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1.6 Over Bore Question

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#1
luvin_the_rings

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Hi guys, 

 

It's been awhile, but I'm glad to be back. 

 

I have a question about over boring my 1.6 SM.  

 

The Block we have is too loose to use standard size pistons. So we are using the .010" overbore allowance.  However i think the tolerance stack up in the Factory Service Manual could put my over the maximum bore size for the larger pistons.  

 

FSM says Standard Piston Sizes range from 3.0690" - 3.0698"  so adding 0.0098" (0.25mm) to that gives us 3.0788 - 3.0796 tolerance range for the overbore pistons.  Is this correct?  GCR says maximum piston diameter allowable is 3.0794

 

The FSM also specifies that 0.0015" to 0.0020" piston to wall clearance.  Is this on the radius or diameter?  Does this mean that my Piston Diameter is 0.0020" less than the bore diameter?

Or is this the radial clearance from piston to wall, making the piston 0.0040" smaller than the bore?  

 

Help, 

 

Thanks. 

 

Z



#2
Brandon

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At multiple points within the GCR, there are values noted with absolutes and ranges.

Some ranges incorporate tolerances while others are to be considered "not to exceed".

 

Valve plunge cuts and other machining items are "not to exceed" (using go/no go tools for example).
Pistons, to my read, are also considered "not to exceed" which means your pistons shall not be larger than that 3.0794" number.

 

What this also means is if you have a block with holes that cannot be perfectly cylindrical without going to larger pistons, you may be in a situation of having an unusable block.


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#3
RWP80000

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Hi guys, 

 

It's been awhile, but I'm glad to be back. 

 

I have a question about over boring my 1.6 SM.  

 

The Block we have is too loose to use standard size pistons. So we are using the .010" overbore allowance.  However i think the tolerance stack up in the Factory Service Manual could put my over the maximum bore size for the larger pistons.  

 

FSM says Standard Piston Sizes range from 3.0690" - 3.0698"  so adding 0.0098" (0.25mm) to that gives us 3.0788 - 3.0796 tolerance range for the overbore pistons.  Is this correct?  GCR says maximum piston diameter allowable is 3.0794

 

The FSM also specifies that 0.0015" to 0.0020" piston to wall clearance.  Is this on the radius or diameter?  Does this mean that my Piston Diameter is 0.0020" less than the bore diameter?

Or is this the radial clearance from piston to wall, making the piston 0.0040" smaller than the bore?  

 

Help, 

 

Thanks. 

 

Z

 

The Piston diameter dimensions you reference from table 4 are the what SCCA has specified in the GCR for the Pistons however the max bore diameter you are looking for is listed in the Spec Line chart which is the very last page of the 9.1.7 SM GCR rules.  The specs listed there for you 1.6 max bore diameters are: Std Bore = 78.13mm (3.076 in), Oversize Bore = 78.38mm (3.086 in). Both dimensions must be within the respective maximum values.  These are the dimensions recognized by SCCA and includes a wear allowance for the bores beyond the Mazda Factory Service manual.

 

Rich Powers

SMAC member






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