All Small Six Cylinder bore and piston measurements

This relates to all small sixes

ncpe2001

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Due to some compression concerns, I'm tearing down my 66 200 I6 to measure the bore and pistons. I rebuilt this engine in the late 80s and had it bored 30 over. In order to evaluate the current condition of the cyclinders and the pistons, what are the tolerances I should be measuring against given that is should be 30 over?
 

B RON CO

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Hi, can you post the compression test results? Did you squirt oil in the cylinders and repeat the test? I'm sure someone here will know the measurement you need. If it has to come apart hopefully you can just re ring it. Good luck
 

bubba22349

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On a fresh rebuild for street use a Cast Piston is set to .0015 to .002 piston to cylinder wall clearance with a Forged Piston the clearance needs to be set a bit looser at .003. Best of luck
 

ncpe2001

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Hi, can you post the compression test results? Did you squirt oil in the cylinders and repeat the test? I'm sure someone here will know the measurement you need. If it has to come apart hopefully you can just re ring it. Good luck
All cyclinders yielded 150psi except #6, it's only 90psi. This is on a block that is bored 30 over and zeroed decked with 52cc chambers. I expect closer to 200psi.
 

B RON CO

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Hi, since you have not posted much here I don't know if you have the tools and experience to fix this yourself. I assume most guys here want to work on their cars themselves. That is why I ask very basic questions. So did you squirt oil in the bad cylinder and repeat the compression test? That can help you diagnose between bad ring and valves. The problem can also be a bad head gasket. It may also be a broken piston ring or a burned piston. If the problem turns put to be rings, you can still do a light hone and new rings on one piston with the engine in the car. Good luck
 

ncpe2001

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Hi, since you have not posted much here I don't know if you have the tools and experience to fix this yourself. I assume most guys here want to work on their cars themselves. That is why I ask very basic questions. So did you squirt oil in the bad cylinder and repeat the compression test? That can help you diagnose between bad ring and valves. The problem can also be a bad head gasket. It may also be a broken piston ring or a burned piston. If the problem turns put to be rings, you can still do a light hone and new rings on one piston with the engine in the car. Good luck
Really appreciate the guidance. Retested with a couple squirts of oil in each cyclinder, cyclinders 1 through 5 went from 150psi to 200psi. The questionable #6 went from 100psi to 150psi wet.
 

B RON CO

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Hi, those higher numbers would imply bad rings. Maybe the rings never seated properly. If this was my engine I think I would pull the engine out and and change the rings and maybe bearings. You can inspect and measure the bearings. If the cylinder walls look good I would hope to hone it with a glaze breaker hone with fine stones and use chrome moly rings for your oversize pistons. Good luck
 

bubba22349

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Do you remember what ring gaps you had when it was assembled? You could maybe go with a file to fit ring set to.
 

ncpe2001

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I'm in the process of validating that the cylinder bore is still within tolerance for being bored 30 over. However, I think I will swap my dished pistons for flat. Where should I go?
 

B RON CO

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Hi, the pistons will probably be broken getting the wrist pins out of the rods. If you want to change pistons maybe you can get another set of rods.
There is nothing wrong with using the pistons again as long as they are in good shape. Many engines have been honed and re-ringed.
As Bubba said there is a piston skirt to cylinder wall measurement you can check with a feeler gauge.
Good luck
 

ncpe2001

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On a fresh rebuild for street use a Cast Piston is set to .0015 to .002 piston to cylinder wall clearance with a Forged Piston the clearance needs to be set a bit looser at .003. Best of luck
After honing and purchasing new flattop cast pistons, I'm looking at .002 to .003 piston to cyclinder wall clearance. How much clearance would be excessive?
 

bubba22349

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With .003 to .005 clearance you could get a little piston noise with a cast piston especially when the engine is cold. Anything more than .006 is getting excessive. You could have the Piston Skirts coated to bring them back to the ideal clearance. Best of luck
 

bubba22349

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Calico Coatings has the Anti Friction Coatings for use on Piston Skirts as well as Engine Main & Rod Bearings this adds a thickness of .0006. They also have the Thermamal Coatings for use on the Piston Tops. Good luck
 

drag-200stang

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Line 2 line coatings, have a abradable coating made to fix pistons that are a couple .001 to small...I hear that some people have good results with it.
 
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