dgberkstr

88 944 Turbo S. Did a search, didn't see this problem: The fasteners holding the AC/alt drive belt pulley to the BS belt drive gear sheared off. Looks like someone replaced the fasteners before and used the wrong ones. Anyway, I had to remove the BS belt drive gear to drill out the broken screws. They came out no problem. When I removed the BS belt drive gear I used a gear puller, and inserted a 9 mm socket into the crankshaft hole to center the puller. When I went to put things back together i found out I bunged up the first couple threads in the crankshaft. Now what do I do? The bolt would not start straight, and it messed up the bolt threads bad enough to require a new bolt. How do I fix the crankshaft threads? I am an amateur mechanic with good proficiency with hand tools. I don't want to drill out the crankshaft and put in a helicoil--never did that before. Has anyone had success fixing the internal threads? I'm really bummed out!
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divil

I would suggest a thread chaser, but I have never seen one that size (I think the crank bolt is 16mm). My chaser kit only goes up to 14mm, and that is the biggest one I have seen. Maybe a tap...you can definitely get those in the size you need, but you'd have to be careful since they are designed for cutting.

Hopefully someone with more experience will chime in.

EDIT: has the size you need afaik (16x1.5mm)



It doesn't look like it goes very deep but if it's only the first few threads you might be ok.

KevinGross

You generally want to avoid cutting, such as would happen with a tap or die. Not sure you can fit a thread file in the crank with the engine in the car, but if you cannot find a thread reformer / chaser in M16, that would be the way to go.

Kevin
Catellus Engineering

harveyf

I would say that if you can figure out how to do it cleanly, you can safely remove the damaged threads without any overall harm. I could give you a fancy formula from ASME B1.1 but as a rule of thumb, it the depth of the "good" internal threads is greater than the nominal OD of the bolt, then you will be fine. I'm pretty sure the crank is tapped substantially deeper than 16 mm, so if the first few threads are removed you'll be fine. You might be able to remove the threads in a clean fashion with a countersink bit or a roundish grinding stone like is used for head porting.

Scott at Team Harco

I snapped that bolt off a couple of years ago. Yes - M16 bolt - snapped off. It took a lot of torque mind you - but it broke off. (Previous owner performed a non-approved repair when the A/C compressor got "removed" from the car (another non-approved stunt))

Most all of the bolt remained in the crank. It took me several months of drilling and griding to get most of the bolt out. The last step was running a tap in and cleaning up the internal treads enough to allow the installation of a new bolt.

Your situation is nowhere near as dire as mine was. Running a tap in to clean up the first few threads should not be a big deal.

sharky47

I am a machinist - if you only just damaged the first couple threads - just tap it. Take your time and make sure it's strait and not cross-threaded and you'll be fine.

You could even make a chaser from a bolt by milling some flats and hardening it with a torch. I made a tool to massage the threads on the cam tower plug holes this way.

Van

Holy smokes!! You could have built a new engine in that time!

Nashio

I had an issue when I stripped the first 4 threads on my crank bolt. I needed to buy a new crank bolt and ended up tapping the crankshaft to get the new bolt to go in. I took my time used plenty of lubricant and quadripple checked the alignment of the tap. All went well, just make sure it is all lined up.

MB968

I agree with the advice to just tap the threads. The tap should have enough of a leadin to easily follow the old threads without trying to create new ones.

And, as was stated above, you should have plenty of threads left (even the first few when cleaned up will be active threads, just not with the same properties as original).

dgberkstr

Thanks, guys. Maybe I'll get to trying to thread it later. I am afraid I will end up overrunning the threads that are already there. I did some research on the net, and decided to try a NES internal thread repair tool. I'll let you know how it works out. I learned to use taps and dies from my dad 40+ years ago. I would not hesitate to buy a 16 x 1.5 tap, but I'm just too scared right now. Taper, plug, or other?

turbodave

I would first use a tapered tap to see if I could grab the deeper good threads to help with alignment. Go slow and check your progress often with a good bolt. When you are satisfied that the bolt will thread properly, use a blind tap and run it clear to the end.

dgberkstr

The threads are fixed. I used a Nes 22 Universal Thread Repair Tool. You put it in the threaded hole, tighten a **** that keeps a small piece of tool steel in the thread path, and back the device out. It did an adequate job of getting a good path from first thread in, but it did not restore the threads to perfect form. I was able to then use an M16 x 1.5mm plug tap to reform the threads. The tap started easily. The Nes tool cost me $70 and was designed to fit threads from 12 to 16 mm dia. or 0.5" to 0.625" dia. I don't know if I would have needed the tool if I had more experience with taps and maybe more courage. The tap did deposit a lot of small cuttings between the flutes. I was then able to easily insert the CS screw and tighten it to the correct torque. By the way, I used the Arnnworx flywheel lock. It worked fine and was cost-effective. It even came with three washers and the correct nut. Thanks fellas for your replies.

Scott at Team Harco

It wasn't every day and it was over the winter. The car wasn't going anywhere (for two good reasons) - so I could have at it in small doses and still keep my sanity (?).

It was a very strange and frustrating situation to be put in. So far the repair has held and I am motoring merrily along.

dg - Good to hear you got your problem resolved. It's always a big relief.