Minimum valve to valve clearance

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Minimum valve to valve clearance

Post  69BOSS on March 6th 2016, 2:01 pm

Hi,

During overlap, the minimum clearance between the valves measured with a feeler gauge is .032". Is this an acceptable clearance?

I came up with this figure by mapping out the closing and opening events, then removing the head and using dial indicators, measuring the clearance at .010" increments as the valves passed each other.

The heads are D0OE-R iron SCJ heads with 2.25 and 1.76 valves.

On the net, I found limited info on this, and some people have stated .030" is okay and maybe less depending on guides and springs and such. I think they may have been referring to inline valve arrangement. So, I am wondering if the canted valve arrangement needs more clearance.
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Re: Minimum valve to valve clearance

Post  c.evans on March 7th 2016, 2:33 am

The number I have heard is .040" between the canted valves when they are seated.  However, it's really a function of camshaft duration more than anything, so it's seems to me that it would vary somewhat on each engine.
What I would do is mock up. Install the proper cam, the assembled heads, rocker arms, pushrods, set your lash and etc..  Then with the engine upside down on the engine stand, and a empty cylinder without rod and piston, turn the cam over slowly and see how close the valves come to each other during overlap. Physically measure the clearance between them with a feeler gauge, at their closest point.
There are several things you can do to reduce a valve clipping problem. Unfortunately we all had to learn this the hard way with that batch of old A-429 heads, where the machining slipped /moved, allowing the valves to clip with big roller cams.  Those were serial numbers 2000 through 5000 if my memory is correct. Things such as opening up the lash, a new valve job sinking the valves a little, also beveling the edge of the valve, margin to face, reducing the diameter of the valves. etc,

Hope this helps,
Charlie

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Re: Minimum valve to valve clearance

Post  supervel45 on March 7th 2016, 8:35 am

Odd you always hear about this with A429 heads but, I don't ever recall hearing about it before and CJ/SCJ's have been around since 1970 with big valves or at least 1.72's. Question

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Re: Minimum valve to valve clearance

Post  FalconEh on March 7th 2016, 10:20 pm

I believe the specs from FRPP for a 521" with a "C" head was .035 min.
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Re: Minimum valve to valve clearance

Post  69BOSS on March 7th 2016, 11:26 pm

Thanks for the info.

Staci
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