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eating up main bearings

7K views 63 replies 17 participants last post by  DoubleVision 
#1 ·
i got my old engine torn down and was quite surprised at what i found , engine only has about 3k miles on it , everything looked good except for the main bearings

center 3 lower halves




center 3 top halves



bottom half front and rear main ( flash washed out the copper showing threw but it was dead center on both)


im curious as to what u guys think is the cause
#1 to much power and its trying to run over the crankshaft
#2 not enough clearance
#3 oil starvation
#4 other ?

the rod bearings were pefect , no other signs of distress in the engine except for those 3 lower mains ( even the crank doesnt have a mark on it )
 
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4
#5 ·
we put the crank in a lathe , and it spun true on the mains, ill have to get a machined piece of roundstock and run it threw the main bores and see if any are out of line

this engine is getting freshened and then put in my truck so i wanna see if i can find the source of the issue
 
#10 ·
For starters there is evidence of a lot of trash moving with the oil. Some of that could be the top plate being removed from the bearing and circulating with the oil. But that is usually babbit which is pretty soft. Are the rod inserts scratched up like the mains?

The three lower middle inserts wiped but no their matching upper nor the end inserts upper or lower are showing distress in the center of the engine. This can one or more of several causes:

- The lower inserts are different size than the uppers. An example could be the uppers are standard while the lowers are +.001.

- The caps are not correct for their location mixing up the center caps as to location is not that uncommon. This throws the register off for any specific cap.

- The block is warped, but one would think the block is bending upward through the center which is opposite of what one usually finds in such cases where the middle of the block sags a few fractions of an inch.

Bogie
 
#11 ·
unfortunatly i didnt build this engine but the caps are marked , still doesnt mean they werent mixxed up

the rod bearings had a few gouges but other then that they were perfect
whatever was in the oil was soft because it only left marks in the bearings and not the crank, the crank came out looking like it was freshly polished
 
#12 ·
Micropolishing the crank in the wrong direction will cause an identical effect, on a microscopic inspection you would see polishing create tiny scales on the surface of the crank...this tends to wipe the overlay off typically on the caps only but it can happen to both halves.

The crank needs to be polished in the same direction of rotation as occurs in the engine, I used to work for a certified Ford engine rebuilder and seen this first hand as a number of engines came back with wiped bearings just like yours. The crank grinder who did the cranks for those engines was putting the cranks on the grinder reversed and polishing them backwards.

It took a long time to figure out how it was happening since the problem wasn't obvious and was on a microscopic scale which required a work flow study and review of polishing cork used etc.

What he was doing was grinding the cranks first and stacking them, then chucking the tail later to polish them. Engines turn clockwise so the micropolishing belt would be rotating against the grain of the previous grinding operation if you do it this way.

Usually it doesn't affect clearances until it wipes the copper off and only affects the Babbitt during starts when the crank is touching the bearings, first sign is low oil pressure...next is cold knock...last is a constant tapping making it hard to diagnose.

One more thing to check, it's very hard to prove unless your there when your machinist chucks the crank to polish it.
 
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#17 ·
thanks guys lots of information and ideas here , i plan on thouroughly checking everything on the new engine

the engine in question here with the bad mains , was running twin turbos with 20+ psi of boost , and a relatively large shot of nitrous (50-100 shot) for the boost number

the new engine im building is going back into the same application but im looking to make about 200 hp more , and the new parts should get me to or past my extra power goal

i just got the block back from being honed , and line bored , we left the deck alone since the pistons are .005 in the hole and thats were i want them

assembly should start this weekend if i get the block back from paint in time im having it painted to match the paint on the car , which is gloss black with a blue pearl finish.

i will start a build thread here later tongiht when i get out of work, i will document every aspect of the build from clearances to part specs etc incase anyone in the future would like to do a similar build
 
#19 ·
i generally stick with 15-40 in my turbo engines if i dont run that or cant get it for some reason i will either run stright castrol 30 or 40, ive found the standard 10-30 oils to allow for a slightly better spoolup but saw much less parts wear with the thicker oil
 
#20 ·
Interesting, I say this because I had A crank go out on A new chevy truck once. One ton dually crew cab 1998 454 engine. Just a little over 10,000 miles. They replaced the crank and I drove the truck to over 150,000 miles before trade. I found out later that GM had quite a few of these engines do that in this year. They never said anything about WHY this was happening.
 
