View Full Version : Characteristics of a wrist pin knock?

01-15-2005, 03:55 PM
Two days ago my motor developed a funny little noise. Its definately a knock, but doesn't act like a rod. It only shows up between about 1500 and 2000 rpm and only when the rpms are increasing or staying constant, never when I back out of it. Until yesterday, it wouldn't make the noise unless the car was moving and the motor was under load, but now it will do it while the motor isn't under load.
It sounds way to loud and deep to be valve clatter, but thats the next thing i'm going to check.
The noise itself sounds kinda like loud spark knock, thats why I think it might be a wrist pin...that and the fact that studes don't have the best wrist pin set up. anyway, if anybody has any thoughts or pointers, please speak up. thanks

Steve Ray
01-15-2005, 03:57 PM
Had that on a car once. Turned out the shaft on the alternator was bent causing thE armature to contact the case. So it could be anything.

01-15-2005, 04:04 PM
you might need to fix it if your going to come into town on the 12th of feb.haha
have you pulled your plugs?

01-15-2005, 04:10 PM
You may need a stethescope to find out where the noise is actually coming from...!

To do this..., Take an old wood broomstick handle, baseball bat, or 2X4 and place it at different parts of the engine (right onto the engine) where you think the noise is coming from and place your ear to the wood..., you'll definately hear the difference when you place the wood onto the affected area...!

That'll give you a better idea if it's internal, external and will at least give you some idea what part is failing!

Good luck


01-15-2005, 04:17 PM
Last night i used a real stethescope and listened all over the motor and didn't hear a thing at all. I listened to the heads, the valve covers, the oil pan, the water pump...everything.
How severe is a wrist pin....will it stay together under normal driving conditions? or should I start preparing for a motor swap. If it will stay together, I can get my fill of going fast and being loud on my bike... and limp the car around for a while longer until I can build another motor.

01-15-2005, 04:46 PM
New info, I just pulled off the waterpump/alternator belt and it still makes the noise. So i guess its officially internal...and most likely from the passenger side bank from what I can tell.
Hey Kenny, what could I learn from the plugs?

01-15-2005, 05:12 PM
Wrist pin knocks fall halfway between lifters and lower end noises. When it get bad it will knock on the upstroke and down stroke sounding something like a lifter. With the engine running and knocking, pull each plugwire one at a time. When you get to the bad cylinder the noise will almost go away if it's a wrist pin. I had a similar noise in my 351C engine in my 53 Ford. The bolt that holds the fuel pump drive cam and cam gear had got loose and the pump cam was hitting the timing cover. If run too long the rod will break. Good luck!

01-15-2005, 05:14 PM
Pull one spark plug wire at a time. Ussualy the noise will go away when you have the wire off, because there is no load on it. This will only help tell which cylinder it's in.

01-15-2005, 05:55 PM
ok, the noise doesn't get softer or go away when I pulled the plug wires, but it did get louder when I pulled numbers 3, 5, and 7 plug wires. So that pretty much means its number 1 right?

The Shocker
01-15-2005, 06:22 PM
Sounds wristpinish to me .I had a simular expierence with a 360 Ford once.The best way to find which one is loose,is to drop the pan,start from one end and try to wiggle each rod one at a time.You will know it when you find it.Mine was number 7,I pulled that one head , replaced that one rod and piston,and drove it 5 more years,then sold it.

01-15-2005, 06:56 PM
that may be the ticket, i've got 112 extra rods and I can get a piston easily enough...i just want to make sure thats what it is...for now it looks like i'll be cruisin the bmx for a while...

01-15-2005, 07:18 PM
Wristpin noise is loudest on acceleration. Sounds like that's what you have alright. The 389 in my 57 Poncho started with a knocking wristpin after a top-end race with a 55 chevy. I dunno what he was running except it was a noisy v/8 but I outran him on I-5 one afternoon. Ten minutes later, I went under an overpass and heard the dreaded knock. I drove the car like that for a few months but it was too embarassing. I retired it.
Yours may last for years or a piston may unravel tomorrow, taking the block with it.

01-15-2005, 07:39 PM
i was thinking that it might be detinating(spelling).
but it sounds like you got it by the balls now

01-15-2005, 07:43 PM
it sounds a lot like detonation, but louder and more consistant. i'm kinda worried cause hyperutectic pistons are more brittle, so it may break sooner. Since the rods are forged steel and the pins aren't full floating (fixed to the rod) the piston is probably bearing the brunt of the fatigue...

