Ford 302 bearing diagnoses

So my engine guy finally got my engine tore down. I was playing out at Gingerman and noticed lowish oil pressure (20 psi) and a vibration above 2k rpm.

He said the front and rear main bearings are toast. All the rest looked fine so he doesn’t feel like it was a loss of lubrication. His preliminary thoughts are that the mains will need to be lined bored. Also all the rod bearings are fine too.

Here is pics of the damage.

I don't know that I would jump straight to line boring.  I would check oiling passages, maybe a cheap crank, maybe a crank with a poor finish on the front and rear journals.  In all the old iron V8s I've built, I've seen dozens of bad cranks but I've never seen a block need to have the crank journals line bored.  The crank is a piece of cast iron that spends its entire life spinning half the speed of sound while getting hammered by huge explosions 15,000 times per minute.  The block is a huge piece of cast iron that is 250 lbs, triangulated in every direction, and doesn't move.

I might take a known good crank and drop it in without bearings and put the caps on.  Just let gravity let it sit.  Use feeler wires to test gaps at the top and make sure they're all the same.  Also check for gaps at the bottom of any journals.  That's where you would get into the line boring.

Wrong balancer/flywheel combo? 
Some 302s are internally balanced, some need the external balancer. The wrong combo will cause some weird issues. 

Typical SBF.  That's the norm and can be rebuilt.  I'd go with a new balancer and flywheel.  Make sure you get the correct imbalance.  302s and 289s were 28oz and 5.0s were 50oz.  I have a strong preference for Ford Motorsport parts for this application.  If it's a manual, the FMS throw out bearing is the only one to use unless you love squeaks and frequent failures.

I'd also think normal SBF that sees a lot of high RPM use.  This got more common with the switch to the 5.0's greater imbalance.  Supposedly the 351W firing order helps, Ford did it to alter main bearing loads.  (Interesting that the 5.0 got the 351W firing order)

I have to disagree that the engine block is solid.  It is a lattice of springy flexible iron that is mainly a bunch of thin cross sections that hold the water jackets to the cylinders and bearing journals.  You can measure the distortion in the main journals from torquing the intake manifold down.  (And people wonder why GM went to low-torque intake manifolds)

I was going to say mean things about 302s and crank bearings, but I thought some people might get offended.

I'm a long time fan of the Ford 302, but I'll concede that that isn't an engine block that is known for rigidity. 

Well, when we assembled the engine, I did have engine guy balance the rotating assembly but I did have to replace the stock balancer since the rubber bit was coming out so I installed a FMS racing one but it was a matching 50oz weight. The balancer going bad might have contributed to this.

Might be time to go with a forged 331 stroker kit that is 28oz balance.

I’m also running the 351/302HO firing order.

In reply to Floating Doc (Forum Supporter) :

Fo sure. A big turbo will split that thing right in half, lol.  

In reply to GPz11 (Forum Supporter) :

A failing damper will definitely contribute to main bearing shenanigans.  The crankshaft is a spindly rope that is twisting this way and that under load.  The fact that it is only the outer half of the front shell seems to want to point to this.

I would look very very closely at the crank for cracking. They like to break in the 1/5 journal area even in 28oz applications. The backyard way is to hold the crank and hit it with a hammer, it should ring like a bell instead of thunk like dead wood.

I saw a pic a long time ago of a bare crank with a damper mounted on it, one can see just how overhung the damper's counterweighting is over the #1 main and how much leverage it has over the crankshaft.  Can't find it now.

I like to be ultra precise with my wording when it comes to engine parts when I can. The thing on the front is a balancing weight, but its role in damping crank harmonics is its key role in any engine.  Many people see it called a balancer and think it is only there for engine balance, you see..