Why does the big end of a connecting rod need to be polish smooth?

[ZenkaiRacerDsM]

I spun a rod bearing my Honda today. I opened it up and the bearing was still there. So the crank was untouched and most of the rod big end was untouched as well. Except for a small portion of the rod was raised up. Don't know if it was assembled like that or what. But I ground it down and sanded it smooth and threw a new bearing in it.

It is no longer polish smooth in that area. I was wondering why it needs to be polish smooth in the first place? Since it's just a seat for the bearing and not the actual area where the journal is spinning.

I know it's a gamble and this bearing might spin again. But I think I got pretty lucky with the way it looked compared to every other spun bearing I've seen where the rod big end has made contact with the crank journal. Anyways I have another good crank and set of rods if I need them.

 

[BogusSVO]

The big end supports the rod bearing, if it is not about perfectly round oil clearances will be off.

Also when you spin a rod bearing, or suffer rod knock, you damage the big end of the rod, even if you do not see it with the eye.

Most rods have a high/low spec of .0007

A 4g63 housing bore spec is 1.8905 to 1.8897 IIRC

So you stand a very good chance of having to rip the engine apart again and replace that rod

 

[Archer Fabrications]

Ground it down? As in with what? Guarantee it is out of round now, especially after you did grinding, and if there was something sticking out? What ever that means.Most likely means when the bearing spun the bore became out of round at that time.

I agree with bogus this will most likely spin again. Good luck with the honda..
I'd get a new rod to be safe.

 

[JacksonAuto]

The housing bore must be smooth to maximize contact area between it and the back of the bearing. This allows the heat to dissipate out of the bearing more efficiently. If it's too coarse, heat will build up in the bearing and shorten its life. Honing or resizing the big end to the proper size is crucial as it dictates how much "crush fit" the bearing has in the housing bore. Too much or too little crush fit will negatively impact not only the bearing clearance but also the concentricity of the bearing which is what controls how the oil flows between the journal and the bearing.

 

[ZenkaiRacerDsM]

Thanks for the info guys. I'll be cringing every time I drive it.

After how many miles do you think it can be deemed good to go? Say it makes it 2k miles, is it safe to say the bearing replacement worked?