Main Cap Walk

[schulte]

When tearing down my motor after my broken crank last week, I found some interesting metal transfer on the main bearing cap mating surfaces.

This motor was in the 1000whp range, but had billet main caps, studs, and a girdle.

Any thoughts on this? Does this block look salvageable?

Not sure how much of this happened after the crank broke; main cap 4 seems to have taken the worst of it.

All of the mains:

Main 1 (front, right near the break in the crank):

Main 2:

Main 4:

All the caps:

 

[PureHybrid]

Its hard to tell how deep those marks are on #4, but its usable.

Ask how much they would charge to skim the mating surfaces and rebore the mains. That will dictate if its cost effective to find a new core.

 

[schulte]

Its hard to tell how deep those marks are on #4, but its usable.

Ask how much they would charge to skim the mating surfaces and rebore the mains. That will dictate if its cost effective to find a new core.

What happens with the girdle I have? Need shims on top of the main caps (assuming they'll be further below the surface of the bottom of the block)?

 

[Dave c]

What happens with the girdle I have? Need shims on top of the main caps (assuming they'll be further below the surface of the bottom of the block)?

Dont use shims. Just deck the pan rails same amount as he takes from the saddle and caps. Cap walk can be from many things, too much stud TQ and too little, poor cap design, or even a uneven cap and mating surface.

 

[schulte]

Its hard to tell how deep those marks are on #4, but its usable.

Ask how much they would charge to skim the mating surfaces and rebore the mains. That will dictate if its cost effective to find a new core.

I've also got a decent amount of money into the rest of this block, as it's bored .020 over for my pistons. If it's savable, I'd save not only the cost of another core, but the cost of re-boring.

 

[Burn Down]

Hard to tell from the pics... If you have a trusted machine shop, I would take it down to them and see what they think.

 

[TheBac]

Dont use shims. Just deck the pan rails same amount as he takes from the saddle and caps. Cap walk can be from many things, too much stud TQ and too little, poor cap design, or even a uneven cap and mating surface.

Question, b/c I dont know, but wouldn't reboring the mains change piston protrusion and/or move the piston higher into the cylinder? Or is it such a small amount of material we're talking about removing that none of it would matter?

 

[schulte]

Question, b/c I dont know, but wouldn't reboring the mains change piston protrusion and/or move the piston higher into the cylinder? Or is it such a small amount of material we're talking about removing that none of it would matter?

I believe you'd compensate for that with different size (OD) bearings, but I could be wrong.

 

[Burn Down]

Question, b/c I dont know, but wouldn't reboring the mains change piston protrusion and/or move the piston higher into the cylinder? Or is it such a small amount of material we're talking about removing that none of it would matter?

It will change the centerline of the crank some... You wouldn't want to take much. A small amount could be taken up with thicker head gaskets... That's why I would take it to a trusted machine shop. They will know the maximum amount of material that could be removed.

 

[Burn Down]

I believe you'd compensate for that with different size (OD) bearings, but I could be wrong.

If you cut the Fretted surface away on the block it has shortened the cap, main bore distance. Once the mains are line bored, half the amount of material removed is the new crank centerline... Make sense?

Edit: Pretty sure the caps will have to be shaved slightly as well... Definitely would talk with a good machine shop.

 

[x MadMAX DIESEL]

If the cap and block are both decked the same amount it won't change the centerline. And it's a tangent circle so I don't think it would need a over bore then made up with oversize bearings. It will just need line bored again. I don't think there would be any issue

 

[x MadMAX DIESEL]

I don't see any dowels in those either, why is that? Did you remove them for the picture?

 

[Dave c]

This process does not change crank centerline at all when done correctly. Line bore/ hone is a must once caps are cut at all. Also will not change protrusion.

 

[x MadMAX DIESEL]

Actually ya know maybe I'm wrong. If you need to take .006 off each to get perfect again. You might need to actually take .006 off the block and .012 the cap. Idk what I was thinking the block won't move only the cap.

 

[Burn Down]

If the cap and block are both decked the same amount it won't change the centerline. And it's a tangent circle so I don't think it would need a over bore then made up with oversize bearings. It will just need line bored again. I don't think there would be any issue

If you remove any material from the block where the cap rides, the crank will move towards cam, changing the crank centerline... The only way to not change the crank centerline would be oversize OD bearings & I'm not aware any exist?

 

[rickaveryjr]

how do you shave material off of a cap's mating surface that has cross bolted mains and not have an alignment issue?

 

[coker6303]

I'm not sure what bearings are available but if you hone the bore of the mains you will lose you pinch where the halves press against the bore of the main and they will walk easier. The notch at the half is for alignment during install, not to hold it when it's running. It's held by the OD of the bearing crushing against the ID of the main journal.

And if you hone the mains, your centerline should remain the same but clearance become looser. You would never hone just 1 half. And if you hone the halves, then cut the face of the mating surface, you create an untrue circle.

I'm no engine builder, but this could get hairy quick. We need a genius in here! Lol

 

[Harbin_22]

My mains were align honed the first time the engine was built, they got aligned honed again when the crank broke. My block has been decked twice. I ended up with .012 protrusion with 16.5 fingers pistons.

 

[juddski88]

how do you shave material off of a cap's mating surface that has cross bolted mains and not have an alignment issue?

You can get away with a very small amount. Mine had to be cut and rebored and off the top of my head, they took .003" off the caps to allow a true bore.

 

[rickaveryjr]

that's definitely adding extra stresses to the bolted assembly without the motor even running. I'd be curious how much this is done. I'd be reluctant to do it on a high hp motor