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Speed pro power forged piston clearance .001!!!

General engine tech -- Drag Racing to Circle Track

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Dan Timberlake
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Re: Speed pro power forged piston clearance .001!!!

Post by Dan Timberlake » Sat Aug 01, 2020 1:25 pm

What clearance would you get with your gage point if the bore was precisely nominal ( Std plus oversize)l?

Are there SpeedPro instructions for the gage point ?

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modok
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Re: Speed pro power forged piston clearance .001!!!

Post by modok » Sat Aug 01, 2020 3:53 pm

The skirt size and clearance are on the box, specified +/- .00025"
they pretty much always match the label. I don't recall if the gauge point is specified.... but it's where I think it would be...if you know what I mean.

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Re: Speed pro power forged piston clearance .001!!!

Post by Dan Timberlake » Sun Aug 02, 2020 4:37 pm

https://drivcat.com/LiveDocs/1104.pdf

Page 5 of this FM/Speed Pro document of unknown vintage makes a farily sharp distinction between forged pistons'relative strength depending upon oil ring oil return design.

On page 3 it briefly discusses the general effect of slotted oil returns vs drilled pil returns.
"(VS 25) Forged pistons with the slot design can be set up at nearly the same clearances as cast pistons."

In the early 60s when Chapparal race cars were using GM hi-performance pistons, those pistons had some version of slotted oil returns.
I envision OEM style 396 forged pistons did too.

The ends of the slot were pretty sharp, being machined by breaking through into a thin section of the skirt, similar to as was done in many stock cast pistons.
A detail that Jim Hall reportedly added strictly for improved durability was a drilled hole at the ends of the slot.
Sounds logical to me.

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Re: Speed pro power forged piston clearance .001!!!

Post by Geoff2 » Mon Aug 03, 2020 5:49 am

I raised the slot v drilled oil ring grooves on page 2 of this thread.

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