A machinist told me that a 350 crank doesn't have to have the bobweight reduced (from 1870 to 1760) to work in a 305. That means that the crank would be overweight by 110 grams (a 51.5:48.5 balance). I know that some people like to balance very high revving motors 52:48, but wouldn't a 305 with 110g overweight bobweights accelerate more slowly?

 

The engine will have a bad vibration.

Although the 350 crank will fit in a 305 block, you would have to get everything rebalanced (if possible) with whatever rods and pistons you decide to use. My 2 new replacement con rods are 25gm too heavy on the small end. This is too heavy and there's no way to reduce the weight and keep everyhting safe. 2 new rods with proper small and big end weights were ordered.

110gm is excessive. The crankshaft weights need to match the bob weight of the rods, piston, pins, rings, rod bearings.

 

He also just asked me if I was boring out for a 350.

 

Might be time to find a new machinist!

 

I was at my machine shop today talking to the guy I deal with. We were talking about head sizes and how mine are small for the size of the engine. Another machinist came in and said I should to go to a 5" bore. We laughed and said I'm already at 4.5" and my block can be easily taken to 4.625" safely.

 

A 5" bore would be a great trick on a block with a 4.84" bore spacing.

 

Why not just bore it sideways?

 

It's too early to be asking questions like that.

What do you mean, "sideways"?

 

Don't the 305s already have the 350 crank in them?

 

305's have a 3.48" stroke just like the 350's, but are balanced differently for the lighter pistons, etc.

with that, and all above said, when rebuilding my current 305, i slapped in my eagle cast crank from my 350 w/o getting it rebalanced for the smaller 305 pistons. we put it in cause the 305 crank rod journals had surface rust on them. i tried to save them with an emory cloth and managed to get the rust off, but it had some minor pits in it. i decided to take my chances with the 350 crank w/o rebalancing (had NO time and NO money) and if it flew apart after 4 weeks, then it did what i needed it to do, which was get me to work and back for the last 2 weeks that i had to work there. slapped it all together, even reusing the main and rod bearings from the 350 cause they are King bearings and are in really good shape still, just made sure they went back in the exact same spot. fired it up and even with the big camshaft and solid motor mounts, the vibrations are marginal at best. since i've had solid motor mounts in the car for the last 3 years or so, i really don't know any better as what it should feel like...oh, and i spin the motor to ~6000-6500rpms...

however, i'll be having the whole rotating assembly rebalanced when i do the piston swap...it's a bit more critical for things to be right when your shoving 15lbs of boost at it.

 

The stroke is the same, but 350 cranks have more mass on the counterweights to balance the larger pistons.

 

i am amused by this here thread

rofl

 

why is it amusing?

 

mm66nova,

It would be very convenient when you get your new pistons if they weighed within a few grams of your old 350 pistons ...might save you the cost of a balance. If you're getting heavier, forged 305 pistons they might be very close to the 350 pistons (if they were the lighter hypereutectic type). A few shops have told me that their "performance" balance is 51/49. If your 350 crank has a 1870g bobweight and you're using balanced, stock weight rods, you'd need 305 pistons around 510g to get that 51/49.

I know that some shops balance motors that are intended for extended high rpm use at 52% / 48%, but in that case they don't care about vibrating. I'd say your 350 crank in the 305 is somewhere around that because the 350 crank is automatically "overbalanced" for a 305. I can see that being ideal for oval track, or some other application where the rpm is constantly high, but for "stop and go" type driving I'm wondering if the extra weight on the crank would affect how quickly the car picks up its revs.

It's only an extra 100 grams or so on the crank's bobweights, but can you notice it when accelerating? Do you think the car picks up it's revs as quickly as a "normally" balanced 305?

Maybe you should grind around the top of the bore with a die grinder, that way you'll relieve valve shrouding, and if you grind away 1/8" all the way around you'll have a 350 at the top, but still a 305 at the bottom. That's good because the 305 needs more top-end power.

Stephen 87 IROC, you should get some 5" rings but re-use your old pistons, that way you won't need to re-bore because you still have the same size pistons.

 

i don't think that grinding around the tops of the bores is necessary. the stocker guys CAN'T do it(rule limitations) and are spinning their little 305's to 8400rpms! i also believe that putting a 1/8" chamfer on the top of the bores will create some serious hotspots and while it'll lower static c/r, you could run into detonation with the turbulence created when the piston is at TDC.

as far as stop and go, remember Casey, this is my only car currently (though i'm building a new DD) and quite honestly, i've not noticed anything down low as far as vibrations. and it wastes NO time getting up to speed. with the regular street tires on, i just completely blow them away till about 50mph. i can drop it into first at 25mph and destroy the tires...i couldn't do that with the first 305 that i built with the 305 crank in it, but i've got better heads, better cam, better roller rockers, and more c/r.

something else you guys have GOT to remember about my combo. i have only about 250miles on the fresh build and only 2 passes. i have not messed around with shift points or anything yet. i haven't done any plug reading so i don't know which way to go with the holley that i put back to stock settings just to see what it would do. and the timing is set based on what this camshaft liked in the bigger engine. i have no idea what it'll do once i get all that stuff taken care of. i'm out of town and haven't even seen my car since the beginning of June.