Mail Truck Engine Noise
 

Looking for opinions while acknowledging how difficult this request is. Skyline Chapter engine man, Reggie, put the stethoscope to the block to try to ID what sounds like a piston slap. Noise isolated to between #2 and3 cylinders, was most notable above 1300 rpm and with full advance. Went away or became very muted at the same rpms but with full retarded ignition. Engine has good compression, pulls hard on the road, no metal in the oil and no exhaust smoke. It runs cool enough when advanced but will overheat with full retard and road speeds of 35-45 mph.

I can understand detonation pinging from running too advanced but this sound is not pinging. It is a slap. Reggie does not think it is a con rod or crank bearing cap issue or babbit wear.

So, slap, between 2 and 3, higher rpm and accelerating, goes away at full retard. Any suggestions for troubleshooting to narrow this down? Thanks in advance!

Re: Mail Truck Engine Noise
 

Louder cold or warm ----- piston slap tends to quiet as pistons warm---- wristpin noises tend to get louder as engine warms and is sort of a double noise usually

"No metal in the oil"?--- has the oil been drained-- and no pieces in the recess of the drain plug----- the pan is off and no debris--- or I rubbed the oil on the dipstick between fingers and it still feels oiled not gritty

Is it louder on reving up or as it coasted down from reving up?

My guess would be center main ----- but not hearing it in person--- it's only a guess

Re: Mail Truck Engine Noise
 

Thank you. The sound builds as rpm increases and it often will recur when decelerating. The sound definitely gets more noticeable when the engine warms up. The oil was changed into an open pan and inspected visually for metal pieces. It was not sent out for analysis though.

From Andrews troubleshooting and the probable location between 2 and 3 it does sound like the center main is the culprit. I have no history on the rebuild of this engine. I’ll drop the oil pan, see if there are shims in the bearings, check bolts, see if removing shims will help. Will be able to see what’s in the bottom of the oil pan as well.

Re: Mail Truck Engine Noise
 

I would drop the pan and inspect the bearings and check the clearance. The last 2 engines in my shop had bad bearings and a pronounced knock at higher RPM's. One had a bad rod bearing and showed lots of bearing material in the oil, I replaced the rods and polished the crank to repair it. The other had a bad front main (unusual) that had gone completely. This showed no metal in the oil but lots in the gear chamber. During the rebuild process, I checked the oil tube in the valve chamber and found a slug of babbitt from a previous failure blocking it. The pump had enough pressure to move some oil past the drive and keep the engine going for a while but not enough to fill the chamber ahead of the front dam so it could feed the front main. I drove a pencil eraser size plug out of the tube and completed the rebuild for a happy customer. This is another reason to run an oil gauge which would have shown an excessively high oil pressure.

Re: Mail Truck Engine Noise
 

Like Kurt, my first guess is the center main is worn.

Re: Mail Truck Engine Noise
 

Thanks to all for taking the time to assist! The oil pan will have to come off but after some summer driving locally. It’s a case of other projects in the shop that must be finished now.