Topic: 150$ for new crank bearings?!

[Finnigan]

I plastigauged my crank bearings tonight and they look to be right at the serviceable limit of .08mm, I'm posting this for my own sanity sake...but the bullet and buy new ones?

[tomkimberly]

I don't think you know how to use plastic gage as I don't see any in your pictures.....

[AJK]

Look at the picture where he's holding the gauge beside the journal. I don't think you know how to look at pictures...

Finnigan - yup, bite down hard and buy new bearings.

+1

[PeWe]

I replaced mine just for sure when the cases were apart, did not measure when they looked scuffed despite only 30.000km's since last replacement (total rebuild).

I did not remove the con rods to check the bearings when I understood that it most likely should end up in a super crank complete with super rods from CycleX or APE. I might regret that later on.

What about the chains? Primary chains + tensioner and cam chain + tensioner.
It cost a lot to open an old engine and put it together with peace in mind afterwards.

[tomkimberly]

Look at the picture where he's holding the gauge beside the journal. I don't think you know how to look at pictures...

Finnigan - yup, bite down hard and buy new bearings.

Just where in his picture is the plastic gage material?  Measuring the gap in the bearing is not measuring the plastic gage.

Tom

[72 yellow]

Look at the picture where he's holding the gauge beside the journal. I don't think you know how to look at pictures...

Finnigan - yup, bite down hard and buy new bearings.

Just where in his picture is the plastic gage material?  Measuring the gap in the bearing is not measuring the plastic gage.

Tom

Looks like the material is next to the .076 mm on the gage.

[DH]

Tom, zoom in real good on the bottom pic

[AbbyRider]

You don't even need to zoom in... I can see it clearly on my phone screen.

My personal opinion is to not replace, but only because I've got a frame kit and have become proficient at taking the engine out splitting cases. Mine gauged out similar to yours, but just slightly less wear. And I kept them in.

[madmtnmotors]

Look at the picture where he's holding the gauge beside the journal. I don't think you know how to look at pictures...

Finnigan - yup, bite down hard and buy new bearings.

Just where in his picture is the plastic gage material?  Measuring the gap in the bearing is not measuring the plastic gage.

Tom

Looks like the material is next to the .076 mm on the gage.

Indeed. Looks like a thin strip of faint gray/green material on the bearing next to the .076 marker...

[Finnigan]

Yah sorry guys I upped the contrast best I could but that light green is hard to see unless you really zoom.  Thanks for the info I'll bite the bullet and add it to the list of primary and cam chains, tensioner and so on 

Should I be checking big end bearing clearance also??

[bwaller]

Yes check the rod brgs. too. As clearances increase, oil pressure decreases and you may see the oil light at an idle.

[Finnigan]

I'll do that, thanks!

BTW I found last night that my PDF manual is missing pages 100+.  Does anyone have a link to a complete download?

[edwardmorris]

I'll do that, thanks!

BTW I found last night that my PDF manual is missing pages 100+.  Does anyone have a link to a complete download?

What kinda bike? Here's the link for the Ks

http://www.sohc4.net/cb750k-manuals/

[Finnigan]

78 cb750F

[edwardmorris]

78 cb750F

Not too sure about the F manuals, but try this

http://www.sohc4.net/cb750f-manuals/

[lucky]

Look at the picture where he's holding the gauge beside the journal. I don't think you know how to look at pictures...

Finnigan - yup, bite down hard and buy new bearings.

OK..I clicked on the photo once and that gets you a larger version of the photo, THEN you can see the squished plastigage.

.08mm=0.00314961  You are close to the limit.
So .076mm= 0.002992126
You are 1 and 1/2 ten thousandths from the limit.


I would get new bearings.
You do not want to start out the life of this rebuilt engine behind the eight ball.
It will just get worse.

[Greggo]

Likely cheaper from Yamiya...

http://www.yamiya750.com/index.php?main_page=index&cPath=21_155&sort=20a

[Jerry Rxman Griffin aka MuthaF'er]

I see VERY little wear in that bearing. It would likely wear MUCH quicker than the crank. I'd guess that your engine has already been apart and the bearings were not resized correctly? But then again there is that old school of thought to just put the loose ones in and they won't be too tight.

How was it running? How was your oil pressure? If it was all good I might be tempted to re-use those bearings if they are all in that good of shape and the clearances are not larger.

OR if you want to be a perfectionist put in new bearings but carefully match the crank and cases then go to the chart. Can you determine what color THOSE bearings are? Look on the edges. If they are the same color as you determine from the chart I'd say replacing would be a waste.   

[Finnigan]

They are all green, the color are very clear.  This crank case, shaft and bearings are all from another motor that I bought from another member when I was picking apart the one I bought (seized up)

Are you saying its not as easy as just replacing them with new GREEN bearings?

[andrewk]

If you're using an OEM manual the 77/8 K and F specific stuff is in an addendum in the back.

Regarding bearing clearance, you need to check the chart on page 49 of the OEM manual.  Its a little confusing I think, but the chart will help you determine both the bearings you have, how worn they are and what bearings to select given the crankshaft clearance.  The letters on the chart give you what the bearing "should" be (case and crank letter stamps), but I would pay more attention to dimensions, as that's what is important.

[Finnigan]

Sorry I'm not familiar with the term "mic", are you talking about measuring them with a caliper?

[Jerry Rxman Griffin aka MuthaF'er]

Calipers are good and easy to use but you really need to use a MICrometer on the crank and bearings. You're talking .0000 measurements.

Have you matched the crankcase measurements with the crankshaft measurements yet? These are marked on the respective parts by Honda. That's a good place to start perhaps before you start mic'ing.

There are 2 sizes thicker bearings for the crank than your green that will get you closer in tolerance. I took one of mine from a measurement similar to yours to about .050 with the use of thicker bearings. With this in mind my crank had been worked on and using the Honda chart and measurements I spoke of only gave me a start. After Plastigauging with the corresponding greens from the chart and getting a .076 or so I ordered new thicker bearing and started all over. 

[Finnigan]

Great thanks for the info!  The crank and cases are from the same motor so I didn't think to match up the markings

[Jerry Rxman Griffin aka MuthaF'er]

If the crank and cases (BOTH halves) are from the same motor and your bearings are that far off then maybe the crank had something done to it?? Thats not a problem as long as you can get bearings to fit. Heavy journal polish perhaps?? Does it look like it's been taken apart before? I'd think so if you can still tell those bearings are green maybe?? The case letters are obvious, it's the crank that could use a trained eye ie knows what to look for. Japanese chicken scrawlings. See what you can come up with.

[madmtnmotors]

I'm kinda with Jerry... those bearings looked mighty pretty without any real signs of wear...

Good advice to have the old bearing thickness measured (micrometer) and compare to the specified thickness of a new bearing.