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#1 |
Registered User
Join Date: Aug 2014
Location: Kansas
Posts: 37
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Well... Picked up my first project yesterday! bought a 1978 400 from a guy I know in town, he said he thought it had a wrist pin knock. After purchase and removal of the oil pan we discovered the knock was from the counterweight on crankshaft HITTING the number 1 piston!
I'll see if I can figure out how to post the picture I took of it. I'm rebuilding the motor so I think we should be able to get it fixed, but my question is, What would cause this to happen? It is only hitting on one piston, it has been rebuilt before I believe, because the Rods and end caps have been stamped 1-8. Just curious to see if any of you had ever run into this problem! I will mic it all out tomorrow ![]() |
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#2 |
Senior Member
![]() Join Date: Sep 2005
Location: Lewisville, Nc
Posts: 10,234
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Re: 400 Small Block Problems...
Wrong rod or piston installed
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David fuller Ase Certified Mechanic Click here to help support our board!! 1971 Chevy c-10 under going a 4.8l LSx swap Build Thread http://67-72chevytrucks.com/vboard/s...d.php?t=421305 2007 Honda Accord my daily 145kmiles 2002 Honda Accord 4 door With 330k(sisters car) 2005 toyota Avalon 228k( brothers car) 2002 Sububran 5.3 245k 2000 Tahoe 5.3l 378 General manager for Marco's Carwash & lube |
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#3 |
Registered User
Join Date: Jan 2014
Location: Sacramento,California
Posts: 696
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Re: 400 Small Block Problems...
Hah! That was my first thought too. Maybe a 350 rod. So, in the old days, if you bored a 350 .080 over, and installed a 400 crank and rods, thats how you got a 383 "stroker" . The 400 had a longer stroke by using a big end crank position further away from center, but that meant it also needed longer connecting rods.
Nm, I see where it's hitting in the pic, I would call someone like jegs or summit and see if they can get you a measurement of a 400 rod.
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Built not bought! My dad always tried to convince me HEI was pointless! Welding is a lot like sex, you don't have to be great with the rod as long as you thoroughly prep the surface and your good at grinding ![]() My build : 68 C10 Short Bed Conversion |
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#4 |
Registered User
Join Date: Jan 2014
Location: Sacramento,California
Posts: 696
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Re: 400 Small Block Problems...
Oh, on another quick note, I have seen this before. .. in a watercraft that took water into one of the cylinders. When the piston came up, it bent the connecting rod slightly ss water does not compress. I couldn't tell what was wrong until I pulled the crank and piston assy. I would think this logic makes more sense if it is only 1. I could see the mistaken rod, but most peeps buy a set of rods, I would think it rare to see 1 being wrong.
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Built not bought! My dad always tried to convince me HEI was pointless! Welding is a lot like sex, you don't have to be great with the rod as long as you thoroughly prep the surface and your good at grinding ![]() My build : 68 C10 Short Bed Conversion |
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#5 | |
Registered User
Join Date: Jun 2014
Location: Glendale, Arizna
Posts: 1,642
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Re: 400 Small Block Problems...
Quote:
Amazing it hasn't done more damage. |
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#6 |
Registered User
Join Date: Feb 2008
Location: Tacoma Washington
Posts: 890
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Re: 400 Small Block Problems...
Stock 400 rods are 5.565" IIRC and I believe the only motor that runs them. I don't care for the 400 blocks because the cylinders are siamesed. Maybe find another block and build a 383?
Jeff
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#7 |
Registered User
Join Date: Aug 2006
Location: Wheat Ridge, Colo.
Posts: 300
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Re: 400 Small Block Problems...
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#8 |
Registered User
Join Date: Aug 2014
Location: Kansas
Posts: 37
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Re: 400 Small Block Problems...
Well if it has stock rods (5.5) and someone put a 5.7 or 6 inch rod in one of the cylinders wouldn't that cause the opposite effect? Like instead of the crank weight and piston hitting, they should be farther appart?
