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CB750 / Bearing Shells Question
« on: September 01, 2018, 01:40:51 PM »
I have my CB750K4 engine apart, am going through the process of checking the bearing shells and could do with some advice.
First, I Plastigauged the four existing rod bearings and got between 0.05mm and 0.055mm clearance
Could not find any evidence of shell colour so working on the crank numbers (‘3333’) and rod numbers (all ‘1’), concluded that the standard shell would be Yellow
At this point, I also measured the crank pins with a micrometer and while finding it hard to get consistent readings due my lack of skill, these came out at around 35.99mm which matches the ‘3’s scratched on the crank
So, reading in the Mark Paris book that a reasonable limit for worn bearings is around 0.056mm (although Honda states 0.08mm as the max.) and with my assumption that wear is mainly on the shell rather than the crank pin I ordered a single pair of Yellow shells from DSS (Honda OEM) to check things out…
Now the confusing part - with the new Yellow shells I get exactly the same values as before i.e. 0.05mm, which is not what I expected!
It appears that the old shells are effectively not worn at all, which is odd given that I’ve done 10k miles on the bike and there is no evidence in the paperwork (or the state of the inside) that the shells were previously replaced. As an aside, the clocked mileage is not relevant as by the numbers it’s ’74 engine in a ’72 chassis.
The follow-on conclusion is that any wear has been on the crank pins and (presumably) the original oil clearance was at the top end of of the standard 0.02mm to 0.046mm in the Honda manual.
The question now is what to do now - leave things as they are or replace with one step thicker shells?
Your thoughts please...
First, I Plastigauged the four existing rod bearings and got between 0.05mm and 0.055mm clearance
Could not find any evidence of shell colour so working on the crank numbers (‘3333’) and rod numbers (all ‘1’), concluded that the standard shell would be Yellow
At this point, I also measured the crank pins with a micrometer and while finding it hard to get consistent readings due my lack of skill, these came out at around 35.99mm which matches the ‘3’s scratched on the crank
So, reading in the Mark Paris book that a reasonable limit for worn bearings is around 0.056mm (although Honda states 0.08mm as the max.) and with my assumption that wear is mainly on the shell rather than the crank pin I ordered a single pair of Yellow shells from DSS (Honda OEM) to check things out…
Now the confusing part - with the new Yellow shells I get exactly the same values as before i.e. 0.05mm, which is not what I expected!
It appears that the old shells are effectively not worn at all, which is odd given that I’ve done 10k miles on the bike and there is no evidence in the paperwork (or the state of the inside) that the shells were previously replaced. As an aside, the clocked mileage is not relevant as by the numbers it’s ’74 engine in a ’72 chassis.
The follow-on conclusion is that any wear has been on the crank pins and (presumably) the original oil clearance was at the top end of of the standard 0.02mm to 0.046mm in the Honda manual.
The question now is what to do now - leave things as they are or replace with one step thicker shells?
Your thoughts please...