235 with unexplainable knock!!!!!

ok so this knock is driving me fuckin nuts! First off I have a 54 235 fully rebuilt by me(not the first motor I have built)and I have a slight knock under load and it's loudest at light acceleration from a dead stop. Ok so I determined that the noise was coming from #6 cylinder by shorting out spark plug wires and raping the throttle. next I removed the oil pan to check rod bearing(it does sound like a slight rod nock but not as loud)removed rod cap to inspect and looks like a new bearing would(because it is)so I plastigauged it twice and its at .001-.002 so I looked at everything else checked piston drag (for slap0 collapsed skirt,debris,dent in oil pan,something hitting,oil pump nothing. so before i put it back together I decided to get another 30 over rod bearing and install it just for the hell of it and put it back together. Noise still there #6 dead shure of it. pulled the pan again ,rod end cap ect. same,so now I decided to plastigauge all the bearings all within spec. pulled the head this time and removed #6 piston to inspect cylinder now I have a brand new 40 over piston rings ect installed. Inspected rings for cracks,wrist pin for play and allignment,piston to cylinder wall clearance,ect. So by now Im about to throw a match in the car and watch it burn to the ground! (just kidding) So I just cant get my head around this . Pulled side cover just to take a look everything is clean and lifters move very well in bores ect.so another gasket set later and she's back together again.the noise is still there after 3 valve adjustments and tearing it down twice!I am completely stumpted someone please help!!!!!

so we have a light knock isolated to 1 cylinder loudest at light acceleration
and its not spark knock either timing it dosent make a difference
I have excellent oil pressure about 60 lbs
compression is perfect 145 straight across
and besides the noise the engine runs great I drove the car for about 2 hours yesterday on the freeway

 

I had a knock in a 235 years ago that stumped me and my buddies for a while. It ended up that the standard trans flywheel bolts had come loose and allowed the flywheel to move around a bit making a knocking noise! It sounded like a "dull" rod knock to us. I know you've isolated it to # 6 piston but that is back by the flywheel so you may want to check it out. Good luck!!

 

For what it's worth you can't trust a plastigage reading the way you're trying to do it. (at least the way it sounds like you're doing it) There's too much "noise" from oil film, other pistons restraining the crank, etc.

For what it's worth part two, rod knock usually appears on deceleration. Main bearing knock on the other hand appears under load. How do the 2 rearmost crank bearings checkout??? How's the thrust/endplay??

Right now I'd be about pissed off enough to run it (HARD) til the noise steps up a decibel or two. Then try the teardown again.

good luck

 

54 engines mostly or all used shims on the main bearings unless the engine mains were line bored,expensive.Is you engine mains shimmed?

 

Like they say, check the mains. They will knock under acceleration, rods don't. If its the rear main, pulling #6 wire will stop it.

 

Sometimes something simple like a fuel pump lever or pushrod can make some mis-leading noises that may come and go.

Does the engine use wrist pin bushings in the rod small-end?

with .040 pistons, I can assume a recent rebore, and not old pistons in an old bore?
Your mention of a collapsed skirt suggested that they may be used pistons??


If you re-ringed with a worn .040 bore, you could still get piston slap or fresh square-edged rings hitting the slightly rounded edges of a cylinder ridge and ready to break a piston land. If your pistons and bores are fresh, you can disregard that.

Did any ring or piston hang up a little on the block when you were installing it?

You mentioned piston drag. Did you check with a feeler gauge without any rings?
If you checked "piston drag" with rings in place, you were feeling just the rings.

Use the correct feeler gauge, NO rings, then check for the correct drag by the skirts at 90 degrees from the wrist pin. (not front and rear, but right side and left side)
That could find any piston looseness at the skirt.

Check and double check the wrist pin bushings and make sure the rod offset is turned the right way or you could be pushing the bearing and the piston at "slightly wrong" angles.

Sometimes a loose flywheel can imitate a rod noise.
CHECK THE FLYWHEEL BOLTS!

 

I had a bent connecting rod that caused a knock...

 

You didn't say whether it's an auto or manual trans., but I'd be leaning towards a cracked/loose flexplate. They will make the death knock sound under a light load just like your describing.

Noobs$.02
Jay

 

How about trying simple stuff first? Check to make sure you're getting a good spark at #6 cylinder and if you are, try changing the #6 plug with a new one or swap a couple. I've had bad plugs from a new set before - not many, but it's happened.

If you don't have good spark, work backwards to the wire (bad wire, wire shorting out, etc.), and then to the distributor (cracked cap,, bad contact on the cap, etc).

