A guy selling a 340 short block states that the motor was running when he received it. He had all the machine work done prior to reassembly. When I asked if the block was magnafluxed, he said no, it was a running engine no need. Risky move/purchase?

It's not a bad idea to have it done, but rarely gets done. A good looking at is usually done here.

Unless you personally know the running history of the block, it's not worth the chance, it's $100-$150 well spent before investing in a boat anchor

Definitely magnaflux any 340. I've taken a part several that ran just fine only to find cracks upon teardown, most were in the lifter galley. They can crack from over heating,freezing, or just from too much stress/HP if raced. unfortunately it's pretty common.

I magnaflux my blocks and iron heads on a refresh, and yes they were running and not hurt. I'd like to know before I put new dollers in an old block.
On the flipside, I know guys that have never paid to have done and have been racing for twenty five years with no problems.

Vinny, Paul here with the '71 Demon. As you know my engine is getting re-done right now. It was kinda running when he got it and he magnafluxes all his rebuilds. He found a inch long crack in one of the valley's. My2C's

A prudent mind tries to avoid doing things twice...

Maybe you could offer him a partial payment pending passing a magnaflux?

Thank you for the feedback! It was strange that the guy had all the necessary machine work completed, but decided against having it magnafluxed.

I haven't had a 340, but how common is it? I have had several 318 and 360 engines and never magnafluxed the blocks?

I have had engines rebuilt over the years and never had it done. Just a visual inspection. I kept my fingers crossed and so far I never have had a problem, but there is always the first one.
I have a couple of 340 T/A blocks that I might be selling in the future and I'm thinking to get a top price on them I would have to have them magnafluxed first.
Just remember that the the inspection is only as good as the person's experience is who is doing it.

Seems crazy to gamble with it. Why risk all the rebuild money and time for $150? I read another similar question on another forum this week. "I'm rebuilding my engine. Should I reuse my rod bolts?" I built my car on the cheap for sure, but I absolutely don't get this line of thinking. The last thing I want to do after finishing my resto is replace an engine that I just built. Hanging over those shiny fenders and keeping things nice is a PAIN.

Pretty common not to have it done. I've even talked to some guys with hot rods way nicer than mine who didn't even know what magnafluxing is.

Did those guys build their engines or pay someone else to?