ChrisWhewell

I bought an engine from a guy last week, it had a spun rod bearing on #8.

A month prior, I bought heads for an LT1 from a guy with a spun rod bearing. A month prior to that, I bought a set of heads from another guy with a spun rod bearing. It didn't occur to me to ask them which rod bearing had spun.

So, I'd like to respectfully request those who've experienced spun rod bearings on LT1's to kindly reply as to which rod bearing had failed in their situation.

I'm wondering whether we'd see a pattern here. If we find that a particular rod bearing is the one that predominantly fails, perhaps it could help us to identify a weakness in say, the oiling system, that we could take measures to prevent. ???

JimiHendrix

Guarentee you that all involved either 1) over-revved 2) ran low on oil 3) spent too much time in the twisties.

My understanding is the that small block chevy is a time proven and reliable engine mechnically if maintained and given a normal supply of oil.

pianoguy

I just spun one last Saturday on my LT4 - I believe it is #8 or #6 (haven't torn down yet). I romped on it a bit on the way to the golf course, nothing out of the ordinary, and all of a sudden, I had the dreaded knock from about 2000 RPM up. I drained and strained my oil - didn't find any chunks, but when I cut open the filter, there was lots of bronze-colored sediment in it.

A guy at one shop told me that he sees this on cars that sit for extended periods, or aren't driven regularly - he said the wrist pins seize up a bit and put extra strain on the bearings. Another guy said it was likely the forumulation in today's oil.

ChrisWhewell

Thanks alot. Please report back which one it was. There's a good chance that with enough input, we might be able to diagnose a currently-unknown cause of this, and through the wisdom of the group, devise a remedy for during a build, to lessen its chances or prevent it.

M.J.L.

This is my SECOND time with a failed bearing. The first time is still a mystery. The second time which was last month I overrevved it... If there was a magnetic drain plug/magnets, would that prevent the bearings from failing? I am confused as to why this happens. I will be watching this thread eagerly.

M.J.L.

Pics of bronze sediment? Size?

ChrisWhewell

Was it the same cylinder's rod bearing that failed in each case ? If so, which cylinder # ??

thanks a bunch.

pianoguy

Sorry, didn't take any pics of it. It was very small (small enough to be suspended in the oil), no big chunks, but a fair amount of it.

M.J.L.

#6..

JAKE

Since I visit several Forums, I've seen this a LOT. I was REALLY surprised at just how many engines have failed from rod bearing issues.

I suspect three things:

The factory rod bearing clearances are too tight. They were set on the tight side in order to use low viscosity of the Mobil 1 oil.

The rod bolts stretch and, possibly

Crank flex.

I'm not all that sure on the crank flex part, but am convinced on the first two.

Jake

West Point ROCKS!

Pete K

I am fortunate enough to be tight with 3 quality engine builders.
I am guilty of running tight clearances with thinner oil on my street cars.
I have looked over the shoulders of these 3 builders enough to see that your suspicion is correct, in my opinion. Every motor I see come apart for a refresh that has loose clearances, looks like brand new (bearings) every time I see it.

ChrisWhewell

maybe we need to build with "20th century" larger clearances and run 10W-40 then ?

What I'm hoping for on this "thread" is that a lot of respondents will state that it was predominantly the same rod bearing; then some focus could be put there. But, if it turns out they're random, there would be useful info in that too.

I have a 92 truck with a 350 in it with 275k on the orig motor, not burning any oil. I beat the **** out of that thing, towed with it, even changed the tranny at 140k but the motor keeps on humming. I did a lot of oil changes on it. Jakes comment makes me wonder about the bearings. I hope I don't find out anytime soon though, until at least the weather cools off in the autumn.

Pete K

Most builders I know do just that. They open up the clearances and request you run thick oil. I see why. It works.
If you are lucky, you may notice a trend on rod bearing failures that occur on the same journal, rather than individually. On a small block chevy, the cam bearings are fed first, then they feed the mains, followed by the rods.

