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Old 06-16-2010, 12:02 PM   #1
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Crankshaft Bolt? Help Please.

I have a early 70's 350 2bolt main with the orig. crankshaft. I was installing my harmonic balancer the other day. My buddy told me to just get the balancer on the crank "snug" & then tighten it the rest of the way with the balancer bolt & an impact. So i did; I also stripped the thread out of my crankshaft! My bolt has serious play in it when screwed all the way in. Anybody ever experienced this(& i def. have the correct bolt for the crank)? I was pretty sure stripping out the threads on your crank was impossible. Anyone know what size to drill & tap this thing out to?
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Last edited by oguin_studking; 06-16-2010 at 12:04 PM. Reason: forgot something...duh
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Old 06-16-2010, 01:08 PM   #2
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Re: Crankshaft Bolt? Help Please.

have no idea what size the bolt is but did you have the pully on before using the impact on the bolt.
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Old 06-16-2010, 01:32 PM   #3
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Re: Crankshaft Bolt? Help Please.

The size should be 7/16-20. I would get a helicoil kit for that size and get a new damper bolt. I dont chance installing them like that anymore. The damper install tool was about $40 but you can probably rent one from Autozone.
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Old 06-16-2010, 01:42 PM   #4
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Re: Crankshaft Bolt? Help Please.

It is possible to strip out a crankshaft bolt by trying to install it with the bolt or turning over the motor with a large wrench on the bolt. That's why they make tools for installing the harmonic balancer. By using a bolt to pull the balancer on like that you're putting all the force on the threads that are engaged not all the threads available. A balancer install tool screws in first all the way then you tighten the nut and washer on the tool itself to press the balancer in place.

When I put my last motor together I used an install tool and a puller a few times never had an issue.

Hopefully you only ruined the outside threads a longer aftermarket bolt may have more thread engagement and fix the endplay issue.

As for the bolt size try this http://www.summitracing.com/parts/ARP-134-2501/

As for tapping it out try something like this http://www.thirdgen.org/techboard/te...er-center.html
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Old 06-16-2010, 06:20 PM   #5
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Re: Crankshaft Bolt? Help Please.

Already have completely installed the balancer. I have bought several different length 7/16-20 bolts the correct size for the crank journal. The problem is whoever put my motor together re-did the crank and had it balanced. It is an older crank so im guessing the threads on the journal are wore out. The crank is hardened steel & i know u cant just use any type of bit. so i just checked that "Replacing Harmonic Balancer....centerbolt broken off inside crank" thread & i think i found my ans.

In your case, you can rethread the crank snout. The OEM bolt is a 7/16-20. You can tap the blind hole in the snout with a 1/2-20 tap (standard size) without redrilling. Use a bottoming tap instead of a plug tap, and lubricate with kerosene or specialty tap fluid. Be careful to pick up the same thread as the original, and back off the tap frequently to clear the chips.

Install a 1/2-20 grade 8 bolt with your original washer, and torque to 65 ft/lb only (on a cast crank). You'll likely have to drill or ream up the washer slightly to accomodate the larger bolt, but it isn't that difficult.

There was a reason GM picked a 7/16-20 bolt thread for this hole, and you just found it. Just use an installer to pull the balancer into position, or you won't have many options for crank repair next time.
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Old 06-16-2010, 06:22 PM   #6
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Re: Crankshaft Bolt? Help Please.

But do I have to drill it out & then tap it, or just tap it out with that 1/2-20 tap?
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Old 06-16-2010, 07:25 PM   #7
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Re: Crankshaft Bolt? Help Please.

[quote=oguin_studking;4581093]But do I have to drill it out & then tap it, or just tap it out with that 1/2-20 tap?[/QUOTE

I'd first try a thread chaser since they remove the least amount of thread which should increase the chance of the new bolt holding the torque.

If the chase doesn't work I'd then try a tap.

If neither works I'd UP the bolt size to a 1/2".

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Old 06-16-2010, 07:25 PM
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