The Stovebolt Site Forums The Shop Area Engine Shop Occasional engine knock at various RPM

Hello Folks,
I'm new to the Stovebolt forums, so please bear with me. I have an interesting problem with my 1955 series 2 pickup (model 3200). It has 76,000 original miles and the motor was supposedly rebuilt by the previous owner. It starts and runs very well. Excellent oil pressure and water temp. Doesn't smoke, no oil use and has a new carb and PerTronix ignition. When I drive it - sometimes it rattles or knocks - at about 2500 RPM in every gear. Sometimes it doesn't. Real loud at 45MPH. If I drive faster it goes away. Same hot or cold. Please let me have your thoughts and suggestions.
THANKS!

Spun or loose timing gear on the camshaft.

Spun or loose timing gear on the camshaft.

????

I would check the damper pulley, the fan and the flywheel. If it is any of these, you want to fix it right away.

Check to see if the load on the engine makes a difference. Pull, "float", and coast conditions will sometimes make a difference in whether a knock shows up or not. This is particularly true of a piston slap, where there is a little too much clearance between the piston skirts and the cylinder walls. Piston slap usually shows up on a float condition, neither pulling or coasting. The comment about the harmonic balancer and/or flywheel being loose is definitely a good one, too. A couple of months ago, I encountered a 235 with a loose balancer which was rattling back and forth on the Woodruff key, and had worn the keyway in the balancer. When we replaced that one, I used the drill and tap jigs I've built to thread the crankshaft snout and install a bolt and washer. We also used red LocTite between the damper and the crankshaft when installing the new damper to resist any slack . Simply bolting a slightly loose damper on doesn't solve the problem.
Jerry

Check the other end too, pull the inspection cover off the trans and look for a loose torque converter/flywheel/flexplate/clutch housing.
I had two friends in different cars have issues with the 235 with flexplates coming unbolted from their converters, sounded almost exactly like a really bad rod knock.

Many thanks to the Bolters who took the time to send me their thoughts.
I've been wondering, if a timing gear was loose, wouldn't it be more noticable at idle or wouldn't the timing go off so much that it wouldn't run right? Or start hard?
There is nothing to be seen or heard at idle. She starts right up withut a knock and drives very well - until the noise can be heard. Then, you can also feel a vibration that goes away if you drive faster. I have a feeling that it is worse when warm - but I'm not sure about that.
A friend has suggested that the two piece drive shaft may be the cause, but how could that make a noise? I've been thinking it may be the rubber in the original vibration dampner or maybe even the heat riser valve....
Thanks Guys. I'll let you know when I find the cause.

Here's an update.
Haven't found the cause yet.
Timing gears are OK. Dampner is OK.
Tach says the "knock" is between 2000 and 2500 RPM.
Changed to 15W-40 oil. "Knock" is still there but a little bit less loud.
No noise at idle. No noise above 2500 RPM. No noise when cold.
Then again, I don't go over 2000 RPM when the engine is cold...
Put a new clutch in with new flywheel bolts. No change but I needed the new clutch anyway.
Does anyone have an idea what the "knock" could be?

Sepp
Does the noise come too when you dont drive and the engine is reved at the same rpm with the differential in nutral?
Also the same in top gear and the clutch pressed in with the car stoped or driving?
That will give you an indication if its an engine or a drivetrain problem to start with.
Jianis

I'm with Jerry on this one, sounds like your hearing piston slap. Often when guys rebuild engines they just hone the cylinders to break the glaze and re-ring it using the original pistons. If you have a Stovebolt with over 50k miles on it nine times out of ten there is gonna be .010" to .020" taper in the cylinders, plus some wear at the skirts. No noise with power on, nothing with power off but that spot in between is gonna give you a loud knock.
Rebuilding an engine with 70k on it would most definitely entail a bore job and over size pistons. I’ll bet that wasn’t done if the guy rebuilt it him self.

Denny Graham
Sandwich, IL

Thanks Guys!
Looks like I've got my work cut out for me.
Am I right in thinking that if it is piston slap, it will get worse (louder) as I put more miles on it? Also, why don't I hear anything when it's cold?

Denny, FYI, it's got 76,000 miles on it and the previous owner says he rebuilt the engine. That's all I know but it just occured to me that if I've got that much play in the cylinders, why doesn't it smoke
and use oil??

Noisy pistons aren't much of a concern, as even a few thousandths too much clearance can get pretty loud without doing any damage. Usually, though, loose pistons make more noise during warm-up, before they heat-expand to closer tolerances. As long as the rings are getting a good seal, there won't be much oil consumption, even with a very sloppy piston skirt fit. Normal piston-to-cylinder clearance will be in the .0015" to .003" range (one or two strands of hair) and they start getting noisy at around .005".

How's the oil pressure? if you're experiencing a noticeable drop in oil pressure at about the time the noise starts, you're dealing with loose rod bearings, not piston fit troubles. That situation will worsen as the oil heats up and thins out. An "owner-rebuild" without doing some precision measuring and corrective machining can be a very short-life situation. It's also possible to put one or more pistons in the engine backwards, causing the offset piston pin to face the wrong way. That creates some rattling noises almost like a Diesel, but it's obvious from the first startup. I walked into a friend's shop one day just as his engine mechanic fired up a fresh rebuild, and it sounded like a Cummins engine instead of a stovebolt. When I commented that all the pistons had to be in backwards, the guy said "No way!" We ended up with a $100.00 bet over it. He dropped the oil pan back off to prove me wrong. I won!
Jerry

Thanks Jerry!
The oil pressure is great. Doesn't change at all when the noise is heard.
I thought about pistons in backwards too. BUT, there is no noise at start up - hot or cold.
I'm thinking it's probably one or more noisy pistons even though there is no excessive oil smoke. In fact, haven't used any oil in the last 1000 miles.
I think I'll keep on driving it and hope for more clues. As things stand, other than the noise, there isn't any reason to tear the engine apart.
Having said that, the noise does make me nervous when I think about driving 50 or so freeway miles to the next club event....
Thanks Again

What's your timing set at? Try retarding it and see if the noise goes away. I had the same thing once. Threw a bunch of money at it. New plugs wires flywheel clutch and even a new tranny. Turned out to be too much advance. Easy thing to check.

New plugs, wires, rotor, cap, condenser
installed and timing set at TDC (ball at pointer). Noise still there.
Changed to a pertronix pointless system with coil. Reset timing to slightly advanced. Maybe 5 degrees static. Noise hasn't changed. It's not louder or different in any way.
I think I'm running out of options...