400 Small Block Problems...
1 Attachment(s)
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 :D |
Re: 400 Small Block Problems...
Wrong rod or piston installed
|
Re: 400 Small Block Problems...
Quote:
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. |
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.
|
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 |
Re: 400 Small Block Problems...
Quote:
|
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?
|
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. |
Re: 400 Small Block Problems...
Quote:
Amazing it hasn't done more damage. |
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. |
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.
|
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... |
Re: 400 Small Block Problems...
Stacked a bearing
|
Re: 400 Small Block Problems...
Quote:
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.... |
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.
|
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.
|
All times are GMT -4. The time now is 04:24 PM. |
Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Copyright 1997-2022 67-72chevytrucks.com