Bearing Shells Question

[gp_st3]

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...

[Trigger]

To measure the journals correctly you will need some fancy measuring equipment. The last bit of kit i bought was around the 400 pound mark, a little expensive for a one off job 
What colour plasti are you using ? Green is correct.
Run your finger nail across the journals to feel for any flaws. Should be smooth with no imperfections.

[Bryanj]

Not unusal to find nill wear if the engine has been serviced correctly, personally stripped a 100,000 mile motor with no discernable wear. That is a conservative mileage estimate as it was originally the bosses bike and was one that had a rebuild by Skellerns of Worcester due to being one of the batch with bad oil hoses. We then knew every owner and every accident with new speedo till high mileage when it needed new chains

[gp_st3]

I'm using the the Red 'UK' Plastigauge (0.025mm to 0.175mm) as unable to locate a uk source for the the Green 'US' Plastigage (0.25 to 0.76) but have now ordered some White PL-X (0.018 to 0.045) so will see what results that gives.

The rod shells' surfaces are all pretty smooth, although one does have a narrow polished ring while the rest have an even matt colour - for the main bearing shells it's a different story with those having a distinctly rough 'sand paper' feel on the forward face between 2 and 5 o'clock so to speak...

Given the oil staining on the inside of the cases I would not be confident that this engine has been very well looked after in the past but if the clearance measures with the new plastigauge are the same then near-zero wear would seem to be the only explanation.

[Trigger]

Green is available from WWW.plastigauge.co.uk  01903 882822 

If you have uneven ware on shells i would also check how true the con rods are.
It does sound like there as been contaminants in the oil but, this is down to how much ware the contaminants has effected the engine. A good sign is the state of the oil pump.
These engines run on a high pressure film of oil and the surfaces should not come in contact with one another. 

I had a engine in a couple of months ago that had uneven ware to the middle main shells. The first thing that run through my head was a bent crankshaft but, on measuring up the shaft it was true. After some time of head scratching i could only conclude that one of the crank case bolts had been over torqued by the last person that built the engine.

There is a very experienced member in Wimbledon that may be able to cast a expert eye over your crank and sells. Try contacting K2 K6 (aka Nigel ) 

[K2-K6]

If you'd like another pair of eyes to look over it with you,  then pm me your phone number to arrange.

Nigel.