I'm a bit confused about the real pro's and con's of 350 mains compared to the standard 400 main in a 4.125 bore aftermarket block. Obviously one can use a "stroker 383" crank if one gets the 350 mains, and this may also be en economicly wise thing to do, but other aspects like streetability, wear and such is more cloudy.

I'm planning a street engine, with a few tries at the track, but it's supposed to be a daily driver. At the same time, I was hoping to be able to evolve the whole setup into a racer in time, trying to preserve mort of it's streetability. So I don't really want to give away to much either.

Any thought's about this subject?

 

It's simple a smaller size main has less friction and thus costs less power. But the smaller size also reduces the strength of the crank. So its a trade off.

 

That's about the size of it. It's a trade-off between a tiny amount of power, and reliability. Risk vs reward.

Use the larger main bearing, unless you plan on racing in a class where the prize money is so big that you can afford to tear the short block down after every pass, like the Top Fuel cars do.

 

But would the larger bearing limit the rpm in some ways? I was thinking something revving about 6000-6500 at the most, not anything wild.

 

At that RPM, the main bearing diameter isn't a limit. Not even close.

Use the larger one.

 

The 400s came from the factory with a larger main journal dia. than the 350s.
Before strokers became popular, guys would use factory 400 cranks in a 350 block, cause at first, there was no aftermarket demand/supply for a 3.75" stroke crank to fit 350s. The main journals on those 400ci/3.75"stroke cranks would have to be turned down to fit the smaller main dia 350 block.
Later, when there became a demand for 3.75" cranks and they went into production, they were offered to fit either main journal diameter.
Hence there ended up being a good number of 3.75" cranks on the market with the smaller dia 350 journal.
In addition to that, some builders were interested in building de-stroked 4.125" bore motors, and in that case there were no cranks with a shortened stroke to fit the large bore blocks besides the factory 350 journal.
All this created a demand for a spacer that could be used to fit a smaller dia journal crank into a lg journal block.

So what I'm getting at here, is that the real reason that there are 2 different journal sizes to begin with, is that which I just mentioned.
Given the choice it's a good idea to choose the crank that fits the blocks existing main journal dia.
But if you already have a crank with the smaller mains the spacers can be used to make it fit.

 

Ok, so the larger bearing is what's reccomended, got it.

Thank's guys.

 

Unless you are turning real high rpm. Think of the larger diameter bearing as having to run around a track. The larger the track the faster you have to run around it to get around it in the same amount of time. A bearing is the same way. More speed=more friction. Larger bearings are good for low to midrange torque monsters.

 

Get the crank with larger bearings. At the speeds the crank will see, the extra friction will not be an issue, but the larger bearings will be stronger, which is always good.

 

Great point. This is where I was going with the comment I had.

The large journal is 2.65"
The small journal is 2.45"

The circumference of the large journal is 8.325"
The circumference of the small journal is 7.697"

A difference of .628"

Multiplied by 6,500 RPM equals 4,082" MORE (340.16 feet)

Multiply that by 5 main caps, and that's just over 1,700 feet more of bearing surface the oil has to lube and cool in one minute of time.

Can you guess which bearing will run hotter, cause the oil to thin out and break down faster, and cause the bearing to wear quicker? The larger bearing.

I suppose if you're not running at 6,500 RPM for one minute, the difference is moot.

 

I would also think that if you had a main bearing fail and spin you could fix the smaller journal buy having it enlarged (line bored) to the larger journal size.
Just a thought.