Main / Rod bearing clearancing

[nuggets]

I'm rebuilding my 6bolt. I got the crankshaft micropolished, block honed, new bearings, and piston rings. I purchased Topline standard bearings for both mains and rods. After checking the clearances with plastigauge, the mains were at .063mm and the rods were at .050mm. As you can see, these are not exactly ideal clearances.

Now my question is, must I order new bearings and if so what size? Or, should I take a couple thousandths off the end caps to achieve the desired clearance?

Also, crankshaft has not been cut down, just to clear that up.




Thanks!

 

[TimG]

Talk to your local machine shop. They will be able to order new bearings and tell you what you should do. I'm not sure what size bearings can be sold as but I don't think anything will match the crank you have right now exactly.

 

[nuggets]

so I can't just go with an undersize bearing if my clearances are a little off from where i want them?

 

[BISHILVR]

No, because undersize bearings will then be way to TIGHT. You are at an inbetween size that does'nt exist. Try another brand of bearing.

 

[nuggets]

okay thanks!

 

[Elvenhome21]

actually you can use one half of a standard bearing normally put in the rod and the undersized bearing half in the cap. it will work because the edges of the bearings are undercut already. Circle track engine builders do this a lot to keep clearances tight as possible instead of constantly regrinding the cranks
as for the mains im not sure but you could probably leave them looser because it doesn't have the slap problem like the rods

 

[VETDRMS]

Plastigauge and actual clearances can vary a fair bit. Do you have a way to accurately measure the clearances? You machine shop should have a fixture to measure the bore diameter, and then you can measure the shaft with a micrometer to find the exact clearance. That is how I have clearanced and built my V8 motors.

I have seen an article that compares plastigauge reading with actual reading and there was a fair amount of variance, your really only looking for something way out of whack with plastigauge, IMHO.

:cheers: