Race Engine Forum UK

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?

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

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.

you mean 0.025mm I trust!

G

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

0.025thou? Metric/Imperial mix-up?

Sorry,

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

G