Author Topic: Crank Bearings (Read 12589 times)

stueveone · « **on:** February 03, 2012, 11:45:43 PM »

Hey guys,
Just took the necessary measurements for my crank bearings and here's what I came up with:

Case letters: AAABB
Crank Letters: JLAAAAA PL1111
Plastigage: All bearing measured out to .051

Still unclear on bearing math/selection etc...

Questions:
1. What bearings to try test with plastigage first?
2.So the case OD is represented by the case letters while the Case ID is represented by the crank letters?
3. Are there no numbers for the OD of the rod's big end?
4.PL refers to the Rod ID?

Am I anywhere in the freakin' ballpark here?!!!
Thanks!

Ernest T · « **Reply #1 on:** February 04, 2012, 07:33:44 PM »

I think the letters on the cases and crank refer to the main bearings, not the rod big ends. The connecting rod and crank pin measurements are in the mnual which is available free on the site under 750. You can measure the clearance with plastigauge, standard clearance is .0008-.0018 and max is .0032 so .051 is way out of spec. Are you sure that is right?

cbr954 · « **Reply #2 on:** February 04, 2012, 08:51:58 PM »

The AAAAA are the crank main bearing journal diameters. The numbers are the rod bearing journal diameters. The rods have a 1, 2, or 3 stamped on them to indicate the rod internal diameter. There is a chart in the honda manual that gives you a bearing color to use with the indicated letters and numbers on your parts. I am guessing your plastigage measurement was in millimeters as .051mm is about .002", which is right where I like to put them.

HondaMan · « **Reply #3 on:** February 04, 2012, 09:52:36 PM »

Quote from: stueveone on February 03, 2012, 11:45:43 PM

Hey guys,
Just took the necessary measurements for my crank bearings and here's what I came up with:

Case letters: AAABB
Crank Letters: JLAAAAA PL1111
Plastigage: All bearing measured out to .051

Still unclear on bearing math/selection etc...

Questions:
1. What bearings to try test with plastigage first?
2.So the case OD is represented by the case letters while the Case ID is represented by the crank letters?
3. Are there no numbers for the OD of the rod's big end?
4.PL refers to the Rod ID?

Am I anywhere in the freakin' ballpark here?!!!
Thanks!

You're at about 0.0020" (like CBR954 said), so there's nothing wrong with that. If the rods all P'gage similarly, I'd recommend leaving them be. Most veteran Big Four riders will tell you their engine didn't run as well before the bearings loosened to at least that much, and as one example: mine were at 0.0018" average at 55,000 miles in 1980, and when I last looked at them in 2005 they were at 0.0022" average with 126,000 miles on them.

Ernest T · « **Reply #4 on:** February 05, 2012, 08:11:46 AM »

Metric system! Doh!

bryanj · « **Reply #5 on:** February 05, 2012, 10:11:12 PM »

Not DOH they were built metric, it's you yanks that are awkward working in inches still

Ducks and steps backwards to avoid the brickbats!!

Have a Nice Day Now

Don R · « **Reply #6 on:** February 05, 2012, 10:58:43 PM »

We tried metric but someone chickened out. Now our new cars require both sets of tools. Good deal huh?

Ernest T · « **Reply #7 on:** February 06, 2012, 05:08:05 PM »

Quote from: bryanj on February 05, 2012, 10:11:12 PM

Not DOH they were built metric, it's you yanks that are awkward working in inches still

Ducks and steps backwards to avoid the brickbats!!

Have a Nice Day Now

Hey, at least we use one or the other!

stueveone · « **Reply #8 on:** February 07, 2012, 06:43:24 PM »

Well, if it's good enough for HM, it's good enough for me! Looks like I'm reusing my bearings.
However, as far as the numbers go still a bit foggy on what they reference. . .

Tell me if this is true:
The case (in my instance AAABB) refer to the main bearing OUTER diameter?
The crank (in mine JLAAAAA) refers to the main INNER diamter?
The crank (PL1111) refers to the con rod INNER diamter?

Ernest T · « **Reply #9 on:** February 07, 2012, 07:25:59 PM »

You got me, I thought the case letters referred to the bearing thickness. When you think of the chart, it references the case inside bearing diameter vs the crank journal outside diameter. However, if you follow the green bearings on the diagonal, they use the same bearings on different case inside bearing and outside crank diameters.

cbr954 · « **Reply #10 on:** February 07, 2012, 08:34:08 PM »

The letters on the case refer to the size of the bore in the case that the bearing inserts fit in. The letters on the crank is the diameter of the main journal diameter. The other set of numbers are the diameter of the journal on the crank for the connecting rods. There is a number on the big end of the connecting rods that refer to the inside diameter of the connecting rod. The different bearing thicknesses have a corresponding color coding, green, yellow, brown and black. The same color of bearing can be used on different combinations of case and journal diameters(as per the honda chart) because the bearing will be crushed a different amount depending on the size of the case and thus change the actual inside bearing diameter. It sounds more complicated than it is. If the clearances plastiguage ok you are good to go.

Ernest T · « **Reply #11 on:** February 08, 2012, 05:03:54 PM »

Quote from: cbr954 on February 07, 2012, 08:34:08 PM

The letters on the case refer to the size of the bore in the case that the bearing inserts fit in. The letters on the crank is the diameter of the main journal diameter. The other set of numbers are the diameter of the journal on the crank for the connecting rods. There is a number on the big end of the connecting rods that refer to the inside diameter of the connecting rod. The different bearing thicknesses have a corresponding color coding, green, yellow, brown and black. The same color of bearing can be used on different combinations of case and journal diameters(as per the honda chart) because the bearing will be crushed a different amount depending on the size of the case and thus change the actual inside bearing diameter. It sounds more complicated than it is. If the clearances plastiguage ok you are good to go.

