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rod stretch |
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hacksaw1971
Supporter of TheAMCForum Joined: Feb/11/2020 Location: hopewell Status: Offline Points: 1404 |
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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.
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its the little things that make the biggest difference
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Trader
AMC Addicted Joined: May/15/2018 Location: Ontario Status: Offline Points: 6916 |
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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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BassBoat
AMC Addicted Joined: Aug/29/2008 Status: Offline Points: 1719 |
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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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PROSTOCKTOM
Supporter of TheAMCForum Joined: Jun/20/2010 Location: Indiana Status: Offline Points: 2458 |
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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
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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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SCRAGTOP
AMC Nut Joined: Jul/03/2007 Location: Chesapeake, VA Status: Offline Points: 367 |
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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
Supporter of TheAMCForum Joined: Feb/11/2020 Location: hopewell Status: Offline Points: 1404 |
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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.
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its the little things that make the biggest difference
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hacksaw1971
Supporter of TheAMCForum Joined: Feb/11/2020 Location: hopewell Status: Offline Points: 1404 |
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Yup... I agree withb100. I got springs to match the Cam per Comp. Shout be good to more than I will ever take it.
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its the little things that make the biggest difference
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