your inputs.

The piston tops were black, witha small clean area near the inlet valves. There wasn't much deposition since the last time I had the head off!

To get rid of the pitting I had to grind away for quite a while. I was left with a 2mm ring of grey with the odd tiny defect of less than 0.5mm. This should give me more area to cool the valve. The valves then passed my petrol test (filling the combustion chamber with the head upside down).

The valve clearances went down by 6 thou or so by the time I had finished. Still plenty of valve margin left though. I did not want to go further and reduce the spring pressure.

As for spring pressure I have no objective way of testing. You just need hefty thumb and finger pressure to move the valve down when assembled. The free length of the springs were all the same - about 1-1.5 mm less than the new length.

I will advance the timing as much as I dare, and go for it unless the springs are an issue.

Dave.

[type26owner]

The clean area on top of the piston adjacient to the intake valve is normal. There should also be an clean band about 1/2" wide all the way around the OD of the piston if the squish deck clearance is correct. If that is not the case then measure what the squish deck clearance actually is by what the manual instructs. Also the head gasket thickness. Having too wide a squish deck clearance is bad and may be your actual problem.

The hemispherical chamber design has fallen out favor for good reason. It's prone to detonate or auto-ignite on the lower octane fuel we have available today.

[charles jackson]

Hi David,

silly question I guess but are you running an air filter?

[david.g.chapman]

I have the original air filter box fitted and use the normal paper filter.

For belt and braces I think I will use an octane booster. I do not want to pull the head again in the near future!

Thanks everyone!

Dave.