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1989 Isuzu Trooper - 230000 miles 1968 c20 - 292 straight six - fourth owner - 96000 1978 400ci small block PROJECT ![]() |
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#9 | |
Registered User
Join Date: Jun 2014
Location: Glendale, Arizna
Posts: 1,642
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Re: 400 Small Block Problems...
Quote:
That's why the 3.75" internally balanced crankshafts require longer rods, because the counterweights are bigger and you have to move the wrist pin up higher to avoid contact between the piston and the counterweight. Hence the longer rod/higher wrist pin location. |
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#10 | |
Registered User
Join Date: Jun 2014
Location: Glendale, Arizna
Posts: 1,642
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Re: 400 Small Block Problems...
Quote:
If that rod is simply bent, and everything else is good, you might get by just replacing that one rod or maybe that one piston as well. Check to see if the piston de-formed in the area of the wrist pin enough to affect the wrist pin clearance, skirt collapse, and piston/cyl wall scoring. In the event you just repair that one cylinder, if the cyl wall is not scored, and if the rings are good it would be best to re-use them leaving the cyl wall alone. Make note of where the ring gaps are when coming apart then make sure they go back in the same way. Of course if you have to hone it use new rings. If you can figure out exactly what the original parts are, and if Summit has them, they are good about selling singles for the same price as multiplied for a set. |
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#11 |
60-66 Nut
![]() ![]() Join Date: Mar 2007
Location: Coeur d'Alene, Idaho
Posts: 23,247
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Re: 400 Small Block Problems...
My guess would be that, that cylinder has a piston for a 5.7" rod mounted onto a 5.565" rod.
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#12 |
Registered User
Join Date: Jul 2014
Location: Seattle, WA
Posts: 267
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Re: 400 Small Block Problems...
Interesing for sure - never seen that on just one cylinder.
have to think the main and rod bearings are scrap...and I'd be kinda worried the main web is cracked. but that's what a clean/pressure test/flux is for... |
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#13 |
Registered User
Join Date: Jul 2009
Location: cleburne texas
Posts: 174
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Re: 400 Small Block Problems...
Stacked a bearing
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#14 |
Registered Truck Offender
Join Date: May 2008
Location: hells training ground (aka Ariz)
Posts: 3,118
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Re: 400 Small Block Problems...
That was my first thought too,,, but the end result is,,, this thing is probably now a very large heavy paperweight. The production 400 blocks are thin wall castings to begin with,, pound the crap out of a bore like has obviously happened here (hammering the piston sideways at the bottom of the bore) and you have cylinder wall damage beyond what will clean up at +.040,,, which is really beyond serviceable in a 400,,, not to mention the rod is junk, you probably have a cracked crank, and as mentioned this all may have stemmed from a block with a cracked main web.
End of this story is,,, take it apart, take the block and crank to a COMPETANT machinist,, have it checked out.... and then check on the price of scrap iron. I surely hope I'm wrong, but only one avenue to go down here....
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Still playin with trucks, even at my age! When you're dead, it's only a problem for the people around you, because you don't know you're dead. .....It's kinda the same when your STUPID. I just did my taxes and reviewed my SS statement. Thanks to the current administration it looks like I will only have to work till noon on the day of my funeral. |
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#15 |
Registered User
Join Date: Oct 2013
Location: Maine
Posts: 2,357
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Re: 400 Small Block Problems...
I agree with post #4, it probably got hydrolocked at some time. It seems to me the piston must be making pretty light contact with the crank. If it was a harder contact the piston would break. The dilemma is to go cheap or go big. I don't know if I would look at that rod very closely and replace it, or bore the block and buy a rotating assembly. Tough call, I would try the cheap way first I suppose.
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#16 |
Registered User
Join Date: Jan 2013
Location: Eastern TN
Posts: 1,921
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Re: 400 Small Block Problems...
As said before the 400 rod is shorter. 400 rods are known for being a weaker rod so I bet that it is bent. If the block checks out a SCAT 9000 crank is pretty darn cheap and alot stronger than the factory crank. A complete rotating assembly can be purchased from SCAT also that has crank, rods, pistons, bearings, rings, and even a flexplate( on some kits) that are quite resonable. Just have to have it all balanced.
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400, chevy, knock, sbc, wrist pin |
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