small block HELP...keep spinning the same bearing

[JohnM]

The crank was brand new..a scat cast crank. that journal will be polished.,..but no marks or scratches...or by feel. Obviously a better inspection once its tore down

The engine ran for all of 45 minutes to maybe 60 minutes top

We did not open the second cap on the same journal...thats happening today

[Rick Parker]

Just an outside chance but with the 69 block which has a marginally shorter deck height I think there is a specific piston pin height for that engine which is different from the later 351's, check into it. Outside of a twisted rod, be sure the rod cap is installed in the correct orientation and the rod fasteners are retaining their clamping force and have not been overstressed and most importantly that the finished bore of the rod is correct, it must create "Crush" on the bearing shell to keep it captive.

[JohnM]

A question now about the rod bolts...we have already ordered new eagle connecting rods, why take chances as the last set of stock rods were reconditioned.

We did use the old ARP rod bolts as the engine has only 3800 miles. Whats everyones thought that when the first bearing spun...it maybe streached or tweaked those 2 bolts??

is it common to reuse the old bolts when so few miles are on the engine?

John

[JohnM]

DA...I was just thinking....the rod bolts are studs, and the new rods will come with them

John

[CobraEd]

If it is a new aftermarket crank, maybe it is not drilled thru from the main to the rod journal. or maybe there is crud in there.


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[JohnM]

Yes it was a new SCAT crank.....I hoped he would of checked that

thanks

John

[sdfletch]

JohnM,

If you get them, pictures please.

f

[vector1]

.002 on mains and rods is pretty tight from what i've read.

[sdfletch]

vector1,

I believe he said the bearing spun out without damaging the journal. How could that happen @ 002? No blueing either.

f

[JohnM]

I will try to get some photos tonight

what does everyone think of those clearances??

John

[CobraEd]

If it had tight clearences but started knocking right away, I would say that journal had no oil. A tight journal will not knock. Musta been starved for oil. I would absolutely check the hole drilled between the main and the rod to ensure that it is clear all the way thru!!!!!


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[sdfletch]

CobraEd,

We will see. That builder is not happy.

f

[JohnM]

CobraEd

not sure until the head is pulled...but I think the knocking was maybe the piston just tapping the head after the bearing moved??

thats what happened last time

with the crank being brand new and all its "surface hardness" being fresh...and the engine ran for no more than 45 minutes. I think thats what saved the crank

the old crank was all chewed up...but it had been ground and I think most of the hardness ground out of it...if that makes sense


John

[sdfletch]

The only thing I can think of is that the rod id is elliptical. That would have been checked by the builder as well as twist, etc.

f

[Gaz64]

Did the engine builder wash the new crank and check for fluid flow through all of the oilways? As others have mentioned, either rubbish in the channel between your failed bigend and the adjacent main that feeds it, or insufficient crush on the bearing.

Did he measure bearing crush? I've dealt with an engine reconditioner that doesn't understand that term.

1 thou per inch of pin diameter is acceptable for bearing clearance.

[JohnM]

Hello All

Latest update.....I am not sure if this is blowing smoke or for real, but the current therory is a bad crank...its a new crank from SCAT, just a cast model

We originaly wanted a Eagle but couldn't get it in time so went with the SCAT

Anybody know anything about their quality etc on the cast models.

It didn't seem to be an oil supply problem...no discoloration from heat, no turning blue etc....

The builder is eating the crank...maybe it can be warrantied....45-60 minutes on the engine I picked up for the new connecting rods...he's doing all the labor, bearings, gaskets cleaning etc. etc.

I feel he is doing right by me

Any thoughts on the crank...I still think it was the connecting rods

John

[CobraEd]

OK, . . . . But what about the crank is wrong?

Journal too big?
Journal too small?
Journal not square side to side?
Journal out of round?
Journal stroke longer than the rest?
Oil passage too small?


It is fine to say it is the crank, but you should know WHY it is the crank, or you are shooting in the dark.


Ed

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[vector1]

so now it is a bad crank. what was it when he did the assembly?

[Jac Mac]

The only thing common to this problem is the block- you have changed crank-pistons-rods etc ,but all still in the same block and the same bearing ( # 3 if I read your posts correctly ) which feeds from the center main bearing. Now you mention that prior to the bearing problem occuring that the car was involved in a minor accident--- could this have hit the crank on the snout & damaged/cracked the center main web to create an internal oil leak and reduce supply to that bearing, remember this is the thrust bearing, any hits etc are felt here---- or is there a end float or spigot brg problem that is causing similar issues. The center ( or Any ) cam bearing can also cause a localised loss of pressure at each main journal if it has delaminated and lost some bearing metal.

[blykins]

From my experiences, Scat is better quality than Eagle. I'd rather have a Scat crank.