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Discussion Starter #1 (Edited)
I'm now measuring the main clearances on a k24 using a .001" bore gauge and .001" micrometer.

The vertical height of the first main is .013". The first main measured with the bore gauge rotated to 3:30 aka approx. 105*, reads .021". The bore gauge is definitely reading correctly; the tangs/oil passageways are NOT causing an inaccurate reading.

I'm thinking this must be a problem but I don't actually know...

The machinist who bored the block also resurfaced the mating surface of the block by an unknown amount - approximately half the depth of the journal code engraving. I didn't ask for this service so please don't attack. Could this be the cause of the out-of round?
 

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yes, could very well be. Don't ever machine the crank bearing ladder and bottom block surface unless you redo the line boring. Same for crank ladders. This is one of the most precise machining step the manufacturers do with accuracies of a few µm between all bearings along half a meter of block length. Most workshops won't manage that even with the right equipment. This is really tricky as on top of top notch equipment and cutting tools, you need temperature control. The aluminium engine block expands by the entire bearing shell thickness adjustment range over less than about 2°C.

Once machined, bearing crush also gets to big distorting bore and shell.

This might work a vintage tractor engine with 4 thou bearing clearance, but not on a K-series with 1 thou bearing clearance.

I found that a micrometer won't work well as the insides of the mains without bearing shells were not cylindrical, but rather inverted barrel shaped. Combined with the oil channels in the middle making a reading next to impossible and the fact you scratch bearing shells measuring them, I resumed to what Honda proposes: Plastigauge.

In your case, I would do plastigauge in 6-8 spots around the journal to get an overview if you can rescue the block. I fear, you won't.
 

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Discussion Starter #3
Should the bearings in a good block measure perfectly cylindrical? In the service manual, a measurement of the clearance at the top of the journal is all that is required for determining clearances.

Plastigauge at the top of the crank would be crushed whereas plastigauge at the sides of the crank would crush, but also smear, causing the platigauge to spread out more. Therefore the the clearance would appear to measure tighter, giving a false impression of a more cylindrical bearing, correct?
 

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don‘t know about the Honda OE shells, but with King XP shells clearance is a tad wider at the sides where the shells touch compared to the middle. This is supposed to help feeding oil into the load bearing part of the bearing. A perfectly new block would be cylindrical, but a used block is almost always slightly out of round. With mounted shells, you can have more complex shapes.
As I wrote, if you machine the crank ladder, the bearing crush can get excessive causing the shells to buckle to the inside. This would likely happen at the aides where the shells touch.
 

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Discussion Starter #5
How out-of-round is a typical used block? For example a crv, element, or accord automatic transmission.
 
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