rod knock at only once specific rpm?

[stevo]

I have an old Ford F150 with a 300 straight six, bought it non-running and just got it going, but it sounds like it has a light rod knock or maybe piston slap at a very narrow rpm. It doesn't do it at idle or high rpm, just around 1200-1500 rpm as the engine passes that range revving up and down, and if you hold the rpm just right and steady at that rpm, you can hear it constantly. I thought if you had bad rod bearings it would knock at all rpm unless the oil pressure came up, and that piston slap/loose wrist pins would make the sound at all rpm. Will one just knock in a narrow rpm range?

Stevo

[ELVIS]

ignition problem?

god bless.

[Rick Modena]

cheap gas homer, add some octane booster with 93 and get back to us with results, if it continues knocking than you have a problem in the crank/valvetrain area.

[stevo]

I just put a new timing gear set, and a rebuilt head on it, new push rods, etc. Old head had two stuck valves and a burnt valve. The piston (tops) and cylinder walls looked great, but I don't know about the bottom end though due to buying it not running.

As far as the old gas/octane, it is an old straight six with like 7.8:1 compression ratio, I don't think it is detonation, but I'll try it.

Stevo

[dville_gt]

i have heard rods bearings "back-rattle" (what i always heard it called) at certain rpms, typically right where you are talking about. buddy of mine had an 98 ls1 that ate some bearings and it sounded fine at idle and through the rev range, only when you'd hold it at steady rpm's 1800 or so, would it have a loud clacking. we tore it apart and the rod bearings were trash.

[BP]

Rod bearings typically rattle worse on decel, rev it up to 3k and slowly back off. You can also kill individual cylinders when it's knocking to determine which bearing it is. If there isn't any change I'd look at piston or wrist pin rattle, and not really worry about it. Drain the oil through a coffee filter and look for metal, if it's clean then just turn the radio up louder.

[butt86]

Rod bearings typically rattle worse on decel, rev it up to 3k and slowly back off. You can also kill individual cylinders when it's knocking to determine which bearing it is. If there isn't any change I'd look at piston or wrist pin rattle, and not really worry about it. Drain the oil through a coffee filter and look for metal, if it's clean then just turn the radio up louder.

^^^^^ this about the killing

my 00 GT mustang rattled the exact same way at about the same RPM. after going throught timing set and kill steps it ended up being the #7 rod bearing.

[Snatch Napkin]

Did you forget a thrust washer or two on the cam gear?

What was your end play like when you did the timing gear set?

Is your truck a manual transmission by chance?

[stevo]

I put the new timing set back in like the old one came out, it didn't seem to have any end play, but it have any thrust washers I pulled them. It is a manual, I thought about the flywheel/clutch being loose, and will check that when I pull the transmission/transfer case for a tranny swap.

I think I'll try to pull plug wires one at a time and see if it stops the sound, see if that can eliminate some possibilities.

Stevo

[Snatch Napkin]

You should be able to hear a rod knock in your shifter handle. Listen to it at idle before you pull wires. Then listen to it after each plug wire to see if its any different.

[strokin]

look into a possible broken or loose converter bolt or flywheel bolt. Had one on a buddies car that only made similar noise at certain rpms and that is what it ended up being.

["MrEd"]

maybe some sludge/trash in the pick up tube screen (slight blockage).
lower RPMs is not pumping enough oil,higher rpms its getting just enough.

[Captain Crawfish]

post some video of what it sounds like.

[mustang87]

look into a possible broken or loose converter bolt or flywheel bolt. Had one on a buddies car that only made similar noise at certain rpms and that is what it ended up being.

This or piston slap Ford stright sixes were bad about piston slap with high milage, just change to a thicker oil.

[stevo]

Just wanted to update this. The idea of it maybe being bad bearings forced me to pull that heavy fucker out. After getting it on the engine stand and pulling the oil pan off, the first thing that grabbed my attention was the brace on the oil pickup tube was broken. The pickups on these models are about 18" long, snaking from the front of the pump to the rear sump. I could grab the pickup and actually move it about a inch each way, and when it banged into the main cap, it sure sounded like a rod knock.

I pulled the rod caps anyways, found them to be a bit worn and on a couple they were worn to the copper where the bearing halves meet. None of the babbit smashed/worn away at the bottom or top of the bearing like you normally see when one starts knocking. The engine has been rebuilt in the past, the crank is turned 010, the oil pump looked good, the piston bottoms were very clean and were probably replaced in the rebuild. I'll probably just put in new bearings and a pick-up tube, and call it good.

I'll say this though, when I first got it running after getting the head and timing set installed, I had the ignition timing WAY retarded, it was idling around 200-300 rpm. Sure was strange to hear one running that slow, it sounded like an old John Deere "Johny Popper" 2-cylinder tractor engine. It is a shame that retarding the timing that much is bad for exhaust valves (and power), cause it idled smooth as butter and sounded cool.

Stevo