I bought a rebuilt short block last winter.
Now after a few hundred miles it clatters quite a bit.
I pulled the bottom cover and the only thing I can find loose is the rods (3 of 'em) have a lot of end play.
They make a noise when I slide them back and forth on the crankpin.
I'm pretty sure they were tight when I got it.
What has caused this?
Sorta like new bands can take a while to seat. The rods may not have been fitted super well to a 100% contact and have worn loose. Only thing to do is tighten them up and sooner or later the contact will wear in enough to last a long time until next adjustment.
That is my theory
Could also be other factors at work such as poorly polished bearing surface of the crank as well as rods that were not straightened, etc....
The rods are still tight radially...it's the end play that's loose.
Bob - This not from experience, but from what I've learned from this forum, and a little more from "who knows where", I think that between you and Eric Bruckner, you might have just about answered the question,......I think.....(???). Hopefully, those more knowledgeable than me will chime in here:
Bob,...you said..."they make a noise when I slide them back and forth on the crankpin".
Erich said,...."rods that were not straightened".
I think that might be the answer. I think a bit of "end play" on the crankpin is normal. However, I'm pretty sure that unless the connecting rod is straight, they can have a tendency to work back and forth and at critical speeds, make that noise (read knock) that you're talking about, whereas a straight rod with normal end play will tend to run more quietly.
Again, I'm not real sure on that, but maybe an engine builder or someone else is and will advise,.........harold
You might check the upper bearing in the piston. The wrist pin should be centered between the two sides of the piston if everything is straight and the crankshaft third main endplay is correct. Take a light and look up at each wristpin while someone slowly rotates the crank turning the engine over. If you notice the that the pin moves from one side of the piston to the other as the rod is pushed up and moves to the other side of the piston as the rod is pulled down you have a bent rod.
Another knock which fools many is not in the engine at all, but it is a loose pin through the crankshaft and front pulley. As the engine idles the pin moves to whichever side is down. When the engine speeds up, it is silent. If things get even looser, the pulley will also move back and forth making even more noise.
Good luck in finding and fixing the knock.
An engine that started out quiet and is now noisy did not likely have any bent rods to start with and are not bent now.
Easy movement longitudinally along the crank is not an issue, but believe that you really cannot determine rod wear based on that assessment. Perfectly adjusted rods will move easily once the car has a few miles on it. Erich is likely right on initial wear in on rod bearings...too bad, as modern methods are available which preclude the need to scrap in new bearings. Either the rods are settling in and need to be adjusted, or the oil supply is compromised and allowed premature wear.
In any event, I'd do two things:
1. ensure adequate oil supply to front of engine by removing passenger/front pan bolt and start engine and ensure that oil POURS out of the hole while engine is running
2. short out plug wires and ensure it's rods...until you do this it's a leap of faith that this is the problem.
3. if #2 doesn't show rods as an issue, I'd be inclined to make sure adjustable lifters haven't slacked off and opened up valve/lifter clearance...that can make quite a racket
Its not uncommon for cranks to be ground so that there is excessive play horizonally when the rods are machined to correct standards. Not uncommon at all. If the builder did not pour the rods himself it is likely that the rods came to him machined to spec rather than to your crank.
My guess is you had a pretty fair amount of play following the rebuild but didn't notice noise because the close fit vertically on the crank journals was enough to prevent banging from side to side.
I also suspect the rods were not "straightened". By that I don't mean to say the rods themselves were crooked, but that the bearing and the piston are not perpendicular. Whenever I receive an engine that has been "rebuilt", I always check for piston verticality with the rod connected to the crank. It requires I remove the crank and rods/pistons and check them on a jig I fashioned on my lathe. Most are not in alignment.
Re excessive horizontal play at the rod bearing, there is not much you can do. Did you measure the clearance? 20 thou wouldn't bother me but 50 definitely would.
Thanks for the help guys.
I'll do some more checking before I tear it apart.
It's noisy under a light load at about 30 or so just like a loose rod.
Crack the throttle or let up and it's quiet.
What are the clearances? Most engines I see before tear down have excessive clearance in the area you describe. If the pistons and rod bearings are perpendicular, some clearance won't hurt a thing.
Re bent rods, when I receive an engine from a rebuilder, I routinely remove the crank, rods and pistons and check alignment on my lathe. Most I check are out of perpendicular when the rod is installed on the crank and the side of the piston is checked for verticality. Considering the piston is fitted to the cylinders with 5 thou clearance, you want less than that in piston tilt. Just makes sense.
One other thing I've noticed. Most rebuilders don't pour their own rods, they order them. So unless your crank is pretty darn good, it is certainly possible the width of the rod bearings even though machined to standard, will have play in the area where you observed. Of course, if the rods were tailored to your crank, no problem, but rods which are ordered come in standard width and if you crank has vertical wear above the radii, then you'll have a lot clearance.
All that said, unless the clearance is really excessive, it won't hurt anything so long as the piston is vertical. So I ask, what are your clearances?
Sorry for the second post. The first did not appear for a while and I though I deleted it by mistake.
Does that mean you will bill us twice, Richard?
*(Richard's profession is known for double billing, etc.)
I know the feeling....we're gearheads...not 'puter geeks!
The rods have around .002 clearance checked with plastigage.
The ones that clank back and forth by hand have maybe 30 or 40 thou clearance.
In some previous life I was a tool & die maker so I can measure stuff.
It makes perfect sense to check that pistons are vertical to the crank. I need to do some serious checking.
Thanks for taking the time to help me out.
Hey Ralph...two times nuthin' is nuthin!
You guys gave me a laugh.
Good luck Bob.
Sounds like too much end (front to back)play at the crankshaft. Note whether the top of the rod is hitting the piston when it is moved back and forth? If it does, that could be the cause of the knock. Anyway 2 thousandths is right on the edge. Try removing a one thousandth shim from one side of the bearing and bolt back up. It might not fix the noise, but not likely to cause the "big bang". That's about all you can do without re-pouring the rods to fit the sideways clearance.
As Norm said, two thousandths is a bit much clearance for the rods. You shouldn't be able to easily move them back and forth by hand on the crank to get a "click and clack." Tighten them up a bit so that it takes a little tap to move them.
I would respectfully disagree with Norm and Mike on this. Two thou is not excessive. One and a half is where it would be new. And a minor point...in my experience, the laminated shims are typically .002" thick per leaf...on the other hand, if you do end up fiddling with the rod clearance, Norm's advice to remove one from one side and go slow at this stage of adjustment is accurate and prudent.
thinking some more, I believe that the shim packs I've used have individual leaves at .003".
Adjust per Mike's advice above.