400 Main Bearings Problem
 

Ok, so here is my issue. I rebuilt my 351M into a 400. While rebuilding I checked my bearing clearence with plastiguage. Being new at this, I thought you were supposed to tear off a piece and place a piece of plastiguage inside the bearing with the paper still attached...yeah I know...stupid me:(. Well even then it shown it as being .003 for clearence. I decided to go with el' cheapo bearing that I got with the regound crank. A few people told me not worry about it, so I assembled the engine. Now...when I fire it up, it shows barely 5 PSI Hot for oil pressure. Unnacceptable.....so.....I pulled the engine out again :'(

Now, I checked the clearance with plastiguage the CORRECT way, and it looks like damn near .0035, close to .0040 for clearence. I removed the crank from the bottom end, mic'ed the journals and bearings, and it measured pretty much the same. I mic'ed the reground crankshaft and shown right around 2.979, which would be .020 under according to my information. Rod bearing shown between .0015 and .0020, so I figure those are good.

So....should I inform my machinist about this problem and order .030 underized bearing and have him regrind the mains, or would I better off ordering higher quality .020 undersized bearing and hope they off a tighter clearence?

The only way to check bearing clearance accurately is with a dial bore gage and a set of mics reading in .0001". Any competent engine builder will tell you "Plastigage is junk". Was the align bore in the block checked ? I seldomly come across a 335 block that doesn't need align honed, particularity later ones.

i bought i rebuilt crank for 252$ at rockauto. the crank weight 72 pounds with the main bearing and connecting rod bearing .010 under, not to bad.

I did try to find a dial bore guage but I could not find one to borrow, and money is too tight to buy one.

My father and I measured the bearing with a telescoping guage and a mic that does read to .0001 . As my father is a machinist, I can trust the reading to be as accurate as what can be done with a telescoping guage and mic. If need be, I can get an inside mic from him and see if that shows a different reading.

After numerous repeated readings on both the bearings and the journals, we had shown a clearence of .0035 to .0041 for all the journals. What this is showing to me, is that it is nearly double what it should be. I can only believe that this is the source of my oil pressure loss.

So what do you call elcheapo bearing?
We have found that because of "tolerance stack" we will
grind our crankshafts larger than what the specs say to achieve the clearance we desire.

Goose, what kinda pressure does it make when you start it up?

what weight oil?

are you doubledarn sure you have the 2 internal lifter galley plugs behind the cam timing sprocket installed?

and the oilpump is mating tight to the block?

if the oil pickup screen is into the bottom of the oil pan you'll see low oil pressure also

i'd fill it with 20/50 or straight 50 and see what it does but i guess you have it apart... i wouldn't put it back together just to try heavy oil

Plastigage has its place!
 

Once found a cracked block 400chevy with a broken main webbing. Was doing a shade tree repair job for a mud race once many yrs ago. The main webbing was cracked but hadn't give it up yet. the plastigage was looking kinda strange and on a paticular journal and then we started looking. the oil pressure was all weird when running and this was a tool we just did some checks with to help us find our answer. The main bearing was worn weird also and also a clue. been building engines for many yrs and plastigage is a good tool in the correct hands. i do own my own mics and everything. Another question is what happens when you can't lift the crank (to heavy and need to do a check). I work industrial engines up to about 3' (feet not inches) diameter mains on occasion and this is a problem. so what do you do? Also do a jack test on bearing clearance. Meaning a a strap around a crank and use a crane or hyd jack to lift crank with indicator on it to see movement in crank clearance. If crank is over 15' (feet not inches) long this is a interesting test. Just thought I would throw this in their for something to think about!! lost in wyoming and lovin it. forgot to mention their are several other techniques to check bearings clearance in the field but won't get into this at this time!

Before you install again get and oil pump primer (5/16 socket on a LOONG 1/4 inch drive extension) and spin the pump with a drill and read the pressure

Ship or big diesel engines are neat but IMO the coolest engines ever made are radial engines. & yes, they do make big mics and bore gages for any size hole.

...or if you don't want to risk dropping something down in your pan you can pull all your spark plugs, then the coil wire, and just crank the engine (10 seconds at a time) until your gauge show oil pressure.

OK, let me stray OT a bit: In engines that size, do they use plain bearings for the mains and rods, or rolling element bearings? It would seem to me in bearings that size, plain bearings would require very high oil pressure, not to mention a tanker full of oil. Just wondering.

Tri-metallic type, non roller. I have never seen roller bearing main or rods on anything other than motorcyles, snow mobiles, etc...

10 PSI per thousand RPM is about the average, but I have seen farm trucks with alot less for a long, long time. A Cat 3406E will run about 15 to 25 PSI per 1000, which still isn't alot.