Stripped crankshaft thread - Vintage Mustang Forums
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post #1 of 28 (permalink) Old 08-18-2019, 12:45 AM Thread Starter
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Stripped crankshaft thread

I'm in the middle of a T-5 swap on my 65 Mustang and while doing an initial torque pass on the flywheel bolts one of the crankshaft threads stripped. I bought a Time Sert kit to repair the threads but have had no luck even drilling out the old threads, likely due to the crank being hardened.

Anyway, I talked to a local old-timer machinist and he didn't like the odds of fixing the crank threads while in the car. His advice was to try a bolt the next size up (metric M12 1.25 in this case). If that does not work he does not see a big issue running with just 5 flywheel bolts for a street car. He said if they are torqued to 85 ft/lbs he did not think vibration would be an issue. Anyone have any experience or opinions on that? Thanks.

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post #2 of 28 (permalink) Old 08-18-2019, 01:02 AM
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This is one of those questions I'd kick myself over too. The bolt's not THAT heavy, and it's also fairly close to the center. How many pounds imbalance would it work out to at 5k rpms? I'm no mathemagician. Probably just a small vibration. But with my luck, that vibration would mess with the crank bearings in the back, and then it'd probably trash my crank, so after a few thousand miles, I'd end up totally rebuilding the engine when I should've just done it right the first time.


The last time I went "Eh, it'll be fine!" I'd pinched the fuel line on the front of my fuel-injected 5.0 HO motor with a crescent wrench while trying to loosen the heater pipe fitting. Not long after, I had a nasty engine fire that burned everything on top of the engine, and totaled my car.


So, being that I am experienced in the "Dang, well, I guess I should've done it right..." department, I'd curse my luck and fix it properly instead. =) Your call! I wouldn't blame you if you take a chance, either.

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Last edited by Grimbrand; 08-18-2019 at 01:05 AM.
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post #3 of 28 (permalink) Old 08-18-2019, 08:52 AM
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If that machinist recommended the M12 x 1.25 bolt, then I'd get a good M12 tap and slowly, backing out often, try to tap the hole to the M12 size. If there's any chance of putting that bolt in, then tapping first should work.
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post #4 of 28 (permalink) Old 08-18-2019, 08:56 AM
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If you're only worried about the weight imbalance issue, then ask your machinist friend this. Would getting a longer bolt the same size, cutting the head off so the remaining stud is the length of the original bolt, then cleaning the hole with brake cleaner to remove oil, and finally using JB Weld thread this stud into the hole. Later it could be removed by heating the bolt to 600 degrees. JB Weld releases at 600 degrees.


This "stud" should restore the balance.
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post #5 of 28 (permalink) Old 08-18-2019, 09:06 AM
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those bolt holes go all the way through in to "oil cavity" . So if you try drilling and tap your going to get metal in the engine ,even if only a small amount. I don't see JB being a good fix
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post #6 of 28 (permalink) Old 08-18-2019, 09:15 AM
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It would be a royal pain, but I would pull the crank and do it right.
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post #7 of 28 (permalink) Old 08-18-2019, 09:25 AM
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Food for thought. If that one stripped tightening to 85 pounds, what about the rest? What's going through my mind is someone in the past over tightened that one, makes me wonder about the rest of them? Even though the rest may have taken the 85 pounds....this time. Those existing threads may already have issues. I have to agree with @LeeFred.
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post #8 of 28 (permalink) Old 08-18-2019, 10:36 AM
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I'd be concerned with why it stripped out. Are the threads damaged ? torque wrench inaccurate ? Wrong type of thread sealant/lubricant ? Correct bolt length and material ?

'65 A-code coupe, T-10 4-speed, 8" 3.25 limited slip
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post #9 of 28 (permalink) Old 08-18-2019, 11:42 AM
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What he ^ said.

I would not run just 5 bolts. A 289/302 running the original firing order has some interesting
harmonics. That's another way of getting to the reality that when I was at JBA, we never
pulled a stock 289 crank that didn't have cracks in its structure. Strange harmonics makes
me want to recommend having all the fasteners connecting a flywheel to the crank in place
and correctly torqued.

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post #10 of 28 (permalink) Old 08-18-2019, 11:51 AM
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Im an extremist...this would be a definite while Im at it moment. The engine would already be out and on the stand.
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post #11 of 28 (permalink) Old 08-18-2019, 12:17 PM
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Replace the crankshaft if you can. If it's not in the budget, bolt it back in with 5 and baby it (no 5k passes)until you can get the funds to fix it. Chances are it'll last year's.

Scott

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post #12 of 28 (permalink) Old 08-18-2019, 04:03 PM Thread Starter
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I chased the other threads and they seem OK. Won't know for sure until I torque them. The bolt in this hole was longer than the others so I think there was already some prior thread damage.

My plan was to pull the engine in a year or two and go with a Fordstrokers short block or possible get their stroke kit and try to do it myself. Honestly, if I have to pull the motor now I'll want to push that up...but I lack time/space/WAF right now.

I probably only drive it a few hundred miles a year right now. I'm leaning towards going with 5 bolts and not doing anything too crazy until I do the engine swap. I wonder if I put the old bolt back in with some red loctite if that would be enough to hold it? At least then the weight will be the same.
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post #13 of 28 (permalink) Old 08-18-2019, 04:04 PM
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If you don't put something in the hole you are going to have a massive oil leak.


I doubt that a HSS tap is going to cut the crank metal if a HSS drill bit won't cut it. Did you try a cobalt drill bit like I recommended in your previous thread on the subject?
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post #14 of 28 (permalink) Old 08-18-2019, 04:41 PM Thread Starter
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Good point. Yes, I tried a cobalt bit. Wasn't much better. Going really slow I got some metallic bits (almost dust) but even if I get the threads out I don't think I'll be able to cut new threads.

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post #15 of 28 (permalink) Old 08-18-2019, 05:43 PM
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If its not completely stripped, I think I would put some teflon tape on the bolt. Wrap it 3 or 4 times and it will probably tighten up enough to hold for awhile.

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