lb7/lbz main caps different?

[DIGGLER]

i need a main cap for my LBZ block. having a hard time finding one... my question is, could i use one from an LB7 block? i am replacing the #2 main cap due to distortion. the crankshaft broke in this engine, and now that cap is loose in the saddle. i want to put another stock cap in there that fits tight and get it line bored.

anyone?

 

[Leadfoot]

I believe the main journals on all Duramax cranks are the same. You should be able to use an LB7 cap but definitely have it checked (you may have to shave it and have it line bored).

How is the saddle in the block though?

 

[DIGGLER]

I believe the main journals on all Duramax cranks are the same. You should be able to use an LB7 cap but definitely have it checked (you may have to shave it and have it line bored).

How is the saddle in the block though?

i am assuming the saddle is ok, since i can swap the #3 cap into the #2 cap's spot and it will fit snug. #2 is loose in #3's spot. comparing measurements with the other caps, #3 and #4 are the same on the external dimensions, but #2 is .011" narrower across the saddle pads. you can see black on the block where the bearing was scooting around, but never spun.
checking across all mains with a machinist's straight edge, it looks like all bores are within .001" of each other in alignment, which is what the repair manual said was spec.

 

[blackdirtymax]

I have a set bud , sorry I didnt get back to you yesterday. I had the same issue with a block I have and just put billet mains in. Number two is
Hurt , but the rest are fine .


Built motor , duel fuelers , 60's overs , gtx4202r

 

[super diesel]

i am replacing the #2 main cap due to distortion. the crankshaft broke in this engine, and now that cap is loose in the saddle.

Make sure you check the surfaces on the block side where they mate up. The one's I've delt with weren't flat any more going from one side of the block to the other (not front to back but the other way). I would see a .007 to .015 warp after such a case. Hopefully this is not the case with yours and the block is reusable. If not, this area may need to re milled to get the cap to fit flat against it again. If it's really bad, then the side blots may not go back in with out some machining on the sides where they go in as well.

Whats the history of the motor if you know it? Has it ever towed much, or been worked very hard for a good period? What kind of oil and how frequent were the changes?

 

[DIGGLER]

the truck was a repo truck for 250k miles. averaged 100k miles per year. driver was hauling a car one day when it started knocking and he pulled over. they towed the truck to a shop where they diagnosed it as a lower end knock. i bought the truck as-is for cheap thinking it was a bad injector. i had no idea these motors were common to break cranks at that time. lol
anyways, it never had a tune or any mods, supposedly and looking at the cylinders/pistons i would tend to believe it. all the bearings came out of the motor looking real good other than the #1 bearings, and that is where the broken crank was see-sawing on it. all the rotating assembly looks great, doesnt even have much of a black stain to anything at all. oil changes were done regularly.

i will see if i can check the block in the way you suggested, thanks for the headsup. i used a bore dial gauge on each of the bores with the caps bolted in place and they all looked nice and round/comparable with one another. the #2 main actually measures decent top to bottom, but when you rotate the gauge around towards the seperation line, it gets tight. you can run your finger across the line and it catches on the main cap on each side. its like the cap was removed, put in a press, and squeezed in .011". which gives .006 on each side of that bore to catch your fingernail on.

 

[super diesel]

it never had a tune or any mods, supposedly and looking at the cylinders/pistons i would tend to believe it. all the bearings came out of the motor looking real good other than the #1 bearings, and that is where the broken crank was see-sawing on it. all the rotating assembly looks great, doesnt even have much of a black stain to anything at all. oil changes were done regularly.

you can run your finger across the line and it catches on the main cap on each side. its like the cap was removed, put in a press, and squeezed in .011". which gives .006 on each side of that bore to catch your fingernail on.

Ouch. That's an issue. Some trucks that have spun the mains or broke the cranks were totally stock. On some trucks this phenomenon can be contributed to over temped oil and the oil jacket between the bearing surfaces is getting squashed out. Then metal to metal occurs and adios amigo. Not saying it was for sure the case, but it only takes once, then your driving who knows how long unknowingly on compromised oil.

Just my 2 cents.