Possible exhaust valve seat recession, and how to prevent?

[SouthSwedeSpider]

The first thing I did after driving my new car home in 2008 was to check valve clearances. The trip home, 400 miles, went OK but the idle was a bit rough. It turned out that there was practically zero clearance at exhaust valves # 3 and 4 (0 and 0,05 mm, respectively). The rest of the valves were pretty OK, all intake and ex # 1 and 2 clearances were around 0,40 mm. When I took the shims out ex # 3 and 4 already had smaller shims than the rest, so after adjustment my shims on these valves were down to 3,10 and 3,40 mm (the others between 4,10 and 3,90). Since then, I have readjusted # 3 once more down to 3,00 mm. The car runs OK, always starts but the idle is still lumpy and I have not been able to get it right.

The question is, could this be an indication on valve seat recession? I've read that both Guy Croft and Paul de Turris at DTR in the UK are saying that valve seat recession, specifically on the exhaust side, is a problem that could show up in the long run due to the use of unleaded fuel, because the valve train isn't really optimised for that.

Has anyone noticed anything similar, and what can be done to prevent it? I'm gonna put on a new overhauled head this summer and don't want to end up with the same problem after a while. I was thinking of adding a sodium based lead substitution compound, could this have any positive (or negative) effects? I driving de-catted, so there no risk of ruining the cat. Any other suggestions? Could there be a risk that the cam lobes have been damaged due to the lack of clearance?

That was a lot of questions/thought, keeping my fingers crossed that you can give me some answers back.

Bjorn

[Daniel]

Hi Bjorn

Sometimes the Fiat Valves stretch this is most likely what happened .

[fiatfactory]

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[SouthSwedeSpider]

Thanks for input. Please bare with my ignorance, but how does a valve stretch? Is it the stem that gets longer, or is it the plate that gets deformed and pushed into the seat?

[fiatfactory]

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[Five40iguy]

Anything under around 3.25mm for the shim size and your going to be close to knocking the edge of the tappet bucket with your cam lobe... you better get that new head on there in a hurry before you wipe out a cam lobe, or the head falls off one of your exhaust valves!!

???????? Why replace the Head? Rebuild the head that you have. I am certain this is a lot more cost effective. Are you mechanically inclined? If yes do you have access to a machine shop? If yes to both let me know via this forum and I'll show you the trick of turning down solid valves to meet your specifications and will outlast maost anything on the market today.

[fiatfactory]

[.

[SouthSwedeSpider]

Yes, I happen to have a rebuilt and gas flowed head on the shelf already, so it will be a
quick swap. Since I do not have access to a machine shop, doing the rebuild myself is not an option. Rebuilding would most likely cost more than what I payed for the rebuilt head (approx 550 USD), and then I can sell the old head as well.