What is a safe pissston to valve clearance on a street motor? I have to decide whether to go with 1.6 or 1.5 rockers on my G1 TW heads and LT4 Hot cam. Using the stock stamped arms and some Playdoh, I measured .075 on the intake and .065 on the exaust. Going to 1.6 rockers should decrease that by .033. So, what is safe?

thanx,
Bob

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Bob '89 Formula CC350
Last(lost)Member of the Elusive Great Lakes Crew
My car's homepage

 

Yikes!

Too close for me already. .100 is a good number to be on the safe side. You can run closer but its also taking chances. I would definitely stay away from the 1.6's unless you have the pistons cut.

 

That's too close for comfort. Get the pistons fly-cut. The stock rockers have so much lost motion from bending and from their ratio never being 1.5 anyway that if you put a set of something decent on there, even at 1.5, you'd probably already be in the negative clearance area.

With that much money tied up in heads, it would be senseless to void most of their benefits by nutting the cam. Definitely get the pistons cut. .100" is the absolute minimum, and I'd run it that close only if you have a rev limiter; otherwise, shoot for .125".

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"So many Mustangs, so little time..."
ICON Motorsports

 

RCR,

Did you take into account the compressed thickness of the head gasket? The extra 0.030" or so might be all you need to get a little more space and "safety factor".

If the gasket was in place or accounted for, you are really tight. I'd be afraid of lethal valve exposure at any kind of respectable RPM. Pumped-up hydraulic lifters would only make the situation worse, which is one of the reasons I like to skimp on the low side of lash adjustment on hydraulics. Another 0.050" from lifter filling could be disasterous. I certainly wouldn't consider higher ratio rockers without relieving the piston tops.

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Later,
Vader
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"No matter how hard you try you can't stop us now..."
Adobe Acrobat Reader 4.0

 

Yeah, that's what I thought. Glad I didn't get the rockers yet. Yes, I did put the gasket in, unfortuneately. Looks like I need to look at some options. Thanx.

 

i try for .100 without the gasket, or something close to it. i'd be getting the pistons notched for sure on your set up.

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ICON Motorsports

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