Valve clearances

[lutorm]

I just learned that the factory cold VW valve clearance of 0.006" assumes the original aluminum pushrods. As the engine warms up, that 0.006" will apparently shrink to about half as the pushrods expand more than the engine, given that the barrels are cast iron.

Chromoly pushrods, which are common and are the ones in the Aerovee, obviously have a lot less thermal expansion, so unlike the aluminum ones the clearances will grow as the engine heats up. For this reason it's apparently common to set valve clearances with chromoly pushrods to as little as possible, which will still open up to more than 0.010" at operating temperature. (See for example https://www.cbperformance.com/Engine-Tuning-s/149.htm.)

This made me wonder if the 0.014" Sonex recommends on the Turbo really is advisable. I remember reading that someone had their rocker arm foot cock itself against the valve stem and hold the valve open? If you set the clearance to 0.014 and this then grows to 0.025 or something at operating temp, that's a lot of clearance. It seems entirely possible it might be large enough to allow something to move.

[SonexN76ET]

Patrik,

I think you may have things backwards.

Try this experiment:

Set your valves per the factory recommendation cold. Then run your engine up to operating temperature. Then when still hot, remove the valve covers and check the valve clearance. In my experience you will find those clearances have significantly decreased.

Jake

[daleandee]

Chromoly pushrods, which are common and are the ones in the Aerovee, obviously have a lot less thermal expansion, so unlike the aluminum ones the clearances will grow as the engine heats up. For this reason it's apparently common to set valve clearances with chromoly pushrods to as little as possible, which will still open up to more than 0.010" at operating temperature.

For your consideration ...

http://www.oceanstreetvideo.com/vwtech/ValveLashHiPoPushrods.htm

From the article:

Designs utilizing these small clearances performed adequately for passenger car use, but consistently burned exhaust valves when used for extended full-throttle operation. To determine the cause, dynamometer tests were conducted utilizing a specially designed machine that could measure valve lash with the engine running at high RPM, at full throttle, and under load. These tests indicated that the exhaust valve stem would expand sufficiently to eliminate all of the valve lash and hold the valve off the seat...

Best,

Dale Williams
N319WF @ 6J2
Myunn - "daughter of Cleanex"
120 HP - 3.0 Corvair
Tail Wheel - Center Stick
Signature Finish 2200 Paint Job
166.7 hours / Status - Flying
Member # 109 - Florida Sonex Association

[lutorm]

Patrik,

I think you may have things backwards.

Try this experiment:

Set your valves per the factory recommendation cold. Then run your engine up to operating temperature. Then when still hot, remove the valve covers and check the valve clearance. In my experience you will find those clearances have significantly decreased.

Jake

Yeah, I was going to do this but since my engine is in parts I can't do it right now. If the clearances decreased with chromoly pushrods, what would happen with aluminum ones? It's not in question that the aluminum ones will expand twice as much.

[lutorm]

For your consideration ...

http://www.oceanstreetvideo.com/vwtech/ValveLashHiPoPushrods.htm

I looked at that page but it was so scatterbrained that I couldn't figure anything out. It seems to just be a bunch of random links to things about valve lash, not specifically for aircooled engines. I don't think you can take conventional wisdom from a water-cooled engine, where the engine temperature is much more stable.

There are several competing effects. Expansion of the exhaust valve stems will make clearences go down, as will the thermal expansion of the pushrods. The thermal expansion of the cylinder barrels and the heads will make clearances go up. It's not obvious to me which will win, but there should be a significant difference between Al and steel pushrods.

[daleandee]

For your consideration ...

http://www.oceanstreetvideo.com/vwtech/ValveLashHiPoPushrods.htm

I looked at that page but it was so scatterbrained that I couldn't figure anything out. It seems to just be a bunch of random links to things about valve lash, not specifically for aircooled engines. I don't think you can take conventional wisdom from a water-cooled engine, where the engine temperature is much more stable.

Although the information presented is not assembled very well ... please note at the top of the article where is says:

the purpose of this article is to address the controversy regarding valve lash settings when running high performance chromoly pushrods in the vw type 1 engine.

Best,

Dale Williams
N319WF @ 6J2
Myunn - "daughter of Cleanex"
120 HP - 3.0 Corvair
Tail Wheel - Center Stick
Signature Finish 2200 Paint Job
166.7 hours / Status - Flying
Member # 109 - Florida Sonex Association

[Onex107]

The push rods and the cylinder barrels are both steel. The thermal expansion of most all steels is .0000113 inches per degree per inch of length. You can calculate the change in length at cyl. head temp. but the cylinder and push rod are going to expand nearly the same amount.
Now aluminum expands .0000234 inches per degree per inch of length. So the head will grow a little more than the cylinder and push rod moving the rocker arm shaft away from the push rods. The magnesium block is also expanding at .000014 inches per degree per inch and moving the cylinders and heads further away from the cam and lifters and that probably accounts for most of any change in the valve clearance. The total expansion change in any of this is only a few thousandths at most. That's why VW says to set the valve lash cold. They know when the engine is hot there won't be any difference to speak of. Is this more than you wanted to know? Mother nature controls these things and there is nothing you or I can do about it. Ref. Metals Handbook, 8th edition, American Society for Metals