I don't understand why the letters would refer to the bore size of the case that the bearings fit in. It's line bored, why aren't all the bores the same exact size? I understand how crank journals can be off slightly because each is machined seperately.

cbr954 · « **Reply #12 on:** February 08, 2012, 09:00:54 PM »

You have to remember that the differences between the different letters are measured in ten thousands of an inch. Most people dont even have a tool that can measure them accurately enough to detect a difference in the bores. Honda bored the holes then measured each one and assigned the appropriate letter for the size, down to ten thousands of an inch, very small differences with occur in mass production.

HondaMan · « **Reply #13 on:** February 08, 2012, 09:43:05 PM »

CBR954 hits the nail on the head: in general machining practice, having cases like these bolted together to a "torque" from a torque wrench produces not-real-even case compressions. After they are align-bored and released, the holes often grow during the second bolt-up, after the threads in the holes smooth out and the torque becomes different. This can easily generate 4 tenths of a thousandth difference.

I first discovered this on Honda cases circa 1974, when we align-bored at K0 sandcast with over 100k miles on it. The holes were a little bigger than the case numbers indicated (which you'd expect by then), but they were not in line, but almost half a thou. So, the cases were bored 0.010" and shimmed with pegged shims, and new Black bearings were installed after polishing the crank journals. The final Plastigaging came in around 0.0015" for the 5 bearings. The most remarkable thing happened, though: that engine was so smooth that we started calling it "the electric 750". It was by FAR the smoothest running 750 I have ever seen. If I had the $$, I'd do it to mine in a heartbeat!

stueveone · « **Reply #14 on:** March 11, 2012, 06:26:37 PM »

Ok, well looks like I'll be re-using my crank main bearings after all.
As far as con rods, I've got a set from another motor however they will definitely need new bearings. I just platiguaged all 4 and came up with these measurements:

With NEW green bearings 3 were all .002 with 1 being .002 with a NEW brown.
Hondaman, you mentioned in this post that .002 was OK for used bearing clearances, but in your book you mention that if the bearings are new they should be half that number, like .001? Is that correct?

Thanks again for your help guys.

stueveone · « **Reply #15 on:** March 12, 2012, 06:04:14 PM »

Any takers? Trying to order these bearings quick!
Thanks!

cbr954 · « **Reply #16 on:** March 12, 2012, 07:37:53 PM »

If they are at .002 I would run them. That is within spec and if I remember right your main are at .002 also so I would say you are good to go.

stueveone · « **Reply #17 on:** March 12, 2012, 09:11:38 PM »

So, then, clearance is clearance regardless of if the bearings are new or old? You wouldn't want to fit them a tad tighter if new to break in or anything?

Thanks!

Grabcon · « **Reply #18 on:** March 13, 2012, 05:25:27 PM »

So this is kind of on topic but a little off. I have a motor torn down that I need to plastigage. Do you put in a bolts in the case and and torque to spec? Or just those over the main bearings?

cbr954 · « **Reply #19 on:** March 13, 2012, 05:31:47 PM »

Quote from: stueveone on March 12, 2012, 09:11:38 PM

So, then, clearance is clearance regardless of if the bearings are new or old? You wouldn't want to fit them a tad tighter if new to break in or anything?

Thanks!

If you checked with old bearings then new ones would be ever so slightly tighter. I dont have a problem with using old bearings if they look ok and are within spec though. In my opinion.....plain bearings dont break in. If you have metal to metal contact in a babbit bearing than the bearing will be toast.

cbr954 · « **Reply #20 on:** March 13, 2012, 05:36:10 PM »

Quote from: Grabcon on March 13, 2012, 05:25:27 PM

So this is kind of on topic but a little off. I have a motor torn down that I need to plastigage. Do you put in a bolts in the case and and torque to spec? Or just those over the main bearings?

I just torque the main bearing bolts but thats just me. I do put some in a couple of places around the cases though and snug them to make sure the cases are pulled together good. I dont install and torque all of the other bolts though.

stueveone · « **Reply #21 on:** March 13, 2012, 05:41:56 PM »

That's the thing, I checked with brand new bearings, which is why I was wondering if i should go a little tighter of bearings since the measurement was .002 with new ones in there.

I did the same as CBR954, I just torqued the main bolts and a few case bolts.

cbr954 · « **Reply #22 on:** March 13, 2012, 06:42:23 PM »

Quote from: stueveone on March 13, 2012, 05:41:56 PM

That's the thing, I checked with brand new bearings, which is why I was wondering if i should go a little tighter of bearings since the measurement was .002 with new ones in there.

I did the same as CBR954, I just torqued the main bolts and a few case bolts.

I would run them. .002 is a good clearance to run.

Grabcon · « **Reply #23 on:** March 13, 2012, 09:22:57 PM »

Out of curiosity where do you buy crank main bearings and rod bearings for these CB750's? What should one expect to pay?

stueveone · « **Reply #24 on:** March 13, 2012, 10:44:37 PM »

I bought mine from Bikebandit. Like 12 bucks per half. YIKES!

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Author Topic: Crank Bearings (Read 12589 times)