Valve clearance, steel valves vs. stainless valves.

[Mats]

Hi

I'm currently building an Alfa Romeo Nord turbo engine. The base is a 2 liter engine and basically everything except crank is replaced. The block is done and I'm now mounting the valves in the head. When I measured the clearance with the old shims during a mock-up the other day I realized I had no idea what clearances to run.
The setup: Stainless valves (stock sizes 44/40), head is unmodified in respect of the valvtrain measurments, catcams steel camshafts designed for Naturally Aspirated (N/A) engines. Clearances spec. as per camshaft maker is 0.2mm intake/exhaust.
Now, as I sat there my mind started to think about thermal expansion and the tight (compared to stock Alfa) clearances in the camshaft spec sheet. Stainelss expands a lot more then regular steel but then again the conductivity is lower, so what will hapen when I start racing with this engine, will the clearance be too tight on the exhaust side since the exhausts will probably be hotter then a N/A engine?

Is this just a problem in theory? Will everything just work in practice and all different properties even eachother out?

[Guy Croft]

As far as thermal expansion is concerned with valve steels you can assume they are the same. But if the guides are cast-iron you can't use plain stainless stems as they will gall (pick up and tear).

G

[Mats]

Thank you.

Valve guides are yellow metal as per standard Alfa Romeo, only 75 Turbo heads had the cast iron guides as far as I know. Valves are bought as replacements for stock part and as a kit with the guides.
I realize this is an unsatisfactory reply but at the moment it's all I got.

I will go to 0.25mm clearance on the exhaust unless you advice against it. Feedback is welcome. Thanks.

[Guy Croft]

you mean 0.025mm I trust!

G

[Mats]

No, I mean 0.25mm valve clearance. About half what the Alfa Workshop manual specify.

0.025thou? Metric/Imperial mix-up?

[Guy Croft]

Sorry,

thought you were referring to the valve-guide running clearance,

G