Bearing size

[WattTheHeli]

I recently replaced the rod bearings in my 76 280Z with stock bearings. I would have done the mains but there were none to be found. An online mechanic friend of mine said I should have used over sized bearings because the engine is nearing 200K. Frankly I am confused about the difference between over and under sized bearings but what I have read said these bearings should only be used after the crank has been turned.

What are your suggestions for me?

Chris

[olzed]

Oversize bearing shells can only be fitted if the crank has been ground to that particular size. Your mechanic friend only told you half the story.

200k is not too much for these engines providing she has had regular oil and filter changes.

Is there signs of damage to the crank.??

The only way to tell if the crank needs grinding is if it measures oval with a micrometer, or if there is some damage to one of the bearing surfaces.

To do this the engine really needs to be out of the car, and the crank removed.

If your oil pressure is OK don't worry. It will be fine.

[Phred]

Chris

Your online mechanic should stay online and away from engines. Replacement bearings are available for both rod and mains in sizes that will match a ground crank. Cranks are ground when the journals are worn beyond the factory specified size range. Datsun cranks are made from very good material and hardend at the factory. Even if your engine has 200K on it, it may still be good if it has been treated to nice clean oil all its life. there is only one way to determin if it needs ground. That is to have the crank in hand and measure it with a micrometer. Not something your average home mechanic has in his box. Thats what engine shops are for. Oversized bearings are sized specifically for cranks that have been ground undersize to a standard undersize. Undersized bearings are commonly offered in .010 ( or .25 in the metric world ) or .020/.5mm. Some companys offer bearings for .010, .020, .030, or .040 undersized cranks. It is common for rod bearings to need replacement befor the mains need replacement. This is because the rod bearings are the last in line to see oil pressure from the oil system. It is sometims possible to tell if there are worn main journals (or bearings) by watching oil pressure closely. As oil heats up, it looses viscosity as the oil thins, oil pressure will then drop excesivly as it bleeds away between the main journal and bearings. When this happens, less oil is available to be transfered through the crank oil passages to the rod bearings, sometimes leading to damaged rod bearings. This can sometimes fool people into believing their rod bearing were at fault, when it was actually the extra clearance in the main bearing which caused the failure. As you can see, there is no easy answer. Only diligent inspection by an engine machinist can determin the actual bearing clearance, and weather it is within factory specs.

Phred

[WattTheHeli]

I was thinking along the line that stock size would be best. Thanks for the info.

Chris

[grannyknot]

Chris, you can just throw in a set of new stock bearings in but that won't really solve your problem, at least not in the long run.

Bearing journals on your crank and snap in bearings should be PERFECTLY matched. Listen to what the gents above are telling you, your crank needs to be measured and possibly re-ground before purchasing new bearings. Short term the new bearings may fill the gap of what was worn away but any high spots on the crank will dig their way into the new bearing putting right back where you were.

Edited July 8, 2012 by grannyknot

[WattTheHeli]

Mine is a project car that is only driven on occasions. It is where I turn when the chores around the house are caught up, which is not often. I have just begun the body work which I am sure will take quite some time, but if it cleans up nicely I will then pull the motor and have it rebuilt at a quality shop as I have nowhere to rebuild one.

I promise to keep it off of the drag strip until I have a worthy engine in it grannyknot!

Thanks,

Chris