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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



#4
luvin_the_rings

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Thanks for the clarification, 

 

helped me figure it out. Looks like GCR allows for the over-bore with 0.015 0.0066" piston to wall clearance on the diameter. 

 

-Z



#5
chris haldeman

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Free tip.... .015 piston to wall clearance is way way way too much
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#6
luvin_the_rings

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Edited my earlier post.  It looks like the maximum bore diameter for overbore leaves 0.0066" of piston to wall clearance when using Maximum Material Condition on the piston and Max bore size.  So that is slightly over the maximum clearance specified by the FSM of 0.006"

 

Machining the bore smaller than that will be necessary to get the piston to bore clearance within the 0.0015" - 0.0020" range for a like new fit. 

 

I'm also assuming that Mazda is machining their overbore pistons to 3.0794" Maximum Material Condition, since that is how the standard sizes are called out in the GRC and FSM (3.0698 is what is Called out in both as the maximum piston size).  

 

If we use the tolerance of the standard pistons from the FSM (+0/ -0.0008), and the piston and bore are at Minimum Material Condition, then the total worst case clearance allowed is 0.0074"

 

So basically, machining your bores to fit the pistons is necessary for proper "like new" piston fit. 

And you will be outside the FSM on bore size before you are outside the GCR bore size.

 

Thanks!



#7
Brandon

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Free tip.... .015 piston to wall clearance is way way way too much

 

This is what we like to call a "loose" motor, amirite?


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#8
Tom Hampton

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