01-15-2005, 08:22 PM
drgnwgn289 ...How about loose carbon in the cylinder ? I have had several more modern motors that have had similar noises that I would swear were a failed wrist pin/rod. Then tried the water down the intake trick and fixed or changed the noise enough that you new that you were on the right track. Also cost nothing to try and might just cure you ailment. This has fixed many a car for me, may or may not do anything for you, just another diagnostic tool. Good luck...H

01-15-2005, 08:29 PM
Snapper has a good point. I too have fixed knocking engines with a little water bath down the carb after warming the engine. The cold water hits the hot carbon and blasts it right out of the combustion chamber. Down side is....if you're wrong, the engine can loose it's weak part [a piston in your case] and make an expensive mess on the shop floor.
Snapper...where in Northern Orygun are you from? I am also from Northern Orygun..

01-15-2005, 11:25 PM
That kinda sounds like the thumping I was getting with two
rod bearings destroyed on my 289.

Hope you get it sorted out soon.

and thanks again for the T5 info.

01-16-2005, 01:16 AM
I had a knock noise in my 340 Buick motor. Started out real quiet then as time went on got progressivly louder. I originally thought that it might be a wrist pin cause it did not affect power at all and it was louder on accel. I drove the ever loving shit out of that engine. One day I was driving back to Santa Cruz from LA for about the 7th time in 2 months. About a 400 mile trip each way. It was running great then I backed out of it to get on another Highway and when I went to accelerate again it was missing two holes. I tried everything I could on the side of the road then realized I had to just get it home on 6 of 8 cylinders. I was about 75 miles out and got the car home with no problems except power loss. The next day I determined that I blew a head gasket btween 3 and 5. Pulled the heads ported them rebuilt them and added a Crower cam Put heads back on. No money or time to do anything to the bottom end. Dorve it about another 7000 miles beating the crap out of it as usual with very common pulls to 7200 rpms where it stil had a strong pull. I finally retired the motor last year to replace it with a 350 chev that I had lyin in the garage. So after all this babling the moral to my story is that after about 25000 miles with a knock it still ran great. I plan to determin the cause of the knock, fix it and run it in a modified project of mine.


01-16-2005, 03:11 AM
Is it a solid rapping noise like a ball-peen tap on solid iron? Not clattering but crisp and distinct? If heard at idle will it sometimes seem to fade out then come back?

When you said it seemed lower down that valve noise it reminded me of a worn cam lobe. I acquired a 70 Merc 428 Cobra Jet with about 64k miles (this was a one year only purpose built ex-CHP pursuit not available to civilians. It was a base Monterey with no power steering and rubber mats) It had not been run for many years but when I brought it back it ran superbly except for the noise I just described. I was able isolate it to number six. As I remember removing the plug wire made no difference. I removed and checked the hydraulic lifter from 6 and it was clean and operating fine and not pumping up.
I found an old service bulletin in reference to premature cam lobe wear on these particular engines.
I drove it hundreds of miles after that including a nighttime SF to LA run with a couple 120 mph blasts. ( I should post a pic of that car)
The noise never worsened. I sold it that way. I should contact the guy I sold it to for some confirmation but I was reasonably satisfied that this was the answer.

Hope this is of help to you in some way.

01-16-2005, 04:22 PM
thats deffinately something I'll try. the sound is very distinct, not like a loose clatter, but a solid clunking noise. i'm just scared to run it because I have a lot of relatively new, expensive parts inside that motor and I don't want to ruin them all.

01-16-2005, 08:07 PM
is it an automatic?
if so ,check your flexplate,and torque converter bolts before you tear into it.

good luck.

01-16-2005, 08:37 PM
naw, its a manual. thanks though. tomorrow i'm gonna adjust all the valves and pray that I find one that is super loose...but I'm not being optimistic. Then I'll try the water down the intake trick. if that doesn't work i'll just get another car.

01-16-2005, 09:07 PM
i call bullshit on you getting rid of the wagon

01-16-2005, 09:34 PM
i'm not going to get rid of it...i'll just get something else to drive. to be honest, i'm kinda burnt out on that car and i really don't feel like fixing it right now.