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Old 08-24-2008, 02:51 AM
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400 Chevy .060 over Too Thin?

I plan on boring a 1975 2 Bolt main 817 Block out to 4.185"?
Intended use is in a Light Pickup.
Target HP is 270
Target TQ is 400
Redline will be 4500RPM
Compression Ratio will be 8 to 1

I figure that since the block is .040 over now with over .006" piston skirt clearance, a .010" reduction in bore radius combined with .001" skirt clearance wont load the cylinders very much.

The Block I'm using is an older casting January 1975 & has the thicker cylinder walls.
I have heard that the late 77-80 blocks have very thin cylinders, & I don't believe this to be one of those blocks.

Have any of you ever bored a 400 .060 over?
Did you ever run into problems?

Since my build is low dollar (under $1400 including the core) I wont be sonic testing & crack checking will be my visual inspection only.
I'm sure I'm not the first guy to do this.

Thank you in advance.

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Old 08-24-2008, 08:13 AM
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You are not the first guy to do this! LOL I've done two and both are fine and still going strong. No heating issues or oil issues.

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Old 08-24-2008, 09:26 AM
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A friend and I pulled apart the 400 he had in his pickup and found that it was .060 over. He had driven it for several years like that without any issues.
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Old 08-24-2008, 10:59 AM
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As a precaution, I would have the steam holes checked for cracks on the decks, usually if they are cracked they can be repaired with a king pin.
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Old 08-24-2008, 11:39 AM
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yes, no, maybe

That something can be done sometimes without negative consequences doesn't mean it is best science. Yes, sometimes an OEM 400" block can be run at .060". No, it isn't the way to go for a high-dollar performance engine. Maybe, for your low-RPM, low-load application it may work.

To get an education about the stiffness of OEM 400" blocks, stand beside one being power-honed to .060" OS. Put a dial bore indicator in an adjacent cylinder and watch it indicate the common wall flex as the hone goes up and down. Scaaary!

thnx, jack vines
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Old 08-24-2008, 05:22 PM
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A .060" over 400 chevy block will work in every application,,,except where they don't...then they break.

Take a small flash light and look through the head bolt holes down the side of the cylinders. There is a notch that is in lines with the head bolt holes that runs the length of the cylinder. You can fab up a crude tool that will check the thickness at this point (it is where the walls are the thinest). Depending on your personality you may not like what you see...

IMO i am amazed that the 500/550 hp .030" over 400's that i build with the stock blocks even last one pass, but they do..

In your application you have a good chance of everything working out. Be 100% sure that you never, ever detonate that engine...

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Old 08-24-2008, 09:28 PM
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Thank a million for the advise.
About the detonation warning, I couldn't agree more.
Since my Dynamic compression ratio will be high (due to a 240 duration camshaft) my Static Compression will be 8.01 to 1

4.185" bore x 3.75 stroke = 845.5 cc Swept vol.
Devided by...
74.5 cc head chamber + 7cc .038 composite gasket + 24cc Generic low buck Cast dish top piston = 8.01 to 1 compression ratio.

I plan on leaving the heated intake port open.
The low cfm carburator will more than likely cause problems.

My backup plan is to aquire a stock 1975-76 262 cubic inch 2BBL intake that has a mount for a larger 287 cfm Dualjet.

Detonation shouldn't be a problem on such a low intensity engine.
It's usually only a problem if the compression ratios are much more (and violent).
Any additional thoughts you may have would be greatly appreciated.
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