One of the engines i tore down last month has a spun main on #1 This is the only bad thing this engine has going for it at this point, everything else is with in factory tolerances. This is the first engine i torn down that had main problems. one of my questions is do i need to line bore this? i got 2.6410" - 2.6390" in two spots using a hole gauge and micrometer with out bearings with the cap torque down. The main saddles have a line its not very deep but still can feel with a finger nail. I suspect the #1 main ran dry and spun there was no peaces in bedded in any of the other bearings, the crank you can feel some groves with a finger nail but not deep. Here is some pics.
Thoughts?

 

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If you can feel grooves in the crank, get it turned. It needs to be perfectly smooth when you run your fingernail across it.

AFAIK, a line bore is only used when the main journals are not perfectly aligned from front to back. I don't believe it would take off enough material to correct something like what you have. But I'd wait for someone with more experience to chime in on this.

 

A line bore would be good insurance. Same goes when you blow a head gasket. The block should be decked to make sure it's flat and square but nobody does it. A line bore makes sure the mains are all perfectly aligned. After a couple of line bores, the crank is moved slightly higher in the block. Shorter timing chains are required to take up the slop. Slightly higher in the block also means the pistons are moved up very slightly. A lot of measurements need to be retaken to make sure everything is good.

Even though a bearing spun, the damage looks minimal. Polish the crank and it should be fine. The block webbing along the surface of the main bearing doesn't look like it was heated up. If there's any indication of black or blue metal then it will need to be line bored. If the webbing got too hot, it will have distorted and will take out another bearing.

The last time my engine was built a few years ago, I had a machine shop put the whole rotating assembly (short block) together. I let them deal with clearances and tolerances. Never had a problem since.

 

So the engines been on the back burner, i happened walk by it today and i notice the main that had spun a bearing that cap wont sit flat on the machined part of block i tried everything to get it to seat by hand. Its like its bowed out and gets hung up on either side.
I don't think i noticed this when i used the hole gauge


here is whats going on.

 

Both of the 350s I rebuilt were like that. A quick tap with a hammer seated them flush. If yours won't sit down with a couple light taps you might have a problem though. Are you sure you didn't mix up the order of the main caps?

 

I number punched everything prior to removal. I was kind of hoping that was the case, and that i could just tap it on. It wouldn't go on by hand. The reason i was a bit alarmed is that its the cap on the main that had spun bearing.

 

Doesn't look right to me... looks kind of like the driver's side surface of the cap isn't parallel to the register surface of the block. Looks like somebody didn't have it seated right, and just ran the bolt down anyway, and tweeeeked it.

I believe if it was mine, I'd be looking to ge it fixed. Or, just get another block, as cheeeep and eeeeezie as 350 bolcks are to come up with.

 

^^^ Good advise.

I can't tell from the pictures. Is it a 2 bolt block?

I'd buy some nice splayed milodon caps, and have the machine shop drill and align bore it and move on.

-- Joe

 

Do you have a good straightedge ruler or a machined table surface like a miter saw table you can set the cap on to see if the two ends are parallel to each other? You could even just use the head gasket surface if it isn't damaged.

Looking closer at the pic, I agree with sofa that it doesn't look straight. I originally thought it was just the angle the pic was taken from making it look like its off.

 

Tap it into place or use the main bolts to suck it down into it's saddle. If they go in too easily THAT is a problem (a problem that can't easily be fixed, by the way). They are an interference fit- it's what keeps the cap from walking side-to-side (not the bolts).

I've not seen a spun main do so LITTLE damage. Usually the metal is pretty well worked when the main bearings spin.

I guess my first piece of advice is to have that main mic'ed in multiple locations. If it's round and within tolerance you might be fine without machine work to it. It's possible it spun just becuase the main cap bolts weren't torqued properly or the bolts are shot (have been taken beyond their yield point or outright defective from day 1).

If that main measures out OK I would suspect a problem with the bolts or lack of adequate torque on them.

USUALLY when you spin a main due to lack of lubrication, etc., it looks really bad. Those bearing shells are clamped in place VICIOUSLY TIGHT by the main cap and bolts. It should take BIGTIME force to make them spin, which is why it typically does so much damage when it happens. Yours looks like they were barely clamped in place when they took off the tabs and started to spin.

I would be reluctant to accept the "usual suspects" in this case. The post-mortem just doesn't look like typical carnage that happens when you spin a main.

Keep your detective hat on for a while longer.

 

Not sure what the deal is, i do suspect that this engine was rebuilt sometime in its life, in fact when i pulled the pan i found 2 or 3 decent size peaces bearings around the oil screen plus the engine was way to clean and the rings looked new so more then likely it was.
Im completely confident its not the block it self.

