valve clearances for different metals

[Dean Southall]

I had to make up pushrods for the Three Wheeler so decided to go with Dural in place of the original steel.
The coefficient of expansion with temperature for aluminium is near enough twice that of steel so I have been doubling the recommended valve clearance from 1.5thou to 3thou.
Am I correct?

[iansoady]

As they used to say, "a happy engine is a tappy engine". So better to err on the slack side than the other way in my uninformed opinion......

[Dean Southall]

Yes Ian, I'd always believed so.

[ducati2242]

Valve clearance increases with heat thermal expansion is the term . In other words its not just the pushrod that gets hot everything increases in size thats why lots of valve clearances are set tight as they get bigger with heat . Other hot parts outway the length of the pushrod gains .  But this isnt always the case . I would do as you have and start big . If its noisy when hot then you can decrease it . I set my gold star to zero valve gap as everything is aluminium and the valve gap increases with heat , but , it could be completely different with cast iron .

[ducati2242]

Made my pushrods from bmw R90 originals .

[Dean Southall]

Thanks for your replies.
The barrels (integral head) are cast iron, which has the same (near as dammit) coefficient of expansion as steel (original pushrods. Hence my decision to increase (double) tappet clearance for ally rods.
If I ever remember, amongst all the other things personal and Trike I need to address after a drive, I will check that there is still clearance when the engine is hot.