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Old 07-06-2008, 11:57 AM
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Why longer pushrods with aftermarket heads?

I'm wondering why you sometimes need longer pushrods with aftermarket heads. I've never heard of anyone needing them with stock heads. I bought a used set of Dart Iron Eagles for a smallblock chevy. The person told me he thought he used .100 longer pushrods. What is the deciding factor for needing loger pushrods? Does cam type or lift affect this? Screw in studs and guide plates make the rocker arm set up higher? I don't want to have to order 2 or 3 sets of pushrods to figure this out. How exactly do you measure and check this? Do you think I would be safe with .100 longer pushrods with my .500 lift cam in my application?
Thanks for your time,

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Old 07-06-2008, 12:16 PM
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Normally the cylinder head manufacturer uses longer valves to allow a spring
that is better suited for high lift cams. Measuring push rod length should always
be done when using any after market valve train parts and is easy to do with a
inexpensive push rod length tool.
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Old 07-06-2008, 12:18 PM
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Hi,
You might try here.
Good luck,
Rich

http://www.compcams.com/information/Products/Pushrods/
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Old 07-06-2008, 12:19 PM
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Simply put, the rocker arm tip should sweep the middle 1/3 of the valve stem. This is because the rocker tip travels on an arc.

Each engine is rather unique in this way, various deck heights, heads, gaskets, valves, rocker ratios etc. will affect where the contact area is on the valve stem.

There is a tool from Moroso you can buy to help with this. (and other companies)
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Old 07-06-2008, 03:40 PM
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Not a true assumption that you will always need longer push rods with after market heads/valve train items. I used Edelbrock heads and Pro Magnum roller rockers and had to get shorter push rods for the particular combo. Basically, unless you are using a known block and OEM parts, you should check your valve geometry on all builds.

Trees
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Old 07-06-2008, 03:48 PM
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Thicker decks also contribute to this? Correct?
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Old 07-06-2008, 04:23 PM
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The head manufacturer makes one casting for a specific port volume , the
difference between the big chamber and a small chamber of the same head
design is the amount that is milled off the deck. Milling the head deck for a small
cc chamber results in needed a shorter push rod, everything else being equal.
Angle milling changes everything, some manufactures angle mill depending on
desired chamber size.
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Old 07-06-2008, 04:47 PM
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Quote:
Originally Posted by RippinRon
Thicker decks also contribute to this? Correct?
Usually not. Usually the decks are cast thicker into the head/water jacket side, not additional material down toward the engine side.
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Old 07-06-2008, 06:22 PM
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Quote:
Originally Posted by curtis73
Usually not. Usually the decks are cast thicker into the head/water jacket side, not additional material down toward the engine side.
i have had several sets of thick deck heads thru the shop, most have been taller than stock by .030-.070.

sam-missle
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Old 07-06-2008, 09:54 PM
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Quote:
Not a true assumption
NEVER ASSUME ANYTHING when building or assembling. This is why there are specialty tools designed for the purpose of measuring. Suppose your block was decked .020, your heads were - .010 and you used a .022 head gasket with too long a pushrod to start... Geometry is critical in keeping the valvetrain stable, getting the most from your camshaft, and not placing undue stress on any of the parts you use. JMO
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Old 07-06-2008, 11:35 PM
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Thanks for the info and help everyone. It looks like I need to buy myself a pushrod tool and check everything before I order any pushrods. I know my block hasn't been decked. I will probably use a head gasket with a thickness between 0.40-0.50. All that said, it would be best to check everything out for sure and not risk anything.
Thanks for the Comp Cams link Rich, it has alot of good information. It looks like it would be easier to check for proper length if you had roller rockers which I don't.
Thanks again,

Last edited by 4speed57; 07-07-2008 at 12:49 AM.
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