My 2¢

 

I have had piston pins knock like that.

 

Hi AKTATOO,

The best note that you have is that the noise gos away when use remove the spark from that clyd. It sounds like you have tried alot of things. From what I have read and if the other findings are true you may want to remove the trans and refire the engine.When you listen to the engine. Make sure you are paying attention to the speed of the knock. Is it crankshaft speed or 1 fourth of that. Spark speed. IF so I would look for a bad valve seat or center. Ugotpk P S remember I am not there to see all that my eyes would tell me. Like a heat valve in the exaust manifold ect. Clyd ridge? Piston Skirt Clearence.

 

Sounds like piston slap or wrist pin to me, a light rod noise would show more on decelleration - Like someone else asked, is it a new piston/bore job? Whether or not, you might try swapping piston/rod from another hole and see it the noise moves.

 

no the mains are not shimed and the bloc was line bored for a .010 over bearing

 

yes i have good spakr as this would have been the first things to check I' way past that

 

I don't mean to be rude but if you've gone over everything, and haven't found it.....

Well it's gotta be something. And you covered everything. Therefore you might need to admit it's something you already covered. It's a humbling thing that trips up a lot of smart troubleshooters. Don't fall victim to it.

good luck

PS, I was serious when saying I'd be mad enough to run it hard for a while. Not very sound advice, but true.

 

I had a 56 Chevy with a 235 auto transmission. It too had an unexplained knock. One of the bolts on the flex plate holding the torque converter loosened up and was hitting something inside the bell housing. Sounded just like a light rod knock.

Joe

 

I went through this with my '50 Plymouth 230... Couldn't figure out where the noise was coming from no matter what I tried. I wound up driving it for another 3000 miles until the noise was bad enough to isolate to one cylinder by shorting the plugs. Checked all the bearings multiple times. Finally pulled the head the piston from the offending cylinder and found that the wrist pin bushing had spun in the rod, and was rocking back and forth.

Super frustrating, I feel your pain...

Pete

 

Well first at this point I would check the flywheel bolts being the noise is in the rear and you don't see anything internaly. If you drop the pan again turn the engine over by hand and watch the top of the rod in the piston. Does it move side to side? If so I would guess the rod is hitting the piston pin boss making the noise. By the way when you line bore the block you do not make the bearing bore larger ,about the only thing you do that to is VWs. You grind a bit off the cap and bore it back round to the standard size.

 

Aluminum timing gear ? They're noisier than the fiber composite version. Thrust plate/cam gear clearance too loose ? Wrist pin to piston fit too loose ? Pistons can be reamed for an oversize pin. Stock fuel pump with a broken return spring ?

 

In my early mechanic days I worked a lot with GM 6s 292, 302. when these mystery knocks would manifest them selfs we would tear them down and measure every thing and what we would find is that it could be anything mentioned above, but it mostly boiled down to either a piston or wrist pin. Good luck.

 

Some talk about wrist pins here.As ya know the Chevy rod has split smaller end with a pinch bolt to clamp the wrist pin.A guy I know had the pinch bolt lock washer break,the bolt loosened,the pin became loose and caused a knock.The pin finally banged around and the noise got real loud when the pin chewed up the cylinder wall.

 

i've just decided to drive untill something breaks then i'll know what it was! hahaha

 

heres a picture of mine after it developed a "knock"...........
funny thing is,,,it STILL ran,,,although the knock was a bit louder,,,hahaha

moral of the story....figure out what it is before it's too late....

 

You haven't checked the crankpin on 6 for out of round yet. Then Check the rear main.

 

Hmmm, couple people saying that rods don't knock under acceleration. Gotta disagree with that one. 30+ years ago I had a Dastun 1300 motor that would knock like crazy under load only, not decel, not idle - just accel or under load. My Dad says "rod bearing". But hey, I was 17 and I listened to the experts that said "no rod bearings don't knock under acceleration", right up to when the rod came through the block!

 

I had one that knocked and it took me a year to find it. It ended up being the valve spring was too short.

 

Pin bushing

 

235s don't have pin bushings in the rod. Rod is split with a clamp bolt to locate the pin.

 

valve spring too short that would make a knock? I didnt know that

 

You say 60lbs oil pressure? Isn't that a little high for a 235? Mine is no where near 60 lbs and it is all new too. Line bored, 60 over bore, balanced, all new etc. A stock 235 is usually around 30+ lbs oil pressure? Just curious.