Topic: Main journals, keep original, or step it up

[kevinvondam]

Alright, after some reading and going through different posts from several years ago i found out how to determine wich main bearings should have been delivered out of the factory.

Since there were no color markings on the bearings, nor could i find the markings on the crankshaft, i took some measurements to find out what the clearences should have been.

Marked from the rotor on, the case shows; BAABA

Measurements taken on the crankshaft;

Journal	Code	1st	2nd	3rd	Avg.	Tol.	Color
1	B	32.985	32.983	32.977	32.982	-0.018	Brown
2	A	32.977	32.977	32.980	32.978	-0.022	Green
3	A	32.976	32.976	32.974	32.975	-0.025	Green
4	B	32.974	32.972	32.975	32.974	-0.026	Brown
5	A	32.986	32.983	32.980	32.983	-0.017	Green

So my question is, since the book shows tolerances up to -0.020, my measurements shows a value up to -0.026, should i use the original color matches or should match different colors, to get tighter tollerances?

[HondaMan]

Alright, after some reading and going through different posts from several years ago i found out how to determine wich main bearings should have been delivered out of the factory.

Since there were no color markings on the bearings, nor could i find the markings on the crankshaft, i took some measurements to find out what the clearences should have been.

Marked from the rotor on, the case shows; BAABA

Measurements taken on the crankshaft;

Journal	Code	1st	2nd	3rd	Avg.	Tol.	Color
1	B	32.985	32.983	32.977	32.982	-0.018	Brown
2	A	32.977	32.977	32.980	32.978	-0.022	Green
3	A	32.976	32.976	32.974	32.975	-0.025	Green
4	B	32.974	32.972	32.975	32.974	-0.026	Brown
5	A	32.986	32.983	32.980	32.983	-0.017	Green

So my question is, since the book shows tolerances up to -0.020, my measurements shows a value up to -0.026, should i use the original color matches or should match different colors, to get tighter tollerances?

Since it is a used crankshaft, I'd suggest using the Plastigage method instead. The crank journals may not be perfectly round anymore, and it is very easy to introduce hand errors in 3-point measurement with hand mics unless you're a real experienced machinist (and even then, many are not that good!).

Test for a bearing clearance (with crankcases torqued to spec) of 0.0008" to 0.0016" (0.02 - 0.04mm) for best performance unless the engine is a post-1976 CB750 or CB550F. On those engines, the clearances were allowed up to 0.0008" to 0.0021" (0.02 - 0.05mm) new.

[paulbaker1954]

This has confused me

I am just about to rebuild my 500 K0 Four and am going to check the main and big end clearances using pLastigage

However I thought the Homda spec was that both main and big end clearances should not be above .008"

Am I missing something as you are quoting much bigger allowable clearances it would seem

[HondaMan]

This has confused me

I am just about to rebuild my 500 K0 Four and am going to check the main and big end clearances using pLastigage

However I thought the Homda spec was that both main and big end clearances should not be above .008"

Am I missing something as you are quoting much bigger allowable clearances it would seem

The CB500, CB750K0-K5, and the CB550K (and first year 550F) all use the bearing clearance of 0.0008-0.0012" new. After the journals are run a while they typically add some clearance from settling, and they end up around 0.0015" to 0.0018" on the same crankshaft. This always seems to return, even if the bearings are replaced with identical or next-step-down sizes, after about 10k miles or so. It's long been my surmise that the intended clearance, and one which fits well with the oil flow quantities, is about 0.0016". The best-running 750 engines I have torn down for other reasons always seem to be 0.0015"-0.0018", too.

Beginning in late 1973, Honda modified the way they characterized these parts. Personally, I think they discovered that Kawasaki was doing OK in their engines by using just 2 bearing sizes, so they seemed to adopt that approach, but kept the other parts available to us all (for which I have always been grateful!). They also seemed to have improved their tooling, as more engines show up with case numbers like all "BBBBB" or "AAAAAA" or even all "CCCCC", crankshafts likewise. This comes from better operator training and less haste, which was certainly NOT how the 750K0-K1 bikes were made! In the later engines, it is completely normal to see clearances like 0.0016" - 0.0018" on the crank and 0.0014" - 0.0020" on the rods, and the engine is happy as can be. When they get the 'tighter' ones, like all "A" crank and "C" journals, they end up tighter and again, tend to stay tighter over time.

The main thing I've found that makes the most difference is: try to get them all to be the same, as close as practical. If they are all within 0.0004" of each other and all under 0.0024" clearance, you'll have a nice, smooth runner.

