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  1. #61
    CB750 Enthusiast ray_'s Avatar
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    Alright. Will figure it out. I was in the midst of choosing bearings for conrods and the crank. Just wanted to check if I understand things right: as per the procedure in the manual, all we need to do is measure the crank pins and main journals right? Then, using honda codes for the ID of the conrods and the crankcase, use the table in the manual to select the right inserts. correct? I ask because I had taken the cases and the crank to a machine shop and they recommended buying the thickest inserts and polishing the crank up to spec instead.

  2. #62
    CB750 Guru amc49's Avatar
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    The problem is complicated by the fact the thickest to thinnest bearing there from one end to the other is not even .001". The bearings rate in TEN thousandths of an inch and they only count based on zero wear which many cranks do not have.

    You are looking for .001"-.002" clearance there. The rods must be torqued at the bolts to check the hole. If the case holes look good you can use the code stamped in case at the back top of top case.You need to measure the crank and rod bores. NO vernier calipers there, you will need a real mike to do the work. Plastigauge can work if you know how to use it.

  3. #63
    CB750 Enthusiast ray_'s Avatar
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    Okay.... but I find one thing difficult to understand. So when honda assembled the engines they stamped the diameter of the holes in the cases and the conrods as alphabet and number codes. Now, as per my understanding, the only wearing surfaces there are the bearing inserts and the crank pins and journals. So why do we need to remeasure the bores as you said? Or you mean measure only rod bores? If so, why that too? Can't we trust the stamping on the rods too (since the bore itself does not wear)?

  4. #64
    CB750 Guru amc49's Avatar
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    If you didn't remeasure using YOUR tools you don't get a true idea of the clearances, different tools measure differently. Suppose the original measure had a tolerance of 3 tenths and yours 2 more tenths, you could be off half a thousandth and not even know it. Another reason is that the holes can move around to settle in slightly different over time. Very common for rod big ends to do that. Enough they get resized when they are rebuilt. They get longer in up and down and shorter in side to side and that is what spins bearings. Rods stretch in use and pull the big end eggshape and after a point some of that distortion settles in to become permanent.

    When you are dealing in measurements that small you always measure things yourself, it backs up what the stamped numbers say, or can show you problems developing that can blow up your new motor, it happens all the time. NEVER assume that measurements made thousands of miles ago and countless heating and cooling cycles later are the same, typically some are not.

    Another point. The holes get honed to final and that means there are highs and lows microscopically there, in use the high points will beat down some making the bore slightly bigger. Take the old inserts and look at the backs, you can see where the impression of load has been impressing on the backs. Look inside the rod big ends too.

    You may get away with not measuring at all, but if the motor explodes you now know why. These will spin a bearing easy as spit if you give a mechanism to allow it. Your choice. Some say screw it and use all yellow bearings to be as loose as they can and run them like that.

    The bearings used at brand new engine were only right when engine was new, often they need to adjust to something else.

  5. #65
    CB750 Enthusiast ray_'s Avatar
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    Okay I understand now. I have been doing some reading and what you suggest seems the most perfect way to do this! Thak you. Will keep you updated.

    On a different note, what do you have to say about reusing the primary and camshaft chains on these? Considering that the engine sat for approx 20 years, would it be okay to reuse those (they seem to be in great condition)? And the chain tensioners get replaced as a good practice right?

  6. #66
    CB750 Guru amc49's Avatar
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    Tensioners get changed.

    Chains maybe not if mileage was low but if any rust got to them inside motor then yes. If mileage over 30K then yes too.

    You are beginning to see why these can be among the most expensive engines to rebuild.

  7. #67
    CB750 Enthusiast ray_'s Avatar
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    Expensive? Oh hell yes! Speaking of the mileage, my odometer reads 11k miles so I guess I am okay. Is the 30K a service limit? Meaning the engine will have to come apart after another 19k miles if I don't replace now? In that case I would just do it now....

  8. #68
    CB750 Guru amc49's Avatar
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    30-35K is about the mileage the tensioners go before they begin showing temperature stress from being in too much heat here in Texas. When you flex the tensioner you will begin to find myriad very small cracks in the rubber when you look very close at those mileages. Those cracks later grow and at some point they break completely to bring the tensioner down and the engine. What happens first if everything keeps running fine. The 750 runs too hot and needs an oil cooler like the 900 has. In say 95 degree temps F if you play hotrod for a few minutes the oil pressure will drop dramatically and will not come back up until you ride sedately at freeway speeds to bring the oil temp back down. I used an oil pressure gauge on mine.

    The chains are fine and go longer if tensioners kept up with, when they lose tension they die quick, the flapping around kills them. Chain flap harmonics are hell in that engine because the design of the front long chain guide is so crappy, it is 3/8 inch away from the chain to literally not be there at all.

  9. #69
    CB750 Enthusiast ray_'s Avatar
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    Hey amc49, I was looking online for parts but it seems they are sort of drying up... And the ones that are available are expensive! Do you have any preferred parts stores/websites that are reputed to carry good parts? CMSNL seems to have everything btw but is expensive again

  10. #70
    CB750 Guru amc49's Avatar
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    Simply put, there are NO cheap parts unless looking for something the aftermarket makes and very few internal engine parts there. The US parts supply ran low to be running out some a while back and many now come from Europe. Those are now running out as well. These early DOHC never had the super long terrn parts availability the SOHC has even now.

