 Why do you set Chromoly Rods to 0 clearance?, Just wondering the mechanics behind it |
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| keske968 |
 Oct 29 2011, 09:25 AM
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#1
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Member  Group: Members Posts: 132 Joined: 5-September 06 From: Papillion, Ne Member No.: 6,778 Region Association: None  |
So I am rebuilding my engine and bought Chromoly pushrods from Raby. I have read that you set the valve clearance to 0.000 instead of the .006 that you would with an alumium rod. I am just wondering what are the physics behind that difference? It just seems that as things expand I would end up cooking valves. Hoping someone much smarter than me (not hard to do) can explain the reasoning behind this.
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| Dave_Darling |
Oct 29 2011, 09:43 AM
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#2
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914 Idiot Group: Members Posts: 14,983 Joined: 9-January 03 From: Silicon Valley / Kailua-Kona Member No.: 121 Region Association: Northern California 
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Aluminum expands a whole lot when heated. Chrome-moly steel only expands a little bit when heated. You set the valve clearances cold, and the clearances shrink as the pushrods expand. When you have aluminum pushrods, you are compensating for their expansion when you use the 0.006" clearance. When you have chromo pushrods, you don't have to compensate for that, so you use 0 clerance.
At least, that is my understanding of the reason. --DD |
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| URY914 |
Oct 29 2011, 10:01 AM
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#3
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I built the lightest 914 in the history of mankind. Group: Members Posts: 120,486 Joined: 3-February 03 From: Jacksonville, FL Member No.: 222 Region Association: None |
^^^ Correct. Rate of expansion is much different.
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| keske968 |
Oct 29 2011, 03:50 PM
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#4
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Member Group: Members Posts: 132 Joined: 5-September 06 From: Papillion, Ne Member No.: 6,778 Region Association: None |
Thank you.
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| Jake Raby |
Oct 29 2011, 08:24 PM
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#5
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Engine Surgeon Group: Members Posts: 9,394 Joined: 31-August 03 From: Lost Member No.: 1,095 Region Association: South East States |
To get those pushrods to expand .001" requires a temperature that will turn your heads into a pile of molten aluminum. The stock pushrods expand 4x more than ours.
All the testing we have done to come up with proper clearances has always supported the fact that zero lash was required. |
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| ArtechnikA |
Oct 30 2011, 05:12 AM
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#6
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rich herzog Group: Members Posts: 7,390 Joined: 4-April 03 From: Salted Roads, PA Member No.: 513 Region Association: None |
The wood pushrods in my 356 engine don't expand much with heat either ;-)
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