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Discussion Starter · #1 ·
i've been thinking about this topic a lot lately. I'm starting to think some of this is overblown.

for instance when putting a new cap on, most everyone thinks the whole motor needs to be align honed/bored.

I don't see why. The engine side of the bearing saddle is still straight and within tolerance. It would be a shame to take material off there and throw the crank location off and mandate a special timing chain.

if the new cap is fitted properly to get the required bearing crush and crank clearance why would anything more need to be done.?

in cases where a block repair is performed causing the main saddle to grow, i don't see why material could not be taken off just that saddle leaving the others alone. If the repair shrinks the saddle, add some material and rebore back to spec.
 

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You have to align bore/hone the new setup because of the variances in each block as they are machined as a set, and you cant replace a cap and expect it to fit.

Its like sex with kobi bryant you can kick and scream, but in the end its just going to happen.
 

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Back to the regularly scheduled programming

When a block is machined, the caps are installed and one of the steps is to align all the bearing bores. If you replace a cap, then you don't have that alignment anymore. The correct way to replace a cap (IMHO), is to mill the very minimum amount of all the caps that you need to get the alignment back. Now, you're not only getting the centerline back, but you're also getting the split line of the cap/block AND the correct crush on the bearings. Plus you're getting the bearing bores square with the rest of the block.

This is one of those places that can be off center in at least 3 different axis, and probably more like 4.

Imagine having six plates that are all 2 inches thick with a 3 inch hole EXACTLY in the center, then cut them in half one at a time and scramble the parts. Try to line the holes all up again and then make the centerlines of the holes all equal with each other and in a perfectly straight line.
 

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Imagine having six plates that are all 2 inches thick with a 3 inch hole EXACTLY in the center, then cut them in half one at a time and scramble the parts. Try to line the holes all up again and then make the centerlines of the holes all equal with each other and in a perfectly straight line.

That's the best anology I ever heard.
 

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Discussion Starter · #7 ·
yea,

the dimension in the up/down direction can be controlled by removing material from the area of the bolt holes on the cap or in the case if an undersized dimension shims could be used.

i would think it would be pretty obvious if there was alignment mismatch in the side to side direction by simply bolting the cap on and torquing it down.

Well in any event, i should be getting a wet rear cap for my block soon and will mike it out. Proving or disproving my theory.

If the bearings plastigauge good and the crank spins freely, i say its good to go.
 

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yea,

the dimension in the up/down direction can be controlled by removing material from the area of the bolt holes on the cap or in the case if an undersized dimension shims could be used.
That works on poured in place babbit. With inserts, any of these creates an elipse rather than a round bearing. NFG:D
 

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0.010 on a circle of 2.65 is going to make it no good?

lets be reasonable here.
When I "bed "a crank on a T, I take out shims, blue it, look for high spots, scrape them with a bearing scraper, then work it till I have 90% or better. The rest will work in. That's a 2000 rpm car, with thick 1/4" poured in place babbit. VERY forgiving.
Take a bearing insert, put it in a vice,squeeze it .010. You will have 10% bearing, and the rest gap where oil will escape, leaving the contact spots dry.
You trying to re-invent the wheel today?:laughing:
 

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Hopefully, the crank is ground correctly, which would make it a perfect 2.65 diameter circle. If a main cap is out of round by .010, then you have a bearing that is incorrectly clearanced because of the crush. So now you've got a 2.65 diameter journal with 1 or more places that the oil clearance is reduced by .010. Using that as a base, and the optimum clearance is .002, you should have a bearing ID of 2.65+.002 or 2.652. Now if the main bore is out by .010 there will be a couple of places that the clearance is 2.652-.010 or 2.642. Now remember, your crank journal size is 2.65 and round, so now it's .01 tighter than it should be. End result, the crank journal is in CONTACT with the bearing and no room for the oil film.

If the bore is cocked, in relation to the crank, then all the clearance will be opposite the direction of misalignment.

