The Stovebolt Site Forums The Shop Area Engine Shop 235 engine rattle

Need help to find my engine rattle problem. Engine was in truck when I purchased it and it had been setting for years however it was not stuck.I have s/n 053322IF255P 235 engine with a rattle at cruise speed. Timing is at 5 degrees with 18 inches of vacuum. Starts, idles and runs good. When I get to cruse speed a rattle developes. Give it fuel or let off and it goes away. The oil pressure is 15 pounds at 180 degrees. Thinking spark knock I retarded the timing to 0 however I could not hear any change. Thank you for any guidance.

A rattle at a "float" throttle setting neither accelerating or coasting, can be loose rod bearings. If tinkering with the timing doesn't make a noticeable difference, it's probably time to drop the oil pan and check the bearing clearance with Plastigauge. I like to see clearance around .0015" or so. As the clearance approaches the .003" maximum, hot oil pressure will start dropping, and the cruise-speed rattle is pretty common. As long as a slightly worn crankshaft is round and not scored, it's possible to install a .001" or .002" undersize bearing to get the oil clearance back in the ballpark.. Be careful not to get things too tight, as it's not possible to shim-adjust the 54-62 insert bearing rods like the 216's and early 235's.
Jerry

You could try having the truck warmed up and running, bring the engine speed up to a fast idle, then tickle the throttle up and down a few times. If you have rods doing the rat a tat tat you can hear it then many times. It takes a light touch and a little practice. It could also be the spring on the arm of the fuel pump is broken. Good Luck.

Hotrod, what parts are actually "rattling" when the engine is under load with the timing too far advanced? I have been wanting to ask you that and figured this would be a good place to ask it.
Carl

There is a condition called "pre-ignition" which is caused by the pressure of the flame front traveling across the cylinder after the spark plug fires. If there is a sharp corner somewhere in the combustion chamber, or a glowing piece of carbon, etc. a secondary flame front gets started across from the spark plug. When the two flame fronts meet, they slam together to produce a shockwave known as "detonation". There is no mechanical contact of anything; the rattling noise we know as "spark knock" or other descriptive terms is actually the hammering noise of the two flame fronts hitting each other. Where the flame fronts meet, there is a huge spike in both temperature and pressure which can melt the piston crown, melt or blow the ground electrode off the spark plug, break piston rings or ring lands, etc. Gasoline engine detonation is the most destructive force known to mankind, short of nuclear. Ounce for ounce, detonating gasoline is more powerful than C-4, Dynamite, Nitroglycerine, or gunpowder.

A highly modified Chrysler 440 engine I was running under full load on a dyno one night developed a momentary lean condition in one cylinder. It was producing 700+ HP at 6500 RPM. I heard a detonation rattle begin, and before I could back off the throttle, it had blown a 2" hole in the top of one piston. Early timing, lean fuel/air mixture, or a combination of both destroys engines! Don't ignore a detonation rattle, especially if the engine is "lugging" a little, where more throttle opening doesn't make the RPM increase proportionately.
Jerry

Thanks everyone, I think Jerry is right on with the rod bearing clearance, I will update you as soon as I can pull the pan and check the rod clearances.

Hotrod, thanks for the explanation on the source of the pre-ignition "rattling". Rattling isn't a good description of the sound, more like a high speed popcorn farting noise.
Carl

Put a handful of BB's in a soup can sometime, and give it a gentle shake. Now, imagine a noise as loud as a 12 gauge shotgun going off several times a second. The noise inside the cylinder is louder than the shotgun, but by the time it makes its way to the driver's seat, the BB rattle is about all we hear. Us old geezers who have a good case of tinnitus can't even hear that. I have to get my son or one of the grandkids to ride along and be my "ears" if I'm diagnosing a spark knock these days!
Jerry

Maybe this is my noise too! Explained just like mine. Only while under easy load. It won't duplicate easily just standing still in the garage. It's really bugging me. It sounds like the speedo cable but it's not that. Maybe I'll change the timing and see if that makes a difference for awhile.

So, if I do change the timing, I'll loosen the set screw, then turn it which way? Clockwise or Counterclockwise? Then I'll adjust the idle. Might this help?

Turning the dizzy clockwise retards the timing. The "octane selector" is the slot with degree markings where the distributor housing is bolted to the block. Minor changes can be made there. Larger corrections are made by loosening the clamp screw holding the vacuum advance to the distributor body. Adjusting for spark knock usually only involves making a slight change with the octane selector, but if the scale isn't centered to begin with it might be necessary to make a rough adjustment to get things in the ballpark with the clamp ring.
Jerry

I fiddled with the timing, and settled on a few degrees advanced. 10-15 or so. It sounds a little better now. I'll just drive it this week and see how it goes.

My shop manual says timing set right on the ball at zero advance. So, is advanced a little OK?

The service manual setting at the "BB" or "ball" is a ballpark guesstimate to get the engine running. If you read further, you'll see that the "octane selector" slot was factory-designed to tweak the timing for variations in fuel quality, altitude, different loads on the engine, etc. Most trucks were designed with fairly low compression and not much timing advance so they could run low-octane (translation "cheap") gasoline, and run full throttle for long periods of time against a maximum gross weight load without detonating. Now that the vast majority of our trucks are Cowboy Cadillac toys that seldom if ever see a heavier load than a right-seat passenger, and we have better quality gasoline than they did in the 1950's, we have a lot more leeway where ignition timing is concerned.
Jerry

I always set mine for a slight ping on a hill..... John