Ryan59

Rods also get a pulse of oil instead of 100% of the time. Cross-drilling the mains helps.

ChrisWhewell

Look at the coolant flow holes in the heads and block, and compare it to the inner diameter of the gooseneck where the thermostat's housed.
Way more flow cross sectional area in the heads/block than enabled by the goose, suggesting it doesn't take much flow rate internally to really cool the thing. The water pump pushes water into the front of the heads and since the size of the holes between the heads and block towards the front are the same as they are in the rear, one would probably expect more flow in the front than the rear. What I'd do, is restrict those towards the front, to put more flow towards the rear. If I can help it, I'd want each cyl to run the same temp, the back ones are known to run hot, probably for this reason. Then again, I'm the type of guy who wants an oxygen sensor at every exhaust port and a dedicated microprocessor for each injector responsive to its output.

Probably same scenario on the oiling system, where more is present at some places than others. Within operational parameters seen by most on the street, its not an issue , else GM would have by now made mods. My guess is that a good racing team's engineer would put an oil pump to a bare block and measure the flow rate at various pressures for every hole in that damn thing and use that info to make appropriate corrections.

BrianCunningham

I know the factory rod bolts are weak

Over rev it and the rod bearings are toast.

I went non factory on my rebuild.

redrose

only trend i've seen is "glue failures"...guys put xx-50 weight oil in and rods or mains 'spin", then they will ARGUE that "joe schmoe uses 50w in his race car, gotta be good for mine"

Pete K

Are they? I am not challenging your opinion, but thinking out loud.
I remember once, tears ago, looking at the psi rating of an arp vs a stock gm bolt, and don't remember there being a huge difference.

My friend has a 67 nova drag car. After blowing up 3 professionally built engines, he was down in the dumps. He requested I help him assemble something from the crap he had in his garage.
It was all unusable, as it was blown up lower ends. We decided to do the following:
Driveway hone a good used block from a 76 stationwagon.
We used a cast crank, and piston combo from a 72 chevy impala.
Stock crank rods, and pistons. We even reused the stock rod bolts, as rods were straight, and sized properly.
Basically, a stock, 2 bolt mid 70's shortblock. The only thing we did other than balance the crank, was polish it heavily, and open up the rod bearing clearances to .0025-0028.

After spending $400 to build the entire shortblock, motor was dropped into the nova. The 4500 stall, transbraked powerglide was hooked to a 5:13 rear, and shift points were set @ 7400 rpm, with an occasional overshift.
Every time that car made a pass, my fingers were in my ears.
Bracket raced for 4 years, every single weekend, was consistant, and placed in the top 15 of the points all 4 years (meaning, the car made many passes due to the points status).
After deciding to build a "real motor", disassembly of that junkyard dog revealed like new bearings. Pretty compelling for me.
Anyhow, just my 2 cents, and a story, imho.

ChrisWhewell

Excellent data point, that's a four-star posting. Thanks.

I'm particularly interested in your phrase about balancing the crank.

I just bought a set of rods, GM part # 14011082

http://www.gmpartsdirect.com/perform...CATID=847.html

The dealer told me I'd need to re-balance the assembly if using these rods. I'm planning to run stock LT1 30-over hyper pistons GM # 10159438 with these rods, but I don't have a crank yet. I'll need to balance it all, but I don't have a friggin clue as to what's involved in that, or what should guide my selection of crank. Any suggestions ? I'd be highly grateful....

Am going with the GM 846 cam and my own version of the 10207643 heads, self-prepped using the principles of David Vizard and personal experience.

Pete K

Balancing involves weiging all components.
Piston
Rings
rod bearing
wristpin
connection rod (both big end, and small end)
Then there is a weight allowance for the oil that will hang on the rod.
Once the math is figured, it is converted to a "bobweight"
The bobweight is duplicated with jigs that attach to the crank.
Crank is spun, and material is removed, or added to the counterweights of the crankshaft to achieve the balance.
What is an acceptable balance deviation is opinional, but most engine builders balance to a tighter standard than a production engine.