OEM Honda cylinder # HM-A2

Discussion in 'Wanted/Looking For' started by Clayton, Nov 24, 2019.

  1. Clayton

    Clayton Member

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    Hey everyone I’m looking for a NOS or slightly used cylinder part number HM-A2 for my K1 rebuild. If anyone has one or knows where I can get one please hit me up. Thanks for all the help.
     
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  3. kirrbby

    kirrbby Well-Known Member

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    I haven't been able to look yet, but I think I have a used one. I don't know what condition it's in tho. 48 year old cylinders are rarely "slightly used"
    I know I have one that has been overbored, honed and ready to go, with a new Honda piston kit. But I am pretty proud of that one...it's really not a part I'd planned to sell.

    But I'll try to find you a good used HM-A2. If I can find a good one, I could even hone it for you.
     
  4. kirrbby

    kirrbby Well-Known Member

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    BTW...HM-A2 is not the part #. It's just a casting stamp. There are HM-A1s and 3s and others that are just different runs of the same casting. You might want a HM-A2 to keep your bike pure, but there are many many correct parts out there, just with different casting numbers.
     
  5. Clayton

    Clayton Member

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    Thanks kirrbby for the info, I’m definitely interested in a good cylinder especially if you could hone if too that would be awesome. Can I PM you for details?
     
  6. kirrbby

    kirrbby Well-Known Member

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    You can PM me for any reason.
     
  7. kirrbby

    kirrbby Well-Known Member

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    I had to hone this cylinder for...maybe a total of about 4 minutes to get some score lines out. They were down in the spigot area. Then I had to make a quick pass with a dry hone in my cordless drill to get a little cross hatching. I think it looks good, but maybe someone else could pipe in about the cross hatching...does it look adequate??

    A machinist friend did some measuring. He came up with 47.04-47.06mm around the top of the bore, and 47.00mm at the bottom/spigot end. Are those numbers good??

    I blasted it clean before I honed it. At this point I'd just spray it down with WD-40 and ship it. You'd need to spray it with brake cleaner before you paint it.

    I think it's a good part, ready to go. But I don't know what the service limit is for a standard bore.
    IMG_20191202_144338784.jpg IMG_20191202_144410401_BURST000_COVER.jpg IMG_20191202_144447103.jpg IMG_20191202_144508723.jpg IMG_20191202_144536256.jpg IMG_20191202_144559008.jpg
     
  8. racerx

    racerx Administrator
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    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.
     
  9. kirrbby

    kirrbby Well-Known Member

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    All good info racerx, thank you.
    Clayton has a piston kit he's hoping to use. I don't have any new, stock bore, rings to use to check end gaps. But that is what I was thinking would be a good way, for me, to be able to check this cylinder, for size, AND roundness, etc. If the end gaps are within new ring specs, and I don't see excessive light coming thru anywhere around the ring...call it good.

    I'm not usually working on stock top ends these days. But if you can get away with deglazing them and running them...well that's good stuff.

    I wasn't sure if I even needed to be trying to remove the scoring that was present. 3 shallow scratches that weren't within the stroke of the rings. I did a lot of extra honing, in order to remove those scratches.

    I did the job on a drill press, and kept it oiled. But I COULD NOT get any cross hatching with that method. But with a clean dry hone, moving fast, with a low speed cordless drill, I got the cross hatch seen very quickly...I learned something. At least I think I did.

    My hone is a 180 grit. I thought that was coarse, as hones go. But, it seemed too fine to me this time.

    ...did someone say rambling?? :)
     
  10. Clayton

    Clayton Member

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    So would I need to up size to a 1 over piston? If so that’s fine I’m just really green on all of this lol. I just need to be told what to do lol
     
  11. kirrbby

    kirrbby Well-Known Member

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    Well it sounds to me like this cylinder is right on the verge of... usable, as is, and needing to be bored 1 over. So I guess it's your call to make. racerx says it sounds like it's just a little looser than he likes.
    I'd say, you could use it as is, and it would run just dandy. But if you're striving for a top notch rebuild, a overbore, with new piston and rings is a sure bet.
    Don't feel obligated to buy this part from me tho. There'll be no hard feelings if you go a different route. I'll take $35 for it if you want it. That makes no account for the time I put into honing it. That was just kinda funzies for me anyway.
    I do know a guy, local to me, who could do a overbore for you if you wanted to go that route. He is a pro. Does great work for a very reasonable price. I could give you his number. Then this part could ship straight to him for the overbore. If I can find a cylinder head for you, and you want the seats cut, that could go to him too.
    racerx is local to me too. He does a fantastic job with cylinder heads. I'm not sure if he is set up to bore cylinders tho?? If not, I'll bet he has someone that he sends them to. I could ship the parts straight to racerx too, if you can work it out.
    I haven't had time to look for a head for you yet, but maybe tonight or tomorrow.
     
  12. kirrbby

    kirrbby Well-Known Member

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    Best advice...
    TRY, to make it PERFECT, because it's never perfect. But if you at least TRY, it will be CLOSER to perfect than it would be if you had never even tried.

    If I were better with words, I'd be able to make that into a better quote :p
     
  13. racerx

    racerx Administrator
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    Clearances have a viable tolerance range. Below that, the piston will soft-seize, above that it'll rattle. It really is that simple. Ring endgap specs leave no room for interpretation, too small, they'll close-up and seize, too big they won't seal.

