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rod stretch

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hacksaw1971 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote hacksaw1971 Quote  Post ReplyReply Direct Link To This Post Topic: rod stretch
    Posted: Jul/03/2022 at 8:34pm
ok ... here is a question to ponder.  
how much dose a cast rod stretch at high rpm? 
 i will be claying the pistons to check for valve and head to piston gap. 
but that won't tell me whats going on when its running hard.
its the little things that make the biggest difference
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Trader View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Jul/03/2022 at 9:55pm
Cast iron connecting rods don't "measurably" stretch like forged connecting rods. They develop mico fractures over time/use and eventually the fractures grow to the point of failure.
The deformation and resizing of cast connecting rods is typically required as the connecting rod bolt has deformed and allowed the cast iron to deform permanently with it. Typically on an AMC 0.002" for 100,000 miles of street use.
A forged connecting rod will stretch and relax back to original shape without the micro cracking for many more cycles, still typically 0.002" or less.
A cast connecting rod that has deformed 0.002" has more, typically 4x micro fractures, then a rod deformed 0.001".
The harder you run a cast connecting rod, the more micro fractures are created to the point of failure.
Many racers preference is the 707 cast connecting rod.
To answer your question, IMO, find 707 connecting rods that have very little deformation, regardless of miles on them, resize, clean up the beams, remove any horizontal marks, balance, and install good connecting rod bolts.
 
Should add that the connecting rods like any other metal expand with heat, but in an engine this is very little:
The 0.060" rule for valve clearance is for high RPM valve float and some piston expansion. Rod expansion is pretty much negligible at operating temperatures.


Edited by Trader - Jul/04/2022 at 7:00am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BassBoat Quote  Post ReplyReply Direct Link To This Post Posted: Jul/04/2022 at 4:27pm
Typical piston to valve should be 080 to 100 minimum on the intake, 100-120 on the exhaust.  Piston to head clearance should be 035 minimum.  These should be safe numbers for street strip engine.  There is a spec for radial clearance of the valve to the relief but I am not 100% sure on that number, I want to say 060.  The piston to head clearance has to allow for relative growth of the piston and rod and block at temperature and rocking of the piston at TDC more than "Stretch"
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PROSTOCKTOM Quote  Post ReplyReply Direct Link To This Post Posted: Jul/04/2022 at 6:28pm
I ran flat top pistons .025" out of the deck in a 7000 rpm drag engine for years. That's only .017" from hitting the head and it was never an issue. So you ask why I did that and it was to get a 12 to 1 compression in a Ford 302 without using expensive domed pistons. If anything in an assembly moves a .001" you have serious issues. What clearances that are recommended and what happens is race engine are two different things.

Tom
Molnar Technologies Full Service Dealer - Crankshafts & Connecting Rods

1969 AMC Rambler Rouge Race Car
1974 AMC Hornet Hatchback, Wally Booth Outlaw Nostalgic Pro Stock Race Car Project
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SCRAGTOP Quote  Post ReplyReply Direct Link To This Post Posted: Jul/04/2022 at 8:39pm
Worry about piston to valve clearance BECAUSE of valve float, not any stretching that may be happening from the thermal expansion of the rods.

Valve float is much more worrisome.  Make sure you use springs that meet the recommended pressures for closed, coil bind, etc.

100 thou is about right.

Just my input.


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hacksaw1971 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote hacksaw1971 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/04/2022 at 9:04pm
Ya...  Clay will tell me all that and I know them numbers.  I was just thinking out loud and looking for info I didn't know...  Thanks for the input.  Good info for the guy new to building a mill. 
its the little things that make the biggest difference
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hacksaw1971 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote hacksaw1971 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/04/2022 at 9:07pm
Yup...  I agree withb100. I got springs to match the Cam per Comp. Shout be good to more than I will ever take it. 
its the little things that make the biggest difference
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