Tech question. Why dont exaust valve stems run dry/coke up

[barana]

Ok, so my question goes something like this: if we know for example that we need to add a bottle of moreys to properly lubricate intake valves when a gas conversion is done, we are told this is because a gaseous fuel is insufficient to lubricate the intake valve stems and will wear them out.
My question is if this is the case, how do exaust valves being quite hot not cook the oil inbetween the stems and the guides, regardless of what type of fuel is used?

[Scorpma82]

Ever thought of it to be honest

I'm not sure of the answer but if I was to take a guess (and I emphasise the word "guess" ) I'd say
1) it is possible that the exhaust valves have gone through some form of hardening process to tolerate the heat of the burnt gases
2) some form of chemical reaction that can only occurr before the combustion process

These are theories.... nothing more

Good question.....

[Superscan811]

Valve stem seals CONTROL the amount of oil that the valve stems receive, not block all the oil.
The guides, like the rest of the head are cooled by the water flowing through the head. This removes heat from the valve stem and allows the oil do it's thing.
Cast iron and bronze valve guides are "somewhat" self lubricating, but do require a very thin film of oil to reduce ware.

Hope this helps.

Cheers.

[barana]

That makes total sense. Cooled by a waterjacket around the guide.
So I'm guessing on a petrol engine on the intakes they understandably cannot keep petrol out of the guides, so they must utilise it as a form of lubrication and reduce via the stem seals the amount of oil admitted to keep things in order. I'm also guessing the exhaust stems must get given slightly more oil as there is only exhaust gasses.
I'd also venture on a petrol direct injection there must be a different rate of oiling on the intakes due to no fuel wash.