It is a 2-bolt block. That's a good idea. but i don't need splayed caps i wont be pushing anything past 400BHP. If i were to think about going that route after buying the caps then drilling / align bore i might as well start looking for another block.


That's a good idea i will have to do that didn't think of that.
its getting hung up on either side of the block when you trying to get it on.



Yea i lucked out and tore into it before it became problematic.
Good points. If i determine the cap is not the culprit then all be looking at the bolts, i don't have a bolt stretch gauge but it probably be cheap insurance to just buy all new ones.



Thanks for all the good reply's so far guys!

 

I agree w Damon; it doesn't look like the shell was clamped tight. Which is consistent with the damage to the cap.

Kind of like, maybe in the most recent "rebuild", the klutz *I mean assembler* got in such a hurry with his impact gun, he forgot to make sure the cap was seated, and strafed the bolt to 75 ft-lbs anyway, and that both bent the cap and held it up from its proper place such that the shell didn't get crushed into the journal properly?

 

Chevy caps fit tight and need to be tapped down no problem there. If the rest of the block looks good then a good line bore and hone will put it back into service. The crank looks as if it need to be turned to me. Have a good machine shop in your aera ?

 

Last week i checked into it more. I laid the cap on the machine part of the block were the oil pan bolts to and its flat. i did get the cap to seat it took a few knocks to get in but it fit fine.

 

#1 cam bearing... looks like it spun, its not aligned with the main oil hole.

 

Been a lot of trash in the oil moving around in there.

 

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R

 

Looks like your bearing surface is completely worn away too. What do the rest of the cam bearings look like?

If the main cap is flat on the oil pan gasket area then it will be fine. Glad to hear it still fits in there.

 

I'm not sure yet i haven't pulled the cam all the way out yet, but as far as i can tell they look fine.

 

Now all have to figure out why the can bearing spun...

 

It looks like the cam bearing is still in its proper position that it would be from the factory. It looks pretty worn, though, like there was a lot of metal and other junk in the oil that ate up the bearings and scored the journal. Id go ahead and replace them all, regardless of what the ones behind it look like. The cam might need some attention as well. That looks pretty rough. If your running high perf. valve springs, youll want to use alum. alloy cam bearings. Those are much more durable, but require a clean journal, proper clearances and clean oil to work properly due to the fact that theyre much harder. Since you spun the main bearing, it might not be a bad idea to pop out all the plugs, and thoroughly clean out the oil passages to make sure there isnt any junk in there,

 

The oil feed hole on the cam bearing is not aligned with the main oil hole, it is completely blocked by the O.D of the cam bearing . which would indicate it was installed wrong or spun id assume. I didn't find any peaces of debure or metal etc pressed in any of the other bearings. I will be replacing all the bearings. The cam is stock so im not worried about that, i will be replacing it with a LT4 hotcam or similar, as far as the valve springs go all be using the LT4 hotcam springs or similar. There is so many types of bearing materiels i was looking into these: http://www.summitracing.com/parts/SUM-173000/
i will be popping out all the oil valve plugs and freeze plugs and thoroughly cleaning the block a few times over.

 

Grit in the oil, excessive heat, metal to metal contact will cause the bearings to spin. If it has spun a cam bearing I would junk the block. Unless it is a rare numbers matching block the cost of repair would not make sense.

 

The cam bearing bores in the block have an oil grove around where the cam bearing mounts. The oil holes dont have to line up with the oil feed hole, and normally should not be installed as such. The front oil holes where located at 12 and 4 o'clock from the factory (with the block upright), as far as I can tell. Yours still appear to be in the right place, and the bearing still looks like it has its oil holes centered over the grove. Its unlikely for them to spin, as theyre in there tight as hell as theyre a press-fit.

Silicon-alum. alloy (bi-metal) is generally on the harder side, and will last longer and tolerate higher loads. The alloy harder than the babbit bearings, which are softer and will fail more easily than the alum ones. The alum. bearings are not as tolerant of dirt, or poor clearance as they will not allow much embedding, or conform to the journal. If contaminates make it in, theyll score the journal, so if you use them, make sure to clean the block real good.

 

i recently bought a sbc 400. guy told me it had been sitting for a long time and didn't know if it ran but thought it was in good shape as he wouldn't have bought a junk engine. took it home and took the mains off to check the bearings. everything looked mint. forgot to torque down the number four main and tried cranking over the engine. noticed that it was hard to spin and took all the mains off again. the number four main bearing had spun about an 1/8 of a turn. it made a tiny groove even less so than the original post's and only to the bearing. put everything back together and now it spins fine. im not looking to spend too much money on this build so im wondering if i can just run it like that. i know its not the right way but will it cause serious damage later on?

p.s. this is also my first attempt at building an engine

 

Your second will be coming up REAL SOON if you try that.

 

can i get away with just replacing the bearings?

 

You could try. Need to check the housing bore to do it right.