    Cylinder concentricity (roundness) isn't usually the issue, though it can be with a worn bore. The issue is getting the rings seated. Even with a freshly-machined bore (which should be perfectly concentric) the rings can compress/expand, that changes their radii. That minuscule mismatch is enough to prevent full contact, resulting in high and low spots. Cross-hatching/deglazing the cylinder increases friction & holds oil...allowing the high-spots to wear quickly and the rings to seat, i.e. make full contact. It works very well but...the rings quickly develop a unique wear pattern. Pull the piston out of the bore, once the rings have seated, and you'll never get them reinstalled exactly as they were...with cylinder now polished from break-in, the rings won't re-seat. For that reason alone, after the first few miles, I install new rings anytime the piston comes out of the bore. The piston, OTOH, is pretty forgiving; it also goes back in the same location/rotation every time.

    I don't really know what grit hones most shops use. Some of the newer-gen finishes are done with #400-600 grit hones but, that's way outside the scope of anything we do...OEM & race. I farm-out boring & honing to a longtime veteran engine builder. It takes him a few seconds, using a drill & dingleberry hone to deglaze, or finish-hone a cylinder. Cylinder boring is one task I shall never do...a new boring mill is like $30K and I've far more years behind me than ahead of me.:cautious:
     
  14. OLD CT

    OLD CT Well-Known Member

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    If Kirby's won't work, I really can't tell by all the rambling. LOL! I have a couple I just looked at. E1 and B4 casting #s. I can check the ring end gap with a new OEM top ring I have.

    Specs are measured at least a 1/2 inch in the top of the bore. .006 - .014

    Mine is around .018 :(...
     
    #13 OLD CT, Dec 5, 2019
    Last edited: Dec 5, 2019
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  15. Clayton

    Clayton Member

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    If I can get a price for boring the cylinder 1 over that would be awesome, iv called numerous places here in Dallas without any luck finding someone who would bore a cylinder for me. They are probably here I just haven’t found one yet lol! Also a head would be great to. I’m in desperate need of a head mine was really screwed up by accident after I took it off. Any number or info would be awesome. Thanks to all of you who have chimed in to help me with this issue.
     
  16. OLD CT

    OLD CT Well-Known Member

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    I think buying a cylinder from dratv or chp honda parts would be the best solution. Why do you think the head is un rebuildable?
    I do prefer building a oem Honda head. The cheap ebay heads are junk.
     
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  17. racerx

    racerx Administrator
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    Aside from the fact that I resemble that opening comment, consider Pat's central point. Look at how much simpler it is to just start with a fresh cylinder & rings(!) Overbored, NOS, new OER or new aftermarket...all plug & play. Compare that to the mental gymnastics of dealing with a used cylinder.

    As for the head, post some pix. He's right, OEM head castings are usually very restorable, with just a good, thorough, cleaning and a proper valve job.
     
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  18. Clayton

    Clayton Member

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    I wanted to keep my bike as original as possible that’s why I wanted to keep my cylinder or have one with the same stamping but now that my head is trash that dream is put the window lol. I guess I will buy a new cylinder from CHP. They are the only place so far that I have found that offer new cylinders that say 72cc instead of 72cm2. They have been out of stock far a while now on there website but I will call on Monday and see if they have an ETA of when they will be back in stock. As fas as the head goes my neighbor was “helping me” and placed my stripped down head in a bucket of muratic acid. He thought he was placing it in a bucket of degreaser. I will post a pic but my head is trash, muratic acid and aluminum react very violently together. It was in there for only a few seconds but it’s gone. In the end I do want a factory head (nothing else) but maybe I’ll ask the folks at CHP if they have a used one laying around for sale. That might be my best bet. I purchased many parts from them for my 74 mini trail I just finished and they were very helpful any time I called or stopped by with a question. I really appreciate all of y’all info and help. Iv learned so much on this forum!
     
  19. kirrbby

    kirrbby Well-Known Member

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    This is a father son operation in Clio Michigan. I can vouch for them 100%.
    Irondog

    14282 N Linden Rd, Clio, MI 48420
    (810) 687-4355

    https://g.co/kgs/qWNbTZ


    This VHMT in Metamora Michigan. I've never had him do any work for me, but I can vouch for him too.
    http://vintagehondaminitrails.com/services.php

    Either one of those places coul help you with labor AND parts too...depending...

    I have a complete head if you want it. It's nice, pretty clean, and when I checked it years ago it was sealing...I decided it was a runner. $75 for that part. 65 if you only want the casting. The stamp on it would I think be early model...70-72 maybe, K0 or K1. But I wouldn't be able to post pics until Sunday probably.
     
  20. kirrbby

    kirrbby Well-Known Member

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    I'll try to post some pics tonight.
    But to be clear...
    I check these by filling the combustion chamber with WD-40. If it doesn't leak within a half hour or so, I call it good...enough to run...runner. I usually check the cam lobes, cam bearing points, and cam followers too. I assume I did all of that too. I offered this head to a member, and they must not have bought it...so I still have it. $75 part PLUS shipping. I don't know your location, but this is a kinda heavy part...I'd guess at 12-15 bucks to ship. Maybe more to the W coast. Seems it's become very expensive to ship far West.

    But if you are doing a proper rebuild of your engine...this head, with a unknown history, should probably be properly rebuilt.
     
    #19 kirrbby, Dec 8, 2019
    Last edited: Dec 8, 2019
  21. Clayton

    Clayton Member

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    Sounds good I’m in Garland Tx ( Dallas ) iv already bought all the parts to completely rebuild a head ( new valves, springs,rocker arms, guides, seals, etc ) because I was gonna completely rebuild my original head.
     

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