#23 ·
unfortunatly i cant answer most of that as i didnt build the engine

i swapped out the flat tops for domed forged speed pros in my driveway to get compression up
stock oil system/pan with a melling pump with the pressure relief shimmed
its on gas with meth injection , this year my fuel system will be capable of running e85 or 100% methanol if i choose to

new engine is going together with a 7quart champion pan and a hv pump with relief shimmed for 50 psi idle hot aiming for 90 psi at a round 6k

im going to aim for 2.5-3 thou on main clearances
 
#24 ·
was the crank balanced correctly cause that will hurt center main cap bearings---or another very common thing nobody has talked about here is detenation will do the exact same thing cause its trying to push the crank out the bottom therefore the center cap bearing get erased---bad octane fuel--to much timing--spark advance will all cause detonation and hurt bearing very quickly?
 
#25 ·
i only detonated it once when i first setup the nitrous tune , it poped the head gaskets instantly , other then that ive been real good about keeping it out of detonation atleast according to my datalogs/knock sensor

my other idea is that my antilag system did this , with only a 2,500 stall i can build around 17 psi of boost off the footbrake/antilag system , ive never heard of it doing this to a crankshaft though i do know its very hard on alot of other parts
 
#26 ·
unfortunatly i cant answer most of that as i didnt build the engine

i swapped out the flat tops for domed forged speed pros in my driveway to get compression up
stock oil system/pan with a melling pump with the pressure relief shimmed
its on gas with meth injection , this year my fuel system will be capable of running e85 or 100% methanol if i choose to

new engine is going together with a 7quart champion pan and a hv pump with relief shimmed for 50 psi idle hot aiming for 90 psi at a round 6k

im going to aim for 2.5-3 thou on main clearances
Try running a harder bearing with your boosted application.
I don't recall what your crank is made of, but with your boost plus the sauce I would run an H series Cleavite bearing with the coating.
Shoot for the larger bearing clearance, run a heaver race oil.
It has been my experience if the caps get mixed up you will not be able to rotate assembly. Even if you could turn it over the bearings would have spun in no time.
When ever I see bearings like yours on an engine tear down it is usually caused by way to many miles on dirty oil that should have been changed years before. Since this is a performance engine I would doubt that lack of good clean oil is the case.
One last thing, if this is a SBC is the plug under the main cap to force all oil to the oil filter first?
 
#28 ·
355 factory 4 bolt block gm forged crank , good factory rods speed pro popup .125 domes which were reshaped to get 9.5-1 with the 72 cc heads ,twin 57mm turbos efi with a megasquirt standalone , 80# prescision turbo injectors , single 20gph meth nozzle, a nos brand single fogger 50-125 shot 92/93 pump gas with the nozzle spraying stright methanol

car runs 12.5 afr's under 12 psi with total timing at 24#
above 13 psi it pulls back to a 12.0 afr and 18* total advance
afr numbers are on gas only when i spray the meth it drops about .75-1 point
16* timing on a 50 shot
15* on 100 shot

all timming numbers have an additional retard amount based on air and coolant temps , air temp based retard starts at 175* air temp and can pull upto 3* coolant based retard starts at 210* and allows for 3* max
knock sensor can pull back upto 5* based on number of knock counts

new engine is a low compression 350 ci ( flat top 4 valve relief ) 72 cc heads
parts list is pretty much follwing the old engine for internals

with an intake and fuel injector / pumps change switching to 120# injectors and twin 340lph pumps , going from a ported gm tpi intake to a stealth ram intake

this is a street strip engine , and sees alot of street usage
 
#29 ·
Can you define good factory rods??
Just kidding.
Lots going on with your set up.
Your CR seems a bit high for your boost application but your problem is not lifting ring lands and burning pistons or blowing head gaskets.
A forged speed pro piston while being very heavy is obviously pretty stout.
How many miles on your engine?
Is it possible you have just wore your stuff out?
A factory forged crank has it limits compared to better 4340 forgings. the 5130 series of forged cranks will bend/whip/snake quite a bit. the heaver the reciprocating weight the worse it will be. (heave piston and rod)
What main bearing? Is it a stock type or a race bearing?
What type of engine oil?
Which harmonic dampener?
 
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#32 ·
Try page 8, not 22. The block portion of the bearing is fine, the center caps are shot- main alignment/bore issue. Caps could have been put in the wrong order or the alignment was just bad to begin with.

If it was a crank issue it would also show up on the block part of the bearing, not just the cap.
 
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