    When you buy from like CMS you are paying a premium for OEM parts sold with even more markup on them, your only choice.

    These were very expensive to rebuild even back in the day and why nobody really used them in racing and such, the engines blew up too easy and way easier to keep a Suzuki or Kawasaki alive and cheaper. Why Honda itself dumped them quick to go to the V-4.

    You could check out Partzilla.com.............

  11. #71
    CB750 Enthusiast ray_'s Avatar
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    Measured my crankshaft today. Based on the markings, was expecting the Main Journals to be : C,B,B,B,B left to right of the engine and the Crank Pins to be: 3,2,2,2. All measurements came out to fall within the tolerance specified in the manual except the C and 3 code turned out to be a B and 2 instead. Briefly put, I have B,B,B,B,B and 2,2,2,2 for the main journals and the crank pins as measured. Was not expecting that. Is that possible? Does that happen in other CB 750 engines too (journals being bigger than expected)? Also, I did find a couple journals to be slightly tapered to feel the slight variance in resistance when sliding the mike down different points along the length of the journals (its not more than 0.002mm as per my measurements.). Will go through the measurements once again in a couple of days just to be sure before ordering parts. Also, the bores remain to be checked....

  12. #72
    CB750 Guru amc49's Avatar
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    The journals of course cannot get bigger. They can wear smaller to then not fit what was new measurements.

    When you measure that small even the air can deposit crap on the mike to mess up readings, why you check and double check and plastigauge will back up what you get. I for one NEVER wipe the parts last with a towel, the fibers and other coming off them will drive you nuts. I use fingers (clean of course) to wipe the mike tips off over and over and recheck my zero at closed mike a lot too.

    Your 'impossible' readings say to concentrate there until you figure out what the problem is. Consider that crank or other might not be the same one the engine came with too, why engine history is valuable.

  13. #73
    CB750 Guru amc49's Avatar
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    As to the journals getting bigger, actually, at those small numbers it may be possible if the journal looks pretty rough. Rough wear can create highs not there before and you possibly need to run a piece of #600 sandpaper over the journal in a motion around it to smooth it up a bit, the numbers may drop back down. I generally do that to the entire crank anyway, a cheap man's polish job, it can bring your variables in a little closer and it can ease in the bearing break-in somewhat. I do NOT do that to the rod big end inside bores or aluminum crankcase IDs! JUST the crank, on both mains and rods.

  14. #74
    CB750 Enthusiast ray_'s Avatar
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    I hope that the reason cause I remeasured the entire crank today and got the same measurements! One more thing though, the journals and pins passed for out of round inspection well but I did notice a max taper value of 0.004mm on the journal that was marked C that turned out to be a B (when measured). That ok? What taper/out of round limits do we work with on these engines?

  15. #75
    CB750 Guru amc49's Avatar
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    .4 mm. is .016", .04 mm. is 1/10th that and .004 mm. is one hundredth that or .00016", small enough to be a non-event. Even the temperature of your fingers will alter that.

    Assuming of course your decimal point is in the correct place.

  16. #76
    CB750 Enthusiast ray_'s Avatar
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    In an attempt to see what kind of clearances I was to expect if I used the chart in the manual and my measurements I made myself a spreadsheet. For the main journals, my measurement and the ID codes suggest using Brown bearings while for the crankpins, its Green and Black respectively. If I stick to the recommended colors, I get a clearance value in the range: 0.02mm to 0.04mm. If I go one size up, using Green for the mains, Yellow and Brown for the crankpins, my clearance goes up to around: 0.03mm to 0.048mm.

    Now, considering that my original main and crankpin bearings are the exact color recommended by the manual chart (except for the two journals that turned out to be slightly bigger), knowing the clearances expected, should I reorder the original colors go up one size as the clearance there is looser and closer to the mid range?

    Btw, thanks for all the help by far!

  17. #77
    CB750 Guru amc49's Avatar
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    Either set of numbers works and why many go to yellow, there is simply literally not much difference in the numbers there.

  18. #78
    CB750 Enthusiast ray_'s Avatar
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    Hi amc49, I am finally done with finalizing my bearing colors and am about to order parts. Just wanted to ask you which bearings are best: Nippon Dia, Taiho or Daido? These are all OEM brands right? Or are they different than the original Honda bearings?

  19. #79
    CB750 Guru amc49's Avatar
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    Just where are you getting them to think you have a choice? It's Honda there and whatever they put in the plastic Honda bags. We lost the better ones when they dropped the 900 and larger bearings that were Kelmut 77 to go back to use up 750 lower quality simple aluminum/babbit bearings. The world has been eating the overstock of bearings for years, you take what you can get.

    If you go outside Honda likely you will not have the sizing (color) choice, most aftermarket bearings come in only one size.

  20. #80
    CB750 Enthusiast ray_'s Avatar
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    Partspitstop an Ron Ayers. Not so much that I have a choice when selecting them but just wanted to understand what I am getting. For the conrod bearings there is a choice to choose between the three brands but for the mains I think they have just one of them. Don't remember which one. But thanks, that answers my question ��

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