Topic: Main bearing clearance assistance

[captaink]

Hi All, I have just made a first main bearing clearance check on my 1977 CB750 engine using plastigauge.
The journals looked in really good order and originally they had Green shells on all 5 journals.
Although the shells looked good and unscored I decided to replace them anyway.
I have fitted new Green shells and the measurements are as follows:
Looking at the engine from the front:
journal 1  is 0.076
Journal 2 is 0.076
Journal 3 is 0.063
Journal 4 is 0.076
Journal 5 Generator end 0.050
I will take the readings again but how does this look
Any assistance appreciated
Regards Keith

[captaink]

Hi All,
A 2nd reading taken and a more accurate torque wrench used, again looking from the front of the engine.
They look more equal but is this correct ?
Journal 1  is 0.063
Journal 2 is 0.063
Journal 3 is 0.076
Journal 4 is 0.076
Journal 5 generator end is 0.063 ..maybe a fraction over 0.063

Regards Keith

[Pat_at_APE]

That seems too loose to me.   The Honda manual says set them to .046mm when new.   The service limit is out to .08mm which is where you are starting. 

Pat at APE

[captaink]

Hi, Original shells were all Green
The old bearing shells looked very good and hardly worn but as it was a total strip it seemed silly not to replace them so as a starting point new greens went back in.
To decrease the clearance what would be, and is there a next thicker shell ? and as its not a daily used bike is it worth doing ?

Cheers Pat

I have lots of APE parts going in both my CB's

[PeWe]

Have you done it correctly? New bearings and those numbers..
Here when I measured mine. Case has B BBBB, crank journals measured too.
http://forums.sohc4.net/index.php?topic=155014.0

I checked first with my lightly used green, changed  to brown (tighter). I have changed back to all green again since I opened the cases again when I had to prepair for bigger bore. The middle bearings looked scuffed.

[captaink]

Hi, Was told that the next size larger shells were the Brown and this was 0.020 larger than the Green.
Am I correct in saying this should put the clearances down to about 0.043 - 0.056 ??
Or am I talking B.......ks

Regards Keith

[PeWe]

Have you measured the crank journals with a micrometer?

http://s890.photobucket.com/user/edhaeuser/media/CB750/20130605_221452a_zpsc4afc7d6.jpg.html
My newer crank I installed have thicker journals than my old that ran OK with browns before.
I read comments from a experienced guy that my values were fine, good oil flow. 0.063-0.076 mm seems to be much.
I got around 0.050mm / 0.0020".
Max clearance according to the Honda manual is 0.046mm /0.0018".

Green shells in which corresponds well with the crank and stamped cases. They green shells have max 6000kms. Old crank is slightly slimmer.
"Case has letters BBBBB, all crank journals have same sizes, 35.980 - 35.985mm"

Measure your journals and you will have more facts. The plastigauge used might be of wrong type?  It is made for current clearance area?

[captaink]

Thanks for this info, Ill check the crank with mic. but this crank and the green shells is original , never been stripped before. that's why im assuming there correct.
Ill check it all out .
Cheers

[captaink]

Question,
Does the type ( colour ) of the plastigauge make a difference ?
Im using red 0.001-0.007    /    0.025mm - 0.175mm  type

?

[PeWe]

Same type as I used
http://forums.sohc4.net/index.php?action=dlattach;topic=155014.0;attach=317281;image
There are 2 brands, Plastigage and Plastigauge. Plastigage for the crank clearance is green
http://www.3sx.com/store/catalog/plastigage-clevite-02-500.jpg

I bought 3 kinds of Plastigauge, all have same color and I got strange values when using the wrong one. Same scales but different thickness of the strings.

Green bearings are said to be the ones in most cases. I heard it back in the early 80's and read it on this forum.
crank journals sizes and crank letters are good to know to verify the plastigauge result with the Honda chart to be sure about the clearance area.

My last experience with checking the clearance was that the Honda chart is correct. Green since crank was in the A area, B stamped on the case. Plastigauge showed that brown should be fine but showed scuff marks after 1000 km that my old greens did not get after 6000 km or original crank with slimmer journals got after 40.000 km with brown.

Your crank and case might both be in the C area and need black bearings?
You need to meaure crank and check letters on case.

[captaink]

Cheers again,
Ill borrow a mic.
I know ill get this sorted in the end
Ill keep posted, thanks for your time mate

[captaink]

More info.
Crank Journals for the mains are 35.990 with one at 35.989
cases are marked BBBBB
crank is either AAAAB or AAAAA ( hard to see the engraving )
either way this should add up to Green shells throughout I believe.
The crank journals appear to be over the standard though.
The journals are in really very good order

Would this appear to be a worn crank ? and bearing in mind the 0.063 oil clearance with green shells what would anyone suggest ?

Cheers Keith

[captaink]

Spoken to the engine builder.
He suggested I get just 2 Black shells and check the clearance again with Plastigauge.
With race engines that he builds he often runs the oil clearances at the top end but of course there engines that run at high revs.
If the acceptable clearances cant be achieved he can have the crank ground and metal sprayed to the exact sizes and this would not be expensive although may take 5 weeks due to work load. Typically a 500 four recently done was £ 120 .
I was informed the 750 cranks cant be built up but this appears not to be the case. Technology has moved on.

2 Black shells to be ordered tomorrow, lets see what it shows !