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What are re-bore limits?

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At what point is an engine block shot and a re-bore unfeasible? Is there a general rule of thumb?

 

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Depends on the motor.. thickness of casting. Some people might be able to speak to recommendations on specific motors.

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l20b can go 4 over, anything more and you'll break into a water way

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For L and Z series you can safely go 1mm, and 2mm is likely, while 3mm is doable but.... Keep in mind that a 1mm increase in bore is only 0.5mm of material removed all the way around. 4mm would be a 2mm shave, about 0.80"

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I'm at 3mm over on my L18.

Machine shop said I was fine, but....

I did a half fill with HardBloc just to be safe(er).

 

But I wouldn't be surprised if something breaks at 8500 either....

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I think a better question to ask is what pistons are available?

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The absolute maximum size piston for L  Z and KA is 95.2mm and the pistons would all be touching.

 

A 90mm (one mm over) Z24 or KA piston in a Z24 would leave 5.2mm or just over 3/8" between pistons.

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My Z22 block (87mm) was bored to 89mm no problem, and why I used a Z22 and not an 85mm L20B or Z20 block.

 

Three mm I would be concerned. Four I would be more concerned.

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Sonic testing is the answer.  Everyone can guess, but no one knows until its sonic tested.  Any good engine builder will either be able to do this for you or will have someone who can.  You could have the odd block with an offset sleeve that's paper thin at 1mm overbore.  You just don't know until its tested.  I went 1.5mm on my L20b and have room for another mm.  I once had a 289 bored .030" over, and it blew in 6 miles due to a thin cylinder wall.  I don't guess anymore.  

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Experience matters too.

 

A typical Z20 block can go .120 over without problems. A Z22 block can go .120, but the bore should be "spread" between the 1-2 and 3-4 cylinders to keep away from the steam ports. L20B blocks have gone .120, but not advisable as the wall thickness is not as great as the Z20. Since the Z20 and L20B blocks are essentially the same, using a Z20 block as a replacement for L20B is acceptable, especially when going large bore.

 

Short deck L motors (L16 and L18) are not usually capable of going any more than .080, but people have gone more. But why? If you're going that big, just build a tall deck motor.

 

As Daniel pointed out, finding an off the shelf piston for large bore motors will be your biggest challenge.

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This is all fantastic information. Thank you, thank you, thank you.  

I am looking at a Z24 with number 4 cylinder showing considerable wear on the rear side of the wall. But I would guess- guess- it is about 1mm defect. I just have little experience with this directly, so I will load it up and take it to the machine shop for a verdict. But I sure appreciate your input.

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<p>A guy over on the ozdat forum has bored out multiple L18s. Many will go to 89mm but all the ones he did ovaled inside of a year of driving. My L18 went to 87mm without a problem.</p>

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For L and Z series you can safely go 1mm, and 2mm is likely, while 3mm is doable but.... Keep in mind that a 1mm increase in bore is only 0.5mm of material removed all the way around. 4mm would be a 2mm shave, about 0.80"

 

 

I realize this is a shoddy test for the deficit I am seeing, but I was curious to see if I could figure it out and it will be a few days before I can get to the machine shop. I have no way of determining how out of round the cylinder in question might be, but I took a machinists metal straight edge and a small trouble light and I chased the straight edge around the cylinder to see if I could measure the worst deficit. When I found it, I used trimmed ends of guitar strings to determine the worst spot was .019 or so. Maybe a little less. If that is anywhere near correct, maybe- just maybe- I am looking at somewhere around .060 clean up bore. (Just a guess it encourages me to go get the machinists verdict.)

 

Given that the damage is localized to #4, one would assume that the #4 rod bearing is also going which allowed the wear to happen in the cylinder. And therefore, perhaps the crankshaft could be damaged to some degree. I toss this out there simply to see if it is a logical assumption.

 

Amazingly, this engine ran pretty darned good, considering. I drove it all over the damn place. I know now that it is supposed to have more power, but that is because the motor is fresh now, and eager to pull a good hill in third gear. Before, not so eager, but it would do it. These engines seem to function despite considerable abuse / damage. Much respect. 

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Cylinder scoring could simply be from a previous over heating and the ring or ring land broke. There is no correlation between this and the rod/rod bearing or crankshaft. If the block is to be bored you would likely replace the rods and mains anyway, I would.

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Oh yeah. It all gets replaced. Thanks!

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<p>A guy over on the ozdat forum has bored out multiple L18s. Many will go to 89mm but all the ones he did ovaled inside of a year of driving. My L18 went to 87mm without a problem.</p>

A long time ago I built a L18 with a 87mm bore and cut down L20B crank, short rod motor. It had an aluminum flywheel too. Just spun the tires. So much torque.

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