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General Minitrail Talk
Wanted/Looking For
OEM Honda cylinder # HM-A2
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<blockquote data-quote="69ST" data-source="post: 184320" data-attributes="member: 5"><p>As best I can recall, 0.00185"-0.002" is the target piston-to-cylinder wall spec, new or with an overbore. For reference 0.002" is 0.0508mm.</p><p></p><p>Not that I'd recommend using this for machining purposes but, new std bore pistons I've measured (caliper slapped) come in around 46.98mm.</p><p></p><p>So-o-o, sounds to me like it should be within wear limits, just looser than I'd like. Problem is, there are too many different specs across the various resources available to us and that creates confusion. Case-in-point...I checked a recent-vintage Clymer's manual for the specs you want. Not only is this not addressed, scanning down the page I noticed the rod-to-crank (big end) side clearance spec is different from the factory numbers...they show .004-.014", with a wear limit of .020". I've gone through a number of NOS cranks over the last 20 years, tightest clearance I've ever seen was .006"...exactly once. The rest have been .008-.012". Straightforward enough, right? Those who've been around long enough to recall my ramblings, specifically that I'd not use a crank with more than .018" rod side clearance, may now have good reason to think "hmmmm?"</p><p></p><p>Apologies if this is creating more stress than it alleviates. These are hard facts, to be used to draw one's own conclusions. IMHO, if ring gaps are within spec (gotta go with new rings, at least) then you'll probably be okay. These engines have a rod-to-stroke ratio that would do justice to an F1 engine, optimal for high rpm. Combined with a long piston skirt, there's a lot of leeway. The Z50 guys have it even better...the piston is significantly taller than its diameter. Between the low peak piston speeds, gentle transition off of TDC and light thrust loading, this is as close to bulletproof as it gets. If all else fails, excessive piston clearance will result in audible piston slap.</p><p></p><p>IMO, overboring and piston clearance is one job best left to a seasoned professional...when it's required. Deglazing, OTOH, can be done by a reasonably talented...and inspired... shadetree mechanic.</p></blockquote><p></p>
[QUOTE="69ST, post: 184320, member: 5"] As best I can recall, 0.00185"-0.002" is the target piston-to-cylinder wall spec, new or with an overbore. For reference 0.002" is 0.0508mm. Not that I'd recommend using this for machining purposes but, new std bore pistons I've measured (caliper slapped) come in around 46.98mm. So-o-o, sounds to me like it should be within wear limits, just looser than I'd like. Problem is, there are too many different specs across the various resources available to us and that creates confusion. Case-in-point...I checked a recent-vintage Clymer's manual for the specs you want. Not only is this not addressed, scanning down the page I noticed the rod-to-crank (big end) side clearance spec is different from the factory numbers...they show .004-.014", with a wear limit of .020". I've gone through a number of NOS cranks over the last 20 years, tightest clearance I've ever seen was .006"...exactly once. The rest have been .008-.012". Straightforward enough, right? Those who've been around long enough to recall my ramblings, specifically that I'd not use a crank with more than .018" rod side clearance, may now have good reason to think "hmmmm?" Apologies if this is creating more stress than it alleviates. These are hard facts, to be used to draw one's own conclusions. IMHO, if ring gaps are within spec (gotta go with new rings, at least) then you'll probably be okay. These engines have a rod-to-stroke ratio that would do justice to an F1 engine, optimal for high rpm. Combined with a long piston skirt, there's a lot of leeway. The Z50 guys have it even better...the piston is significantly taller than its diameter. Between the low peak piston speeds, gentle transition off of TDC and light thrust loading, this is as close to bulletproof as it gets. If all else fails, excessive piston clearance will result in audible piston slap. IMO, overboring and piston clearance is one job best left to a seasoned professional...when it's required. Deglazing, OTOH, can be done by a reasonably talented...and inspired... shadetree mechanic. [/QUOTE]
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General Minitrail Talk
Wanted/Looking For
OEM Honda cylinder # HM-A2
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