what is a safe redline for a stock (rebuilt) bottom end 350? my 350 was definietly rebuilt by the previous owner, without a question. however, i have no idea as to what the extent of the rebuild was. so what would be a safe redline/power band when choosing a new head/cam combo?

 

That's a tricky question. The problem is the rebuild. If the original pistons and rods were put back in their correct location and the rod nuts torqued correctly, it'll spin to 6k with no problems (original factory balance preserved). The problem is you can't tell if it was re-balanced if the pistons were replaced.
If the pistons were changed when it was rebuilt and it wasn't balanced, then it's a crapshoot how far out of balance it is.
Some unbalanced setups won't go past 5500 without making the gauges blur due to vibration.
My 350 has a stock cast crank, 2 bolt mains and stock rods. I balanced it since I changed the pistons (forged). I've spun it to 6700 before and hit 6500 regularly when I run it "*****-out".

So in a nutshell:
My 'stock' bottom end will run 6500-6700. (because it is balanced)
An original untouched 'stock' bottom end will run 5500-6000. (factory balance)
A 'stock' bottom end that someone has bored and stuck pistons in, may not be safe above 5000. (not balanced)
Are ya confused yet?

 

no im not confused... lol. that is about what i thought. i know this is kind of a hard question due to my lack of knowledge about this engine, but thanks for the response. any other opinions are welcome!

 

I spin my poor ol' wore the frack out 305 to 6k when I'm racing it. I've actually had the peg leg break loose hitting second gear and it spun up to 6500 before I could get out of it

 

I could be totally wrong here, and way off base, but I find it EXTREMELY difficult to believe that the factory balanced any of the rotating components, except for maybe the crankshaft - and only then balanced it (the crank) to itself. Have you ever weighed a set of stock OEM rods that all came out of the same engine? You can tell with your naked eye that there is considerable discrepancies.

In my mind the weak link in a stock rotating assembly, when it comes to OEM parts, is the rod bolts. I personally wouldn't go over 5500 on a regular basis.

 

The way the factory "balanced" motors, was they weighed rods as they came off the production line, and put them in an "n-dimensional array" of bins, sorted by total weight, each end weight, length, and so on; and when a bin filled up, slapped them together with a crank set up for that bob weight. What they "look like" is deceptive: they're usually within a couple of grams, easily, even though they "look" mostly random.

They DID NOT do what we out here in hot-rod land would do; and take 8 random rods and 8 random pistons and a random crank, and hand-match them from the ground up. But they did sort of balance them, just the same.

Their tolerances are MUCH wider than ours, since their end goal (survive the warranty period) is MUCH different from ours. On the other hand, wneh was the last time you got into a brand-new car, and the motor had an out-of-balance vibe? So maybe it DOES work, after all.

I agree 100% that the weakest part of a stock bottom end is the rod bolts. With OE ones, I would not run a 350 over 6000 RPM, and THAT only on rare occasions; not an every-weekend track toy. With good rod bolts though, the rest of the parts are good to 6000 reliably and 6500 or more on rare occasions, until the crank breaks, which will usually take quite a while. The stock cast crank is the next weak link: it will break at the front somewhere, usually in front of the #1&2 rod journal. You can help that out DRAMATICALLY with a good crank damper, specifically an ATI (not a cheeep CAT or similar "stock replacement" whether SFI-cert or not); but that's kind of dumb really, unless class rules or something specifically call for a stock crank, using a $300 damper to protect a $75 crank. Kind of like using $15 transistors to protect $0.50 fuses.

 

The stock rods will look very wierd. Some with tall balance pads on either the big end or small end, and some with short.
The factory balance is actually better than you would think. It's a "match" balance based on deviation from a standard. A piston that is -8 is matched with a rod with a small end weight of +8 would be the same as a piston and rod that are both at "0" deviation. With enough pistons and rods to match from, both reciprocating weight and rotating weight assemblies can be made to match the crank requirements. Again, each crank throw will get an assembly that will balance it.
This is why I made such a big deal out of changing pistons turning the factory balance into a mess. Let's say that the original pistons were:
-5, +12, +4, -8, +2, -2, -6, +1 .
You put in a "perfect" set of new equal weight pistons that are all "0" deviation.
Can you see how the engine would now "have to" be out of balance?