400 Small Block Problems...

[Bobby277]

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

[truckdude239]

Wrong rod or piston installed

[no1udknow]

Originally Posted by truckdude239

Wrong rod or piston installed

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.

[no1udknow]

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.

[mechanicalman]

Originally Posted by no1udknow

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.

I agree, when looking at the color of the piston it would appear it's ran awhile, leading me to believe it spent the first part of it's life without the knock. The fact that it's only one made me think immediately that it had a bent rod. Putting the engine in a hydraulic lock does strange things to a rod, bent, twist, what have you.

Amazing it hasn't done more damage.

[imjeff]

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

[xaircav]

Originally Posted by imjeff

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

He's right. I think somebody put a 5.7" rod from a 350 in one hole

[Bobby277]

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?

[mechanicalman]

Originally Posted by Bobby277

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?

Yes, you are right, longer the rod the farther the piston is away from the counterweight.

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.

[mechanicalman]

Originally Posted by Bobby277

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.

I can tell by the thickness of the piston skirt that it's at least had the pistons replaced. Before you go all-out to rebuild the engine, maybe you should assess the overall condition of it.

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.

[Captainfab]

My guess would be that, that cylinder has a piston for a 5.7" rod mounted onto a 5.565" rod.

[DBD]

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...

[78maliburat]

Stacked a bearing

[Marv D]

Originally Posted by 78maliburat

Stacked a bearing

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....

[cadillac_al]

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.

[burnin oil]

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.