[BPellerine]

while we are on the subject,mark,do you often find shells marked std!found two sets in two diff sets of rods,seem to be the same as green in Honda speak.bill

[HondaMan]

while we are on the subject,mark,do you often find shells marked std!found two sets in two diff sets of rods,seem to be the same as green in Honda speak.bill

Right you are, Bill!
And, I'd lay odds that it is in a post-1973 engine? Or else, that they were replaced sometime after about 1995?
They 'reduced' the bearing shells to 2 sizes, but both are marked "STD". The smaller (like Brown equivalent) ones may be marked "STD-A". So, it becomes easiest to just rely on Plastigage, under you're real good at (and have long enough bore gages to) measuring the ineer, empty, case holes, when torqued. It's a lot of work!

[MRieck]

Currently American Honda lists only 1 size of main bearing as available for the 550. You can measure but what good will it do?

[BPellerine]

while we are on the subject,mark,do you often find shells marked std!found two sets in two diff sets of rods,seem to be the same as green in Honda speak.bill

Right you are, Bill!
And, I'd lay odds that it is in a post-1973 engine? Or else, that they were replaced sometime after about 1995?
They 'reduced' the bearing shells to 2 sizes, but both are marked "STD". The smaller (like Brown equivalent) ones may be marked "STD-A". So, it becomes easiest to just rely on Plastigage, under you're real good at (and have long enough bore gages to) measuring the ineer, empty, case holes, when torqued. It's a lot of work!

one was a set of k2 rods and the other was a set of rc rods,dont know when they might have been changed,did not reuse ,but no markings except paint on the new ones from Honda.bill

[paulbaker1954]

A further question, the block and Crank from the factory are marked with letters and numbers right.

eg on the block will be AABBC etc. IN the Honda manual it says never use these as part of service.

This seems odd to me if I was replacing the mains wouldn't youjust replace with the right coloured shells for the original factory measured sizes.

Or are these letters and numbers nothing to do with the crank journal measured diameters

I am going to Plastigage them anyway but I am just curios.

[HondaMan]

A further question, the block and Crank from the factory are marked with letters and numbers right.

eg on the block will be AABBC etc. IN the Honda manual it says never use these as part of service.

This seems odd to me if I was replacing the mains wouldn't youjust replace with the right coloured shells for the original factory measured sizes.

Or are these letters and numbers nothing to do with the crank journal measured diameters

I am going to Plastigage them anyway but I am just curios.

That's interesting about your Honda manual: mine states clearly that these letters MUST be used to select bearing shells. Then it goes on to explain how they were sized to become those letters (and numbers), and which shells apply. It further goes on to explain how to Plastigage them, too. But...my manual is for the CB750K0, the first bound edition allowed out to Honda shops back in those days. I don't think I've ever seen another one like it, only the K1 and multi-model versions.

Most of the time, after the bearings are bedded, it is beneficial to step down one size if you want to add some smoothness. It goes through another break-in period, but they seem to last a lot longer this way. I just did this at 140k miles on my own, after doing the same-size bearings in 1980 (at 55k miles) just 'cuz. In the 1980 rebuild, the bearings Plastigaged only .0002" different old to new, though, even being the same size. The one thing I did note then: my old shells had NO GROOVES in them, while all the shells I have seen since have grooves. Interesting stuff!

[MRieck]

A further question, the block and Crank from the factory are marked with letters and numbers right.

eg on the block will be AABBC etc. IN the Honda manual it says never use these as part of service.

This seems odd to me if I was replacing the mains wouldn't youjust replace with the right coloured shells for the original factory measured sizes.

Or are these letters and numbers nothing to do with the crank journal measured diameters

I am going to Plastigage them anyway but I am just curios.

Correct. That is what it says in the factory 550 manual. Only greens are available for the 550 mains and browns/greens for the rods.

[kevinvondam]

Alright, since the brown bearings were on backorder, it took a little while for them to deliver the whole set. So today i had the complete set;

Journal	Code	Color	Plastigage
1	B	Brown	0.051mm
2	A	Green	0.051mm
3	A	Green	0.051mm
4	B	Brown	0.051mm
5	A	Green	0.051mm

So the good news, all of them are the same, i did measure them twice, on top of the crank and on the bottom of the journals. Both came out pretty much the same.

Since the book says 0.020mm - 0.046mm, i was wondering if this would be an issue in the long run?



You can see the little green stuff

Ps. i used new stainless steel A2/70 bolts at 19Nm for these measurements.