Exhaust studs just spin

[davedidit]

I have an exhaust issue with my Taotao EVO 150. It seems as if the threads where the studs are screwed into the block are gone. I can't get the studs to screw in properly, they just spin and spin. I was told by a motorcycle shop to use J B Weld on the studs, screw them in and wait 24 hours before putting the cap nuts on. That worked fine to actually tighten the cap nuts, but it lasted less than a mile till the studs dropped out again. I was wondering if I should try tapping the bolt holes hoping to "re new" the threads. I'm probably going to have to have someone weld the studs onto the block and go from there.
I was wondering if anyone else had this problem and what they to to fix it.

[rks]

To fix it right, the head needs to be removed.....and if you are going to do that, you might as well replace it with a new one..... $30 or so bare, $60/$70 complete with valves, springs, seals, etc.

[Shadowfire]

The correct fix for the stripped out head, is to install appropriate Heli-Coils into the head (something along the lines of https://www.amazon.com/Helicoil-5546.../dp/B0002SREP4).

This means taking the head off, since you have to drill out and tap the old holes.

Also, JB weld on an engine head (especially around the exhaust port) will be subject to temperatures that will destroy it. At this point, running a tap through the holes is not going to renew them; there is obviously missing metal, which the tap won't fix.

[davedidit]

Originally Posted by Shadowfire

The correct fix for the stripped out head, is to install appropriate Heli-Coils into the head (something along the lines of https://www.amazon.com/Helicoil-5546.../dp/B0002SREP4).

This means taking the head off, since you have to drill out and tap the old holes.

Also, JB weld on an engine head (especially around the exhaust port) will be subject to temperatures that will destroy it. At this point, running a tap through the holes is not going to renew them; there is obviously missing metal, which the tap won't fix.

Thank you for your reply, Shadowfire.
I came to realize today that I would probably need to get a heki-coil repair kit because I got a drill bit/tap set (one size up from original threads). I could only do 1 hole because the frame is in the way of the other one. I drilled the one hole I could, and tapped it. I used a 1/4 x 2 bolt. It would tighten down all the way either and fell out in less than a block,

Do you (or anyone else) know what the right heli-coil kit to get for the exhaust holes. And do I have to take the whole engine out to do this repair. Or can the head be removed without dropping the engine.
Any replies would be welcome.

[Shadowfire]

I don't know what thread it is - but, it's a pretty safe bet that its some sort of metric thread.
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Before you proceed, I -highly- recommend you spend an hour or so browsing through Fastenal's Technical Reference; at the very beginning, it has a lot of great information about screws.
https://www.fastenal.com/content/doc...renceGuide.pdf
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Once you've had an opportunity to review the screw reference, Fastenal also has a recommended torque calculator which will answer the question "What is the hardest I can torque this screw/bolt down?" Keep in mind that this calculator only applies to the screw/bolt; you may not be able to go that hard if, for instance, the head has aluminum threads without any kind of steel inserts.
https://www.fastenal.com/en/83/torque-calculator
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"Take one of the stud bolts down to the local hardware store and identify the diameter/thread pitch. Get a pair of replacement studs. Get the appropriate heli-coil kit, including the correct drill bit and tap" is what I would say, except for the fact that you've already drilled one of the holes out and tapped it to an English thread size.

At this point, you've almost certainly screwed the pooch. Unless the drill size for the heli-coil is the same/larger than the OD of the 1/4" tap that you used, the only way out now is to replace the head.

If the frame is in the way, you really have no choice but to drop the engine out, unless its a vertical engine. Also, make sure that you are using a torque wrench to install the cap nuts; on my Xciting 250, which uses M8 stud bolts, the service manual says to torque them down to 14 ft-lbs (this compares with the calculator, which states that for a dry coarse M8 grade 8.8 stainless steel screw permits a maximum of 18.8 lbs, derated to 80% = 15 ft-lbs). Also, make sure to use anti-seize compound on the studs. I would suggest tightening the studs into the head first, by putting two nuts on them, tightening them together, and driving it in with the outer nut. After you can use a wrench on the inner/outer nut to separate them and put the exhaust on. Do this on the bench before putting it back on.

[Ebuddy]

In my experience, there are 3 things that you can do:

1. 1. Switch to bolts/lock washers that are 1 size larger than the studs. This is the least desirable solution however and there are risks. It is the cheapest however. Some folks have been able to have this solution go a long way. I've done it successfully, but only on a scoot that could die tomorrow and I wouldn't care. Suckers held for a couple thousand miles before I got rid of the scoot.
2. 2. Use heli-coils (or similar) as directed. I've actually only had only 50/50 success (1 for 2) with this method but the failure was the first attempt so I could chalk it up to my error/inexperience.
3. 3. The new head solution is probably the best, especially if the scoot has a few thousand miles on it. Gives you a chance to look at everything and get an idea if something else might be going bad.

[blueboy5000]

The studs are pretty crappy stock. I always drill out the stud holes and tap to the next size up and install quality bigger studs from NAPA.

I disagree with Helicoil for any exhaust repair, and for that matter, any repair at all.

In fact, I have never used a helicoil on anything related to a) aluminium or b) a gy6.

I cannot recommend against Helicoil anymore than I do. Just drill and tap.

[Shadowfire]

A heli-coil is actually a GOOD thing in a soft substance like aluminum or plastic. You can torque smaller bolts down harder with a helicoil, because the weakest link isn't, say, an M8 thread in the aluminum head... it's now an 9.5mm diameter thread in aluminum, which has more surface area and can withstand greater forces.

[blueboy5000]

No. That's not how heli-coil works. It's not going to allow better torque of studs, as it's not very tightly attached to the bored hole you have to drill to insert the heli-coil.

Heli-coil falls under the category of compression fittings: absolute junk that actually takes longer and costs more than doing it the right way.

It is always better to drill a stripped thread one size larger. New threads are always preferable to inserts, ESPECIALLY on exhaust, where sealing is critical.

[rks]

Still thinking a new head is the way smart money would go.

[stitch_man87]

Originally Posted by rks

Still thinking a new head is the way smart money would go.

Couldn't agree more, an entire top end rebuild can be done for under $100