Will a flex plate like this hold up to 500hp? Picture is for reference only. I had a 408 roller engine built from a 5.9 magnum and builder said he was including a flex plate that was balanced with the rotating assembly (all forged internally balanced) I hate to question a shop but it looks kinda flimsy to me. I assume this is what he normally uses for this combo and hasn't had any failures..I am a four speed guy and rarely mess with autos. Am I worrying too much?

Yes and no, both good answers depending on the rpm at which the horsepower is transmitted. At 10,000rpm. I am sure it would withstand the torque. But at 5,000rpm the torque necessary to generate 500hp has doubled.

Now to put into practice. The original load for this flexplate was max torque of a 5.9 Magnum engine, net of 345 lb-ft. Add the extra cubic inches of a 408 and using the same ratio we get 391 lb-ft. The factor of safety for this part has to be at least two, I'd use three if I was designing. We've increased the torque by a factor of 13.3%, well within the limits.
My conclusion is the flexplate will work, no problem.

Others will ask, if the crankshaft assembly is neutral balanced, what difference does it make if you substitute flexplates?
Answer: If it was indeed internally balanced neutral, it shouldn't matter a bit.

R.

Looks to be balanced somehow by those 1/4" holes at the top

I guess if the OP wants to change something he should ask the machine shop guy. If the metal is 0.125" and the holes are indeed 1/4" we're talking about .278 oz and with a radius of 4.5", we get an imbalance of 1.25 oz-in. The 5.9Mag's imbalance is something like 14.5 oz-in, and the 360 is a little more than 18.

I just found another picture of a stock neutral balance flexplate and it had the same kind of holes in the same place, just not as many.

I'm saying that is a neutral balanced flexplate and can be swapped for any other neutral balanced flywheel or flexplate.

R.

The one thing I'm hanging up on is "balanced with the rotating assembly". If the flex plate is balanced by itself, then dogdays is correct...any alone-balanced flex plate is OK with an internally balanced engine (they're both balanced neutrally).

However, if the engine's balance INCLUDES the balance on the flex plate it's a different story. Maybe I'm just hanging up on this since my big-rod motor is balanced WITH the flywheel & harmonic balancer, so I can't change flywheels without rebalancing. I had a shop check my flywheel balance & the machinist said it wobbled like crazy (not alone-balanced, so to speak...it's part of the engine balance).

Bottom line...dogdays sound knowledgeable in this area and it sounds like it would work fine. If it breaks (aka sounds like a rod knock), then it's not too tough to upgrade it later. I would suggest you ask your machinist if it is "alone-balanced" though just to be sure (aka neutral balanced separate from the engine).

When asked he said he was supplying the flexplate that they balance with the rotating assembly.I didnt ask whether he meant with or by itself. It does have a weight welded on. The one pictured is not mine just one like it. The style and thickness seems flimsy to me, but I'm used to dealing with flywheels. So I was curious if anyone had used a stock type on a 408 similar to mine. I'm having trouble getting hold of him after the email I sent complaining about the fact he didn't put the fuel pump eccentric on the cam bolt, after I reminded him multiple times.

Yes Dog is the man on this stuff.

If you are running it in a magnum factory EFI aplication you have to use a similar one for the tone wheel. However, that one in particular is for a V6 and will not signal the V8 computer properly anyhow but is built the same as the 318 plate otherwise and the 318 plate is the same as the 5.9 plate minus a big weight welded on one side.

I would not wory about it as far as strength, with all the extra metal around the perimiter it has to be a lot stronger than the old factory ones on 340s or even 440s and such.

Those balance holes look factory, I have seen tons of those flex plates and some have few or no holes and some have a bunch like that one, it is just to get it to a neutral balance. Stamped sheetmetal is not something that tends to come out balanced very well without some extra effort.

If you really feel the need to replace it with something else AND it really was really balanced with the rotating assy, a competant machine shop could see what the balance factor is on that plate and balance your new plate to the same spec. However I think the only out of balance they may find is a few grams that would have been within the factory "neutral balance spec-range of tolerance".