my bike threw a rod bearing on #3 piston rod, took the head off, took the cylinder sleeve off , my question is first- can i replace the bearing without taking the motor out and splitting the crank case and second question is can I reuse the piston rod and bolts
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Threw Rod bearing 84 nighthawk 650
- Thread starter mhw0508
- Start date
Dave
CB750 Junkie
#1 - No.
#2 - Yes, if they aren't damaged.
#3 - The crank is probably damaged too.
BE WARNED! - This engine is another fine example of Hondas "See what fits and put that in there" philosophy. Every bearing on the crankshaft has been selected to fit in that exact spot and only that exact spot. Also, the rods can be different weights. The top end is like this too. Pretty much any part that there is more then one of, there is more then one size as well.
I found a manual in PDF format an uploaded it to mega for you. See "Crankshaft/Starter Clutch - Section 12"
Filename: 1984 Honda Nighthawk CB750SC.pdf - Size: 14.0 MB
Link: https://mega.nz/#!sMpVyCha
Key: !r1b4OY-vnhvLFp0TMJqoohOhlTJclIGCvljhEcUpjUQ
FYI - Wrong forum, you should be in "The DOHC Garage (1982+ Nighthawk & CB750F2)"
#2 - Yes, if they aren't damaged.
#3 - The crank is probably damaged too.
BE WARNED! - This engine is another fine example of Hondas "See what fits and put that in there" philosophy. Every bearing on the crankshaft has been selected to fit in that exact spot and only that exact spot. Also, the rods can be different weights. The top end is like this too. Pretty much any part that there is more then one of, there is more then one size as well.
I found a manual in PDF format an uploaded it to mega for you. See "Crankshaft/Starter Clutch - Section 12"
Filename: 1984 Honda Nighthawk CB750SC.pdf - Size: 14.0 MB
Link: https://mega.nz/#!sMpVyCha
Key: !r1b4OY-vnhvLFp0TMJqoohOhlTJclIGCvljhEcUpjUQ
FYI - Wrong forum, you should be in "The DOHC Garage (1982+ Nighthawk & CB750F2)"
amc49
CB750 Guru
It is pretty much impossible to simply replace a bearing only and go back to it, the rod and crank generally get some type of damage there and often not visible to the untrained eye. but there to make it do it all over the first time you rev it up hard again. I'd say out of 100 engines then you can get away with it on 5. HIGHLY unlikely. I've done a few like that but most, no way, you will simply tear up engine again. Pretty quick.
Look at the bearing differences in the color codes. only like a couple ten-thousandths difference in each code to the next. Meaning maybe not even a whole thousandth difference in the entire color code range there and the engine wears crank more than that generally if it has lived at all. Making all bearing selection by color pretty much worthless. MEASURE parts to make sure but like we do on the DOHC, if parts are useable within wear limits then you generally use what will be the loosest bearing color you can get on all of the positions and thinking yellow IIRC in the case of DOHCs. The later year model may well work out to be the same way but careful measuring is in order, that will put the lie to the changing bearing only thing too.
They really pay close attention to new motor setup for max longevity but not really needed to run long and far if you can read measuring tools. Or Plastigauge.
You must split cases. Any rod that has spun a bearing is VERY highly likely to do it again. I don't care HOW good the part looks. Also, they generally spin for a reason, if not addressed then that underlying condition is probably there to spin it again as soon as you get it running. Odds are you may well find the other bearings to be severely worn too.
Rods have a weight code letter stamped on them; you match replacement rod with same letter as your other 3 to be same weight. Each letter has parts weighted to be within 5 grams IIRC. Read the manual. Don't be surprised if you have to balance rod, the older the engine is the more the lighter rods get bought to run out of those then nothing but super heavy ones can be bought and then you have to grind on it to bring the weight in line with the others or engine vibrates. I've had to match F weight ones to B's before. Grind city. Or find like ebay rod of correct weight.
Better hope crank not damaged like said, that'll end the party for a bit. Spinning bearing often tears it up and you can't regrind them like with cars.
Look at the bearing differences in the color codes. only like a couple ten-thousandths difference in each code to the next. Meaning maybe not even a whole thousandth difference in the entire color code range there and the engine wears crank more than that generally if it has lived at all. Making all bearing selection by color pretty much worthless. MEASURE parts to make sure but like we do on the DOHC, if parts are useable within wear limits then you generally use what will be the loosest bearing color you can get on all of the positions and thinking yellow IIRC in the case of DOHCs. The later year model may well work out to be the same way but careful measuring is in order, that will put the lie to the changing bearing only thing too.
They really pay close attention to new motor setup for max longevity but not really needed to run long and far if you can read measuring tools. Or Plastigauge.
You must split cases. Any rod that has spun a bearing is VERY highly likely to do it again. I don't care HOW good the part looks. Also, they generally spin for a reason, if not addressed then that underlying condition is probably there to spin it again as soon as you get it running. Odds are you may well find the other bearings to be severely worn too.
Rods have a weight code letter stamped on them; you match replacement rod with same letter as your other 3 to be same weight. Each letter has parts weighted to be within 5 grams IIRC. Read the manual. Don't be surprised if you have to balance rod, the older the engine is the more the lighter rods get bought to run out of those then nothing but super heavy ones can be bought and then you have to grind on it to bring the weight in line with the others or engine vibrates. I've had to match F weight ones to B's before. Grind city. Or find like ebay rod of correct weight.
Better hope crank not damaged like said, that'll end the party for a bit. Spinning bearing often tears it up and you can't regrind them like with cars.
APE can repair crankshafts, have had them do it done several times.
amc49
CB750 Guru
While that can work I don't care to do it unless forced. Some of the drag cars I worked on had such expensive cranks or heads you had to but if one can simply find another crank in good shape on stock OEM stuff I would be going there first. Doing that job is expensive as it has to be done right and I've seen some butcher work although I would expect somebody like APE to do a good job. You never know, I've seen high end expensive work that turned out to be butcher job done by some of the supposedly finest shops in the world.........
Don't get me wrong, so far the stuff I've seen from APE is top of the top. They are the only people I trust with bore work, gave up on the local machine shops around here, all they can do is wreck parts.
Don't get me wrong, so far the stuff I've seen from APE is top of the top. They are the only people I trust with bore work, gave up on the local machine shops around here, all they can do is wreck parts.
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