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General Minitrail Talk
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'72 CT70 winter restore project
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<blockquote data-quote="69ST" data-source="post: 184814" data-attributes="member: 5"><p>Octane requirement is a tricky thing. I'll spare everyone yet another longwinded dissertation, with this. First off, 10.8:1 static CR should be quite compatible with 92 octane pump premium. Due to the tiny bore diameter, there's some question as to whether premium is even needed. Cutting to the chase...at 100mpg, running premium is cheap insurance.</p><p></p><p>Let's be clear about the term "compression height". This refers to the location of the wrist pin, relative to the top of the compression ringland, i.e. the area above the top piston ring. Dome is another element. You're correct, longer stroke does pull the piston lower inside the bore. But, it's possible to have the same stroke...with different connecting rod lengths. And that's where compression height matters...a longer rod needs a "shorter" piston, to keep it from standing proud of the cylinder at TDC (a.k.a. "negative deck height") and hitting the head. Likewise, that shorter piston, combined with a shorter rod would leave too much deck height at TDC, leaving the piston way down in the bore...killing compression.</p><p></p><p>Honda went to a longer connecting rod, with the 12v era motors. The wrist pin had to be move up, closer to the piston crown, to match.</p><p></p><p>With aftermarket cranks, you have to pay attention to which configuration is used...6v/short rod or 12v/long rod and match the piston accordingly.</p><p></p><p>I suggest ordering your stroker crank, then test-fitting the original (6v) piston, sans rings (to make things easier), to check deck height. The original cylinder could be used for this, even if you replace it, the same, stock, 63mm length is required. If any of the top ringland protrudes above the deck, at TDC, you need a 12v style piston. Then, you order the correct piston without guessing.</p></blockquote><p></p>
[QUOTE="69ST, post: 184814, member: 5"] Octane requirement is a tricky thing. I'll spare everyone yet another longwinded dissertation, with this. First off, 10.8:1 static CR should be quite compatible with 92 octane pump premium. Due to the tiny bore diameter, there's some question as to whether premium is even needed. Cutting to the chase...at 100mpg, running premium is cheap insurance. Let's be clear about the term "compression height". This refers to the location of the wrist pin, relative to the top of the compression ringland, i.e. the area above the top piston ring. Dome is another element. You're correct, longer stroke does pull the piston lower inside the bore. But, it's possible to have the same stroke...with different connecting rod lengths. And that's where compression height matters...a longer rod needs a "shorter" piston, to keep it from standing proud of the cylinder at TDC (a.k.a. "negative deck height") and hitting the head. Likewise, that shorter piston, combined with a shorter rod would leave too much deck height at TDC, leaving the piston way down in the bore...killing compression. Honda went to a longer connecting rod, with the 12v era motors. The wrist pin had to be move up, closer to the piston crown, to match. With aftermarket cranks, you have to pay attention to which configuration is used...6v/short rod or 12v/long rod and match the piston accordingly. I suggest ordering your stroker crank, then test-fitting the original (6v) piston, sans rings (to make things easier), to check deck height. The original cylinder could be used for this, even if you replace it, the same, stock, 63mm length is required. If any of the top ringland protrudes above the deck, at TDC, you need a 12v style piston. Then, you order the correct piston without guessing. [/QUOTE]
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