Plastigage is a great tool to just do a quick check of clearance. If you want to be right, the ONLY way to do it is use an inside mic and an outside mic, a piece of paper and a pencil. Measure the ID and OD in 3 places (every 45*)around the ID and OD. Then see what the real clearance is. But all of that is null and void if the bearing bore(s) are not in alignment with each other- assuming the crank is straight. I've gone thru 5 cranks from a rebuilder until they gave me one that was straight.

If you're building a street engine, you can really get by with very little machine work. Straight edge check the bearing bores, clean everything up, and drop a set of bearings and a crank in it. If you're building a HP engine that's going to see high RPM and you expect it to live, you really need to evaluate how long you expect it to live and act accordingly. I know machine work isn't cheap. Good parts aren't either. Would you put a $600 crank in a block that's questionable? and then use a set of stock production rods?
 

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Discussion Starter · #12 ·
i was just browsing,

check this out

http://www.chevyhiperformance.com/t...rain/0707ch_main_bearing_clearance/index.html

i consider CHP to be a pretty good source.

1/2 way down the page they say bearings are not round so don't worry about the clearance on the sides. Thats what i thought. I know this applies only within reason.

OH, and noone has commented yet on why my bearings are apparently pinned. Notice the hex key going through the bearing into the cap. I have not seen this before.

Is that just extra security to keep them from spinning in the bore?

 

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Looking at your picture a couple of things came to mind.

First, that's the very first time I've seen a main cap with 3 bolt holes on each side. A "6-bolt" main?? or has someone added a register pin? And someone has added the pins in the lower cap. Gm didn't do that anytime I'm aware of, even on the stuff they were sending to the race guys. I would think tha drilling a hole in a main cap for a hollow pin like that would take some strength away from the cap.

And they (CHP) are correct. When you torque the caps with the bearings installed, not only the ends of the bearings crush together, but they also try and squeeze out- toward the crank. So you take a flat file and chamfer the cap and the block at the bearing split line to keep the diameter at that point from becomng smaller.

I just looked at the picture again-- that's a splayed bolt block-:surprised that's why there are more holes. And most likely aftermarket caps too.

When they did the original installation, the block was probably not machined. They should have milled the caps and the fit them to the registers in the block, then line honed to get everything straight. Minimum removal of material on both sides so a short timing chain is not needed.
 

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Discussion Starter · #14 ·
Looking at your picture a couple of things came to mind.

First, that's the very first time I've seen a main cap with 3 bolt holes on each side. A "6-bolt" main?? or has someone added a register pin? And someone has added the pins in the lower cap. Gm didn't do that anytime I'm aware of, even on the stuff they were sending to the race guys. I would think tha drilling a hole in a main cap for a hollow pin like that would take some strength away from the cap.
.

the hole in the middle is for a dowel pin. Its a Rodeck aluminum block with splayed 4 bolt mains.

I guess it may take some strength away but they may be made of a stronger material. they definitely feel heavy. The dowel pins may help prevent cap walk. I'm still learning as i dig into it.
 

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bearing shells are not round for a reason, when you torque the caps down you crush the bearings and the ends butt together and deform inwards. If the bearing was round it would create a tight spot right there, that's why they are sort of arc shaped but NOT when tightened down, they will be more round than when they came uninstalled. Still, it'll be wider there than on the full thrust face but still. Get the crush all wrong and it could get ugly.

Have you researched if the cap for an internal pump will even fit? I think the hole where the pum shaft goes through is drilled and tapped for a cap bolt in these dry sump blocks. Also, are the oil passages are most likely not even drilled.

Better check that block out very carefully before sinking any money into it, a lot of these things are cracked in the mains or damaged one way or another and welded back up. They'll probably hold for a modest perf. application but if you want to make some serious numbers it'll come back to haunt you. Doni't even think the 350 is made anymore, yours is a 350F right? (it has a fitler pad and no extra lightening, which the 350L has)
 
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