Topic: cb750 main bearing clearances

[D DIAMOND]

hello just stripping my cb750k2 engine the crank pin diameter is a bit near the serviceable limit  the crank measures av 1.4160the journals are y-g-y-g-y will it be ok to fit the same colours y-g-y-g-y or is there any alternative
thanks DD

[69cb750]

New bearings shells, check with plastic guage.

[D DIAMOND]

hello hot shot should i use plastic gauge on the old  shells or new shells
DD

[drumstyx]

You can use the plastigage on the old bearings to check clearance, assuming they're not scored and in otherwise fine condition. Make sure everything is clean of oil and give er a shot. I didn't need new shells, though I don't recall what my crank mic'd in at.

[PeWe]

Letters on case?
Letters on crank?
The latter can be tricky to see, need to look in a specific angle and illumination.
Written close the primary, cam chain cogs on crank, the weight.

[D DIAMOND]

thanks for your reply I know the coding for the mains and big ends i am waiting for my crankcases to come back from cleaning but i have been checking the big end shells with plastic gauge they are all green codes i have 0.050on the plastic gauge will new green shells take the clearance down by how much or what is the ideal clearance i can get away with DD 

[69cb750]

i have been checking the big end shells with plastic gauge they are all green codes i have 0.050on the plastic gauge will new green shells take the clearance down by how much or what is the ideal clearance i can get away with DD 

The factory built engines with .0008" to .0018" clearance
.050 mm = .0019" within service limit.
New shells may return to new engine clearance.

[D DIAMOND]

thanks for your reply hotshot  that is good  does the main bearings use the same limits thanks again DD

[D DIAMOND]

have got the crankcases cleaned i have put the plastic gauge in and torqued them up i get 0.065 on the plastic gauge chart when i order the new shells will i be ok use the same colour shells DD

[drumstyx]

have got the crankcases cleaned i have put the plastic gauge in and torqued them up i get 0.065 on the plastic gauge chart when i order the new shells will i be ok use the same colour shells DD

There are markings on the crankcase to tell you essentially what the bore size is. If I recall correctly, you can cross reference these markings with the measured dimensions of the crank with a chart in the service manual to tell you which bearings to use.

[69cb750]

.065 mm = .0025"
Your crankshaft has wear, buy two brown shells, buy two black shells, test with plastic gauge.

[PeWe]

Still not mentioned letters on case and crank.
Old shells might be wrong.

One old procedure was to always use green. One size fits all
When I bought an extra case, my    condition was BBBBB marked case.
My other cases on my bikes have that too and my extra crank will match that with green as I have.
I think Honda had really tight specifications.
I had that up in a thread when I checked my crank. 0.0020"
Better oil flow is one thing with more clearance.

Oil pressure is really fine.

http://forums.sohc4.net/index.php/topic,155014.msg1774464.html#msg1774464

Edit: That engine got green back in later. Oil pressure is fine at idle and hot engine.
I think this modified K6 engine prefer oil flow as better cooling.

[bryanj]

Way back when if you wanted to race you fitted yellow if you were willing to run the engine in properly you fitted black

[D DIAMOND]

hi did mention the colours on the first post  the crankshft left to right yellow green yellow green yellow DD

[PeWe]

Colors of the shells might not tell the truth as I wrote earlier, twice.
Letters on case and crank is the first step.
Measure crank with micrometer next to verify the markings on crank and eventual wear.

This will give you a start to use correct shells confirmed by the practical test with plastigage.

[Don R]

 I asked the guy that assembled my sandcast when it was new, he is still in the Honda business. He said, just buy green and forget it. Russ Collins reportedly bought the loosest ones for his race engines. Yellow?