I took my motor to the machine shop and the guy there is saying i will need 30-over bearings for rod and main.

Turns out the engine's been rebuilt before and the rod is currently 10+ and the main 20+.

Asked the guy why go 30+ on the rod and not 20+ since it is now 10+ and his reason was so that all the bearing sizes could be the same and that it is better than going 30+ on the mains and 20+ on the rods...might as well just make them all 30+. Does this make sense??

Also...another machine shop is saying that the bearing size effects performance....the bigger they are, the less power the car makes. Again, does this make sense or is it complete BS??

 

Find another shop, and take a uncut, [stock] crank w/ you.
As for the Hp difference.. That guy has been reading too many Nascar magazines.

 

RUN away from both shops.

Far as bearing size, in racing sometimes yes they will run smaller bearings BUT not a thick spacer crank undersize bearing but rather a combination of crank with smaller journals and connecting rods with smaller bigends. Lighter is a big part of why that is done and in no way accomplished by using a crank cut undersize with thick bearings. There is some element of reducing rotation speed at the bearing surface but that is well outside the scope of this discussion.

Again RUN from both shops.

 

Aha...OK, could you plz shed light on where the flaw is with what the first shop is saying?....and what should be done?

 

He may have meant the performance of the crank. Cutting that much into the journals takes away the surface hardening treatment of the crankshaft. It's probably not going to make the crank junk or anything, but a stock crank is cheap to replace.

 

Realistically you can probably find a used stock crank without an undersize and have it polished for what it would cost to have yours blindly and excessively turned undersize.

Hell that begs the question why does he want to turn it down again? Are the journals damaged? A polish is usually cheaper and all that is needed unless there is damage. If so are you looking at ending up with rods resized twice???

I wouldn't use a shop that wants to take extra off the journals unnecessarily. Why cut .020 off if .010 would clean it up? Might not hurt anything but why do it, why cut so deeply or possibly through the heat treatment?

Maybe you should be just looking for a decent used motor, I wouldn't want to deal with a cheaply re-re-built motor.

 

Quite the opinions here. All you men are correct in one aspect or another. Not tooting my horn, but I did have 4 years in a Cup engine shop. Yes, smaller diameter does increase performance, but for the street it is not going to make a difference you will notice. And yes, a Cup block with large mains will use spacers to enable use of a smaller main crank. The block and crank don't know what size the other is, just that they are working together. Now as far as rod bearing sizes, the Honda journal is so prevalent and affordable, this would be feasible for the street guy, but small mains don't mean much. Remember with the Honda sized rods: smaller big end equals less weight spinning. Which improves rpm capability and power. Just my humble opinion.
2Barrel

 

I guess I could have separated main journal discussion from rod journal, but what was being suggested was not worth that much discussion. Then as you understand running a Honda journal rod is one thing, running a stock crank and rod with .030 under bearings is nowhere near the same thing, you haven't taken any weight out of the equation, well maybe a miniscule amount if the bearing material is lighter than the crank material it is replacing but nothing meaningful particularly on the street.

Far as thick spacers used in Cup, wonder how that works for 50K miles instead of just 500hard ones? Not saying it wont, just saying people compare racing to the street a bit too much. In another section just yesterday someone said 4 quarts 0w-10 and no windage tray was quickest in the 1/4 mile. Doesn't mean I will even consider running a street car like that even though I do believe he is correct.

 

Yes the difference between race and street is often brought into question, but don't forget that the OEM engineers take about 80% of their forward directions come from the advancements in motorsports. Such as: smaller journals, dry sump systems, coatings are the big ones, etc.
and yes, the spacers for rod journals are not practical, just added weight in a different place makes no improvement. And the block spacers can be a long term fix depending on application and installation quality. A Cup engines cylinder pressure and crank load are a lot higher than most street driven cars will ever see, unless it is forced indiction. As far as our opinions go on power-making differences, don't forget: There is always more than one way to skin a cat! And we all have our own ideas and budgets. I'm not one to call someone an idiot over their ideas, and I look forward to learning from you guys on here.
2Barrel