stripped out crankshaft threads
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Elder User
Joined: Oct 2003
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From: kingman az
stripped out crankshaft threads
i bought a used good running 390 then i found out 2 holes in the crankshaft were stripped out and they wont take new threads the guy i bought it from said its okay to run it like that .the local machine shop said the flywheel will be very out of balance .and the flywheel will destroy the motor and maybe come into the cab and hurt me .
i want to be safe but i have a problem i paid 300.00 for it .
if anyone can help thank you
i want to be safe but i have a problem i paid 300.00 for it .
if anyone can help thank you
Elder User
Joined: May 2003
Posts: 520
Likes: 1
From: california
Sometimes, a guy will say some convincing things to get you to buy. I know 3 points make a plane, but I wouldn't leave ANY bolts off of the flange. What exactly do you mean that it won't take new threads? Do you mean that you can't chase the threads with a tap to straighten them out? I'll see if I can stop by to see my machinist friend and ask if heli-coils will be safe to use on the crank flange.
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Elder User
Joined: Oct 2003
Posts: 702
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From: kingman az
stripped out crankshaft
i tried to chase it with a tap no luck .
i had a idea if i got longer grade 8 bolts with nuts put the longer bolts thru the flywheel and put nuts on the back side with loctite of course;/
another idea
put the two bolts thru and have them tack welded in would that make the flywheel too unbalanced.if the other bolts were tack welded in the same place would that balance the flywheel.
i dont want to buy another 100.00 crankshaft and another 160 .00rebuild set to fix 2 strippped out holes .
thank you .
in trouble in tucumcari
i had a idea if i got longer grade 8 bolts with nuts put the longer bolts thru the flywheel and put nuts on the back side with loctite of course;/
another idea
put the two bolts thru and have them tack welded in would that make the flywheel too unbalanced.if the other bolts were tack welded in the same place would that balance the flywheel.
i dont want to buy another 100.00 crankshaft and another 160 .00rebuild set to fix 2 strippped out holes .
thank you .
in trouble in tucumcari
Elder User
Joined: May 2003
Posts: 520
Likes: 1
From: california
I just talked to my machinist friend. He said a helicoil will work if installed correctly. I would nix the longer bolt with nut idea. That would definitely throw the balance off. Not sure if there is even enough clearance for a nut. I would try the helicoil first along with loctite. Filling (welding) the hole would be tougher because you'll need to be dead on when you drill and tap a new hole.
Senior User
Joined: Sep 2003
Posts: 238
Likes: 0
From: Bridgeville,CA
One thing to consider is that by the time you take enough stuff apart to be able to properly helicoil the hole it might be a better to just get a good used crank and replace it. As long as you leave the top end alone putting in a new crank should be pretty straight foward.
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Elder User
Joined: Oct 2003
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From: kingman az
is it worth it to put in a 250.00 crank
question
is it worth it to put in a 250.00 crank in a 300.00 used 390.
is it worth it to put in a 250.00 crank in a 300.00 used 390.
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Elder User
Joined: May 2003
Posts: 520
Likes: 1
From: california
I would not place a used crank in without having it magnafluxed, ground or at least polished. You'll also need to factor in the cost of new rod and main bearings and gaskets. I wouldn't try to reuse the old bearings as the clearances most likely won't be correct.
Post Fiend
Joined: Jun 2003
Posts: 8,209
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From: Black Hills of SD
Here is a solution I have used many times in this situation. Take one of the original bolts to a fastener store or good parts store and find the next size larger in metric. Very often they are barely larger and you can run the corresponding tap in without even drilling. If you had a 7/16s bolt I think a 12 mm would be a little larger but it may take the next larger size. Just remember to get grade 8 bolts for replacements. Helicoils are good too but if you have never done one there is a bit of a learning curve with them. If you are really concerned with balance you can weigh the bolts and remove material until they weigh the same. Unless you intend to run high rpms the difference that close to the centerline if the crank probably wont be noticeable as long as the bolts are basically the same length.
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