Crank bearing size question

[Gabby]

Hi. I am replacing the crank main bearings in a K7. They aren't badly worn but while it's apart I figure it can't hurt. I have not measured anything so was just going to replace with original sizes. Is that a reasonable thing to do?

Can someone also confirm if I have interpreted the manual correctly please? The case markings are all B. The crank markings are all A. So it looks like the original shells must be Green according to the table - correct?
Thankyou

 

[Gabby]

Hi again. Is anybody able to confirm I have these crank journal sizes right please?

 

[dirtdigger]

Yes your correct with the size according to the markings...but, the only way to know for sure what size the journals are is to measure them with a micrometer. More than likely they are worn some, you need to either mic the journals and/or use plastigage with new bearings to see what the clearance is. Odds are you will be close with new green bearings but if the journals have some wear then you could end up with too big of a clearance. I would plastigage with your current bearings and see what the clearances are...if they are in spec then there is no reason to replace the bearings. Many times I fine tune bearing clearances to my liking by using a new bearing and a slightly used bearing or combinations of different bearings until I get them perfect. If the bearing isn't trashed then they are most definitely still usable.

 

[Gabby]

Thanks for your reply Dirtdigger. I'll try to get a decent micrometer and/or plastigauge then, just to be sure. Cheers for that.

 

[Gabby]

Back again. I measured the clearances with plastigauge and they are fine. The crank is also showing no wear but since a couple of shells had some marking I have obtained a complete set of new shells the same size as original.

Just a question on installation: the manual states that the top of the bearing should extend above the case mounting flange by ...0.068-0.098mm. Obviously the bearing in the other half has to extend by the same amount on the other join. Is this simply so that the bearings do not meet at the flange? If I install a bearing in the top case so that the oil holes align, one end of the bearing does extend above the flange (see attached photo) but by a fair bit more than the manual states.


Should I just install the bearings firstly in the top case with the oil hole aligned in this way and with a similar amount extended on the other half before bolting it together>
Thanks, G

 

[dirtdigger]

No the purpose of bearing extending above the joint is for bearing crush. The bearings are ever so slightly bigger then what the inside diameter of bore in the cases is. This causes the bearing shells to be tight in the cases when the cases are bolted together. If you didn't have crush the bearings could spin in the cases...not good. There is nothing special you need to do when installing the bearings other then making sure there is the little bit extending up like you are describing. Doesn't matter which half goes in first just put them in, you are thinking to deeply about it but if you have new bearings there is no reason for you to worry about it.

 

[Gabby]

Good one, thanks dirtdigger. Yes, thinking too deeply can be a problem sometimes. Cheers