harmonic balancer attaching bolt : no problem to remove ??

Hi all ! I need to check ( or perhaps change ) my timing chain : So I need to remove the harmonic balancer attaching bolt : I think to use an impact wrench ( strong with 500 N-m), but i' m little afraid to do that : no risk to break the bolt (in place since 1959..) ??? Have you experience of that ( my engine is in the car ) .
Thanks . Phil.

Fire away with the impact, won't hurt a thing during the removal process.

Do NOT use an impact putting it back on though!

Thanks for your reply . I believe the bolt strong , and I don't know another method to remove it ..

There's an old method for removing and installing the balance bolt that works well, just takes a bit of time:

Remove one of the spark plugs that's within easy reach. Turn the engine by hand so that piston is at the bottom of the cylinder on the compression stroke (both intake and exhaust valves closed). Thru the spark plug hole, feed a length of nylon rope (it MUST be nylon!) into the cylinder until no more can be fed but let a bit stick out for easy retrieval. With the nylon rope inside the cylinder, it will be compressed by the piston preventing the engine from rotating when removing the crankshaft bolt. Use this method to reinstall the bolt but be sure use a torque wrench. I believe the proper torque spec is 160 foot pounds but verify for sure. I'll dig out my manuals and look it up the spec if you can't locate it. Hope this helps!

Oh yes , I know that method , and I tried it ( but perhaps not with nylon rope ...) But unfortunately the bolt is tighten so strong (160 lbft , like you say) : I can't unscrew it manuelly , even if I put an extension on my torque wrench ! I thought to heat the bolt with a blowtorch, but i preferred to buy an impact wrench with a regulation possible until the good couple for the tightening : actually I wait to receive the good sockets by Ebay ( I live in france , and here we can' t find impact socket SAE) . Now you know where I arrive : I DON' T want to break this bolt !And also my fear is that someone has already disassembled and reassembled with a threadlock !!

Some news : i got it look my timing chain , completely distended . Odometer give 89 351 ..
https://youtu.be/z2gfy3cmR-M?t=6

Congrats on getting the bolt to break free.

Before I disassembled my engine, I measured the slop in my timing chain out of curiosity and came up with 12 degrees. During disassembly, I could lift up the timing chain and freely move the cam gear either direction.

Mine is 18 degrees . In acceleration, I heard an annoying BRRRRRRRR. For a long time , I thought it was coming from the power pump steering ; Then after some research on you tube and forum , I began to think i need to check timing chain ; after new chain installed , I 'll tell you if my BRRRRRR left !!

new timing chain installed : approx 5 degrees

The job is finished , no problem .. but the BRRRRRRR is always when accelerating !