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Piston to bore clearance tsm typo? |
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RobinPDX
AMC Apprentice Joined: Dec/07/2020 Location: Oregon Status: Offline Points: 184 |
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Aluminum whether Cast or forged does NOT expand and equal the coefficient factors Cast iron does!
Aluminum is 21-24, (greater amount) Iron in general is 9.5 - 11. Might want to ask the piston MFG “their spec gap” recommendation. And ask a few machine shops a general question being coy and ask them about this as if they were to build it for you do they go by the book #’s you Think is right or Piston MFG. you will get your answer there!!! I have never seen these tight clearances in my decades or even or hear of these like this. Thermal Expansion (10-6 in/(in oF)) Edited by RobinPDX - Jan/27/2021 at 11:00pm |
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Rob Wiley
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WesternRed
AMC Addicted Joined: Aug/03/2010 Location: Australia Status: Offline Points: 5787 |
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71 TSM has 0.0005-0.0013" for all 6 cylinder engines.
304 is 0.001-0.0018" 360 is 0.0012-0.002" 401 is 0.001-0.018"
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I've finally given up drinking for good...........now I only drink for evil.
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farna
Supporter of TheAMCForum Moderator Lost Dealership Project Joined: Jul/08/2007 Location: South Carolina Status: Offline Points: 19608 |
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Just for kicks I checked my 65 TSM. The earlier books sometimes have more details. 65 was the first full production year for the 199 and 232. Piston to Bore Clearance Top Land - .027"-.031" Skirt Top - .0009"-.0025" Skirt Bottom - .0009-.0015" Skirt Bottom measurements are very close to the 1971 measurements. I'd say the tight clearances are correct. Also looked up the 4.0L specs. Looks like they are measuring at the skirt bottom, the tightest spot. Piston-to-bore clearance. 1998 and earlier. Desired. 0.0013 to 0.0015 inch. Allowable. 0.0008 to 0.0015 inch. The "modern" AMC six clearances are tight! I had one that a machinist bored to Chevy V-8 specs with 0.006" clearance and it caused early excessive wear. Had to rebuild at 70K miles, and that was probably 10K over when I really should have. Anything around and over 0.002 can cause slight piston rocking in the 4.0L, which is indicated by a "tick" that sounds like a valve tick, but is lower in the block. Won't hurt to run it that way, but the sound can be disheartening. Chrysler increased the clearance to reduce rejected blocks shortly after taking over AMC. After a rash of warranty returns they told dealers it was "normal" and wouldn't warrant the engines. A few people who knew what was going on got great deals on traded in Cherokees in the 89-90 model range! The engines will run 200K or so with the tick -- which usually developed after 60K or so miles. I knew one person who did that and had over 200K on it last time I talked with them... must have been over 10 years ago now.
Edited by farna - Feb/17/2021 at 6:26am |
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Frank Swygert
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FSJunkie
AMC Addicted Joined: Jan/09/2011 Location: Flagstaff, AZ Status: Offline Points: 4741 |
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AMC engines have really tight fitting pistons in general, but the sixes especially. You can feel it when you drop the pistons down into their bores.
You can feel it while they are breaking in too. Most of mine ran hot and didn't like high speeds for their first few thousand miles until they loosened up a bit. After that they ran much cooler and started loving high speeds. AMC wasn't kidding in their owner's manuals when they specified a break-in procedure that slowly worked the vehicle up to higher and higher speeds. Chevy engines are sloppy. They just are.
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1955 Packard
1966 Marlin 1972 Wagoneer 1973 Ambassador 1977 Hornet 1982 Concord D/L 1984 Eagle Limited |
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First_Gear
AMC Addicted Joined: Jan/18/2010 Location: Mukilteo WA Status: Offline Points: 644 |
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I finally got round to inspecting these pistons. They all had cracked skirts and I threw em out. I ordered new forged pistons. I was surprised to find a maximum piston to bore clearance of .0035. it seemed excessive but the machine shop told me that because they are forged pistons they machine them slightly under size and they expand. I can't find the manufacturer s spec sheet since they are new old stock from ebay. All the forged pistons measured about 3.748 My cylinders were about 0.0015 over the nominal 3.7500 bore diameter at the worst one so I think I will run it and if it piston slaps itself to death I'll pull it out and have them overbore it but hopefully that won't happen. I do think the cylinder to piston clearance is not a typo.
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tomj
AMC Addicted Joined: Jan/27/2010 Location: earth Status: Offline Points: 7522 |
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Don't forget most pistons are cam ground, so they're oval to compensate for expansion at operating temperature (one of the important functions of a thermostat). THe steel inserts and the fat metal wrist pin support etc cause expansion to be non-linear.
I don't really know but I would guess that the tight clearance is 90 degrees from the wrist pin? I'm too lazy to google it, lol. |
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1960 Rambler Super two-door wagon, OHV auto
1961 Roadster American, 195.6 OHV, T5 http://www.ramblerLore.com |
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farna
Supporter of TheAMCForum Moderator Lost Dealership Project Joined: Jul/08/2007 Location: South Carolina Status: Offline Points: 19608 |
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All forged pistons expand more than cast, so the bores are always larger for forged pistons. It will likely make a little noise (piston slap) until it comes up to full operating temp, so for 5-15 minutes depending on outside temps. After that the pistons should have expanded to full size and sound should be as a normal engine. Brother had a steet/strip car with forged pistons. When first started it almost sounded like a diesel! It was intentionally bored "loose" though, looser than actually required. Supposedly spun up a bit quicker that way, but had to be rebuilt every 20K miles or so though... wore rings out quicker, and the bore wore more also.
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Frank Swygert
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JGRANTAMX
AMC Addicted Joined: Jun/02/2017 Location: Maine Status: Online Points: 1663 |
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I have read back in the sixties Chevy used forged pistons in some performance engines 327, 396 new they were known to be noisy when cold so they changed from having the wrist pin centered to a slight offset that took care of the noise depending on application street or race only that offset might still be used today?
J
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wittsend
AMC Nut Joined: Apr/15/2020 Location: California Status: Offline Points: 413 |
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I have a 1985, L-98, Corvette 350 in my '64 Studebaker. It has forged pistons from the factory. Reference: http://https://www.corvetteforum.com/forums/c4-tech-performance/895558-forged-pistons.html The 1988 T-Bird Turbo Coupe engine in my Pinto wagon also has factory forged pistons. Yes, both cars need to be warmed up. Even with the slightest acceleration they can be heard when cold. But then they remind me of one of my favorite movie lines. Airport (1970) "Mr. Petroni didn't you hear him? He said to shut down and get out. - I can't hear a thing..., there's too much noise." (everyone in the theater audience cheers!)
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'63 American Hardtop
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