251 Broken Crankshaft / Noise on Rebuild

[Herding Goats]

Hi all, new member here. Wondering if the unusual problem with my friend's truck is anything specific to a 251. The background: His crankshaft broke in half under the #5 connecting rod. He sourced a replacement crank and had the engine rebuilt by a local builder. It runs well but as soon as the oil pressure comes up (a couple of seconds after start) it has a knocking sound at idle which does not go away as rpms increase. Oil pressure is about 20 at idle. Removing #4 plug wire changes the noise, but does not eliminate it. No change with the clutch in or out. Pulled the pan and rod and main bearings look ok. He drove it short distance (brave or foolish?) and the noise disappeared. Turned off engine, went in a store, came out, restarted and it reappeared. It's got a couple of pretty experienced guys stumped. So asking the experts here if there is anything specific to these engines that should be looked for before the engine gets pulled back out. Thanks!

[kencombs]

I don't know how oil pressure coming up could be related to the noise.    But, IME the most common cause on a newly assemble engine of a consistent sounding knock at all speeds with good oil pressure in a foreign object on top of a piston.   Odd as that sounds it's really common.  Something dropped in the carb or open spark plug hole, carb butterfly screw etc.

 

Maybe worth grabbing a bore-scope attachment for your smart phone to take a look before ripping it out and apart.

Edited December 23, 2023 by kencombs

[Scott in ct]

I’m no expert. I recently watched a vintage dodge technical bulletin video. From this site. It tells of a knock caused by the oil pump pick up pipe hitting the site of the oil pan sounds similar to a bearing knock. Good luck with it .  Scott

[kencombs]

4 hours ago, Scott in ct said:

I’m no expert. I recently watched a vintage dodge technical bulletin video. From this site. It tells of a knock caused by the oil pump pick up pipe hitting the site of the oil pan sounds similar to a bearing knock. Good luck with it .  Scott

That reminds me, the oil supply line, pump to main gallery, not the pickup, will fit swapped end to end.  But, it will not avoid the crankshaft when installed backward.  

 

But, that should not be affected by the oil pressure rising.  That part is really odd as knocks are almost always evident before the oil pressure comes up.

[Dave72dt]

I'd consider something that tightens up as it gets warm.  Maybe a wrist pin loose in a piston or the bushing or a slightly tweaked connecting rod since it lost some of its support when the crank broke.  Loose valve?

[Herding Goats]

Thanks all for the great feedback. The timing about the knock and oil pressure may be more of a coincidence than a direct relation, just to say it doesn't seem to knock immediately upon start up, but a couple seconds later. One new piece of info (which I wish I'd known earlier), is that the knock developed after the clutch was replaced, which was very soon after engine installation as the first new clutch was slipping. So it looks like we have to cast a wider net. We will be back at it again soon, explore the above suggestions, and report back!

[lostviking]

Does the speed increase with RPM? Does is match the crank speed, or twice that (CAM)?

[Herding Goats]

Speed increases with engine speed and matches crank timing.  

[Merle Coggins]

On 12/26/2023 at 9:44 PM, Herding Goats said:

... One new piece of info (which I wish I'd known earlier), is that the knock developed after the clutch was replaced,…

 

Are you depressing the clutch pedal to start the engine, out of habit, (no knock) then releasing it just after it starts (knock starts)? If the noise came from the clutch replacement that’s where I’d be looking first. 

[Herding Goats]

Hi Merle, that is a very good question. I think best have transmission in neutral and not to step on the clutch when starting, but I digress... I wasn't the one starting it, so this will be added to the list to check out. I do know pushing in the clutch after it is running did not change the noise, however agree the cutch is where we need to look next.  We haven't been over to work on the truck as one of the guys got covid for the holidays, but will be soon and I'll report back. Thanks everyone for the great ideas and happy New Year! 

[kencombs]

20 hours ago, Herding Goats said:

Hi Merle, that is a very good question. I think best have transmission in neutral and not to step on the clutch when starting,

I've had, and driven others, that would not start in cold weather in neutral and clutch out.   To much heavy weight oil resistance for the starter to overcome.  Main drive gear, cluster gear and 2/3rd slider all turning cold 90w!  

 

One, an old Chevy with synchro  4spd, had to be warmed up good to even let the clutch out in neutral after running.   Could have been the 140w in there to slow down seal leaks!

 

and these were all way before synthetics so it was all dino stuff.

Edited January 2 by kencombs

[Herding Goats]

Hi all, been a while and we still haven't fond the problem, but I thought I would post an update. Despite the noise the truck has been used a fair bit and now the noise only occurs at idle. We've gone through pretty much everything in the bottom end and even explored the clutch as that was a recent change. Our latest working theory is a sticking valve. So we will be digging into that. Will report back!

[Jerry Roberts]

Fuel pumps have been known to knock too . 

[Scott in ct]

Here’s is a dodge tech. Paper  Scott.                    https://www.mymopar.com/downloads/mtsc/034.pdf

[MBF]

I find this thread most interesting.  Two years ago I broke the crank in the 251 at the number 6 rod cap in my ‘49 B2JA.  After machining and

rofessional rebuild by long established machine shop it ran fine.  .030 over, crank turned, new main, rod and cam bearings.  New oil pump, block boiled twice (before and after machining).  3 angle valve job, new guides and springs,, cut flywheel and head, new timing chain/gears et al.  After clutch replacement it Developed a knock at idle as soon as oil pressure came up that went away as soon as rpm’s increased.  No noise at run speed.  Machinist dropped pan, rechecked all main and rod bearings all looked fine.  Checked chamfers on crank journals, no signs of anything hitting inside block.  Crank end play fine, depressing or releasing clutch has no effect on noise.  Checked bypass valve and spring.  Took fanbelt off and installed block off plate on fuel pump noise still these. Told to run the truck and if it fails he’ll repair/ replace at no cost.  As of now noise is becoming more intermittent at idle and goes away above idle.  When engine is up to temp sometimes noise will reappear at times after idling for awhile.  Appreciate the article and input on this.

Edited July 21 by MBF

[rustyzman]

Have you pulled the head yet to check for carbon buildup or a foreign object?  Worth a look.  Carbon knocks can be quite loud and startling.

 

Also, correct spark plug?  Not too long and getting hit by a valve opening?

 

It would be weird, but a wrist pin/connecting rod bearing or even a cracked piston could be an issue.

 

The oil crossover pipe for sure could cause noise if installed backward, at least in the 218/230's.  Crank will hit the tube.

 

Pulled the belt and checked the water pump?  I have had a few bad water pumps that you would swear was a rod knock.

 

Distributor shaft bent, rotor hitting cap on one terminal?

 

Just a few ideas.

[Herding Goats]

Thanks everyone for the additional ideas and resources. I love these old tech papers @Scott in ct! 
@MBF I think this may be the same vehicle we're talking about. A friend of a friend! I'll send you a message. 

[MBF]

Yes, we've eliminated the water and fuel pumps as the sources, checked the clutch mounting bolts, changed distributors and replaced the new oil pump w another new one. We've been inside the engine from the bottom and found nothing.  The noise only appears at idle and goes away as soon as the engine revs up a little.  It is getting noticably better.  Strong running engine that I'm convinced would have come apart by now if it was going to.  The saga continues............

[John-T-53]

Piston skirt, perhaps....

 

Use a section of heater hose, about 36' long, one end up to your ear, and probe the engine in various locations until the sound is the loudest. 

Edited July 30 by John-T-53