Topic: CB750 oil delivery/rod end bearing question

[janderstang]

Hey guys,

Quick couple questions. I have a 1978 CB750 and the #4 piston was hitting the head. Turns out #1 and #4 bearings spun. 2 and 3 are fine, as are the crank bearings. Does anyone see any reason why it would be foolish to just replce 1 and 4 and re-assemble? The bike has 35,000 miles on it.

Second (and more importantly) does anyone know why 1&4 would spin? Is there an oil delivery problem on these bikes that you may have heard of? Any help would be greatly appreciated.

Jeff

[TwoTired]

Second (and more importantly) does anyone know why 1&4 would spin? Is there an oil delivery problem on these bikes that you may have heard of? Any help would be greatly appreciated.

Speculation?  I'm game!!

If the 1-4 ignition timing was early, this would place extra pressure on the rod bearing.  Pound it enough times and...

[janderstang]

sounds about right. The PO had it all jacked up so that may be what happened. I know the timing was off. Thanks ,I feel a bit better now.

[janderstang]

So I'm trying to order rod-end bearings and the drawing has four different ones yellow, brown, green, and black. Does anyone know how to determine which one to get? I have attached the image since I can't figue out how to insert it in the text (I'm a disaster, I know).

[Jerry Rxman Griffin aka MuthaF'er]

Best way would be to use Plastigauge after you've determined the starting point. The rods were measured at the factory and have a marking on the side of the big end. The crank is also "etched" but you'll swear it isn't on there or you just can't see it. Match those up and refer the the chart. Get the bearings and install with Plastigauge. Measure the width of the Plastigauge after you have torqued the bolts down and carefully taken the rods apart again. If the measurement shows you have the proper clearance then torque 'em back and go. If not, order the correct bearings and start over. Just don't torque the rod bolts any more than necessary since they strech and weaken slightly each time.

[janderstang]

I've done some more reading and the diameter of the crank pin should be 36mm and the rod end should be 39mm. There are tolerances on the chart at this website, but the way it reads I am supposed to be measuring thousandths of a millimeter!?!?!?

It would make more sense to me if it were hundredths of a millimeter. Anyone know what a standard wear number would be for crank and rod end bearings?

If I do the chart in hundredths, it works out to be about .003" from one bearing to the next which makes sense. If you look at it as printed, I am understanding that 3 different bearings have been manufactured to cover various gap ranges MAXIMIZING in .0009". that seems a bit too high-tech for 1978.


http://www.bikebandit.com/houseofmotorcycles/honda-motorcycle-cb750f-1978/o/m9455  

[janderstang]

Best way would be to use Plastigauge after you've determined the starting point. The rods were measured at the factory and have a marking on the side of the big end. The crank is also "etched" but you'll swear it isn't on there or you just can't see it. Match those up and refer the the chart. Get the bearings and install with Plastigauge. Measure the width of the Plastigauge after you have torqued the bolts down and carefully taken the rods apart again. If the measurement shows you have the proper clearance then torque 'em back and go. If not, order the correct bearings and start over. Just don't torque the rod bolts any more than necessary since they strech and weaken slightly each time.

Good advice, I'll definitely use Plastigauge when I assemble it. I just posted while you were replying witha question on the average wear versus the chart. Does that seem like a mis-print or do they really measure in thousandths of a millimeter?

[Jerry Rxman Griffin aka MuthaF'er]

The best thing that chart can be used for is matching bearings to crank and rod markings FROM THE FACTORY. I don't think it's going to take into account wear after the fact which is what you'll use Plastigauge for. Can you read your crank "scribblings" that were inscribed at the engine factory? The rod markings on the big ends should be obvious. I'm away from home with no refernce materials. I have 3 manuals and a Honda shop manual at home that will give more detail.

[bryanj]

If the shells span in the rods the rods are toast and probably the crank as well

[janderstang]

If the shells span in the rods the rods are toast and probably the crank as well

What do you mean if the shells span? The bearings fell out when I dis-assembled the rods on 1 and 4.

[TwoTired]

Bryan meant spin, not span.

I was going to mention the same thing.  A spun rod bearing has damaged the rod and crank journal every single occurrence I've encountered.

The crank has to be replaced (or turned down under-size), the rod replaced (or ground and re-bored), and a new bearing fit to the proper clearance.

The "bed" the bearing shell fits onto in the rod must be pristine to hold the shell in proper place.  Not even fingerprints can go there.  If the crank journal isn't finished smoothly, the replacement bearing will simply grab the new bearing and spin it, too, or the rod will snap, and the remaining pointy bits will poke holes in the crankcase.

Cheers,

[janderstang]

Uh oh, well, thanks guys for the advice. I guess I'd better start looking for a crank and a set of rods.

[bryanj]

Span as in past tense of spin!

Just cos the shells fell out dont mean its trash show us some pictures of the shells, both sides, and well give you a better idea.

[754]

I am thinking, that rods could be resized IF you are careful.

Bearings can be pinned.

Generally on a Honda four there is no undersize bearings, but in a few cases.. with slight wear it may still take a different coded Honda bearing shell.

So generally if crank is worn (like spun bearing), you cannot grind and find a shell to fit, unless you build up the journal and regrind.


_____________________________________________________

As far as other bike engines go, on a Beemer I have spun a bearing, replaced it without removing the crank and it ran fine.
They have very hard, nitrided I think, cranks.

I bought new bearings for mine tore into it in the evening.. spent forever polishing the crank with crocus cloth, and then flushing it out to clean out the crankcase.Got it buttoned up at 5 am.

Seeing as I had planned to go to Strurgis in a few days, I had to test it to make sure it worked, so I went for a ride. Headed to a town 125 miles away, was nearly there hit rain so I turned around,, got about 5 more miles and hit the wall so to speak.. snoozed in the ditch for 1/2 hr or so, then rode home.

Got home at 11am, had 200 miles on it, seemed fine so a few days later I hit the road for a few weeks.

[janderstang]

I wish my stupid digital camera would work, but it's dead. Anyway, I simply cleaned the crank pin with 2400 grit polishing cloth and felt no damage whatsoever, it was simply cosmetic. The rod end looks great, it may not have actually spun, but the bearing is definitely trashed and was causing the piston to hit the head. I ordered a new bearing per the factory marks and I am going to give it the old college try I guess. We'll see how it plays out. Wish me luck (I have a good feeling about the crank and rods being good to go). Thanks for all of the advice, I'll know if conventional wisdom or luck will win the day.

[754]

If you mike the crank and compare with the chart you will quickly see if you should buy the bearing..

[janderstang]

yeah, I took a few measurements with a digital micrometer, and they all looked good, but it only went to the second decimal place. To get thousandths of a millimeter I would need a dial depth guage and a rig fixture. I don't have all that. I really think it happened and didn;y run much after that and I am good to go. I guess we'll see. Any advice on actually installing the bearings? Should I freeze the bearings and heat the rods? I have to go pick them up tomorrow. How should I prep the surfaces, will brake cleaner suffice?