So I rebuilt a 400, I think without cutting corners, with Aussie heads, bore .040 over, stock bottom end, lunati cam, roller rockers, hei distributor, Holley truck avenger, headers, clevite bearings, hi-vol oil pump, etc... I broke the engine in for 20min and drove an hour or so with open headers (loud!!!), then changed the oil. I then put exhaust on it. Now with the exhaust being quiet I noticed a knock. I checked valve/liter adjustment and rockers etc.. And all seems well. Timing good, no exhaust leaks that I can tell. It sounds like a rod knock. I don't hear it at idle, but above 1200-2000 rpms in neutral it is there, and while driving at normal highway speeds while barely touching the gas is worst. Can't hear it during accel or decel though. I don't think it could be detonation, as I am running 100LL aviation gasoline in it, since I'm a airplane mechanic, it is readily available to me. I have a 4 speed manual tranny, and the truck is a 74 F250. Any help would be appreciated before I go and pull the pan. Thanks!

 

Hows the oil psi ? Did you get the block align honed ? Rods redone ? Were the bearing clearances checked with a good dial bore gauge that reads in .0001" ? I'd dig the old filter out of the garbage, cut it open and see whats in it.

 

Oil psi good (stcok dummy gauge), block was line bored, rods, resized and measured by machinist. I didn't use plastigauge though.

 

Also I did cut the filter open, and no flakes of any kind, a tiny bit of steel shavings but very small amount

 

I put a timing light on it and it "appears" the knock jives with the light, but things are happening so fast, its hard to tell

 

Things to check:

Clutch fan if you have one. If it's worn out it'll be loose and make a knocking sound.

Timing, make sure the vacuum and mech advance are functioning and not stuck.

Belts, misalignment or if you have ever used belt dressing it's a no no. I have seen people tear engines apart when the noise came from belts that were grabby due to belt dressing.

Other accessories, the water pump alt, etc. if worn out.

Harmonic balancer, if it's slipped it will give bad timing readings.

Good luck

 

Pulled the pan. All rods are tight, no metal in the pan. The search continues...

 

If you didn't check bearing clearance then you took the hand that was dealt to you, never a good idea when your dealing with clearances measured in .0001". What kind of pistons, forged or cast ? Do you know how much clearance is on them ? Without it being here it's hard to say. Too much piston clearance can make a rapping sound under the circumstances you describe but then again too much main bearing clearance could as well.

 

Cast stock pistons. Ruled out bottom end of engine when I took the pan off. I'm going to pull the tranny and look at the flywheel and clutch assembly next.

 

Well, found the problem. Turns out it was #1 rod bearing. Pics attached of #1 and #5 bearings, which share the same journal. Crank has tiny grooves in it on both journals. Are small grooves normal after 5 hrs of operation? should I change all rod bearings? sand/polish and measure crank? With what? What would cause this?

 

Oops! cant figure out how to post pics

 

So only the front two bearings are worn into the copper (#1 and #5). #5 has very minor grooves. #1 has worse grooves. #1 journal is about .009 smaller than it should be, but isn't really out of round. The question is can I polish it down carefully to achieve a .010 undersize and use an undersize bearing? Would #5 also need to be undersized? Or am I just screwed to replace the crank?

 

Your just screwed. I wouldn't use a crank that any journal was anymore than .0003 out of round and that is pushing it. I wouldn't push a rod bearing clearance anymore than .0028 max. There is no way to sand a crank down. Now if the crank is standard, a journal was within .0003 but came up loose on the oil clearance you can buy undersized bearings in .001, .002 to make up some difference but you need a good bore gauge and mics reading in .0001 to get the job done right.

 

something broke loose and got in oil system. their is actually a way to undersize but the cost of doing it is more expensive than replacing crank.

 

Where is the best place to get a new crank?