Fitting Billet Main Caps and Line Boring on a CNC Machine Center

turbodave231

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May 24, 2001
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Macomb, MI
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Here are a series of quick videos on the installation of new billet main caps.

In the first video I machine the side(s) of the cap to fit the register of the block.


In the second video I chamfer the side of the cap. If you don't do this, It makes it really hard to start the cap into the register


The pictures show the caps installed after fitting. You can see the large mismatch between the new cap and the original bore. This is why the block must be Align Bored. If you tried to Align Hone the block, the mandrel would catch on the huge mismatch.......(not good)

photo 2.JPG

photo 3.JPG


photo 4.JPG


The last video is line boring new billet caps done on a CNC Machine Center.
200 RPM and 1.0 IPM (Inch per minute)
The cut goes quick, the setup time is extensive.

This is the roughing pass A second pass was made to bring the bore to 2.6830". The last .004" was finished in the line hone. After honing the final bore size ended up at 2.6872-2.6873


DR
 
Last edited:

MCH86GN

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Aug 20, 2008
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Nice upload Dave. This is really educational. I actually didn't know what align honing the main caps meant. So this gives you a precise or more precise geometry to work with. I should start my rebuild.
 

turbodave231

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May 24, 2001
2,227
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Macomb, MI
www.geocities.com
The factory specification for the main bore is 2.687-2.688. Align boring and align honing always return the bore to the factory dimension (not an oversize).

I prefer to keep the bearing bore on the low side of the tolerance. On this block I ended up at 2.6872-2.6873 on all 4 bearing bores.

Keeping the bearing bores on the tight side of the tolerance improves bearing crush and heat transfer.

The bore size obviously influences bearing clearance, but it is never a good idea to use bore size to achieve your desired bearing clearance. You run the risk of spinning the bearing (bore too large) or buckling the bearing shell (bore too small, too much crush).
 
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3.8L V-8 eater

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Jul 22, 2006
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The factory specification for the main bore is 2.687-2.688. Align boring and align honing always return the bore to the factory dimension (not an oversize).

I prefer to keep the bearing bore on the low side of the tolerance. On this block I ended up at 2.6872-2.6873 on all 4 bearing bores.

Keeping the bearing bores on the tight side of the tolerance improves bearing crush and heat transfer.

The bore size obviously influences bearing clearance, but it is never a good idea to use bore size to achieve your desired bearing clearance. You run the risk of spinning the bearing (bore too large) or buckling the bearing shell (bore too small, too much crush).
Curious Dave, how do you align bore and keep to factory demensions???
 

turbodave231

Moderator
May 24, 2001
2,227
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Macomb, MI
www.geocities.com
With new caps, they are made small and you machine them to size. With stock caps you cut material off the parting line of the cap and raise the centerline of the crank .002-.004"


Sent from my iPhone using Tapatalk
 

MCH86GN

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Aug 20, 2008
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Keep up the educational pictures/vids. A picture or a video is worth a thousand words.
 
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