Stripped heli-coiled front barrel studs and time-sert or big-sert

I am working on my son’s 71 750 Commando, does anyone have experience with repairing the holes for the 5/16” front barrel studs with a time sert or big sert?
A PO had converted the holes to 5/16-24 via helicoil and threaded in the 5/16-24 studs in about 1/4”, unfortunately when I was torquing the nut down to 20ft/lbs it stripped out the heli-coil. After researching options it looks like a “sert” product is the right answer to repair the heli-coiled hole and that a big sert is designed specifically for this scenario, but given the limited material around the stud holes, I was wondering if anyone has faced similar and which “sert” worked.

thank you for your insight and help.

Been using Helicoils for a LOONG time and never had any trouble with them stripping with the appropriate torque for the fitting. 1/4" is not much of an engagement for a 5/16" bolt/stud. Not saying that the serts you are referring to won't work well but I have installed a lot of helicoils in aluminum/cast iron/steel without any difficulty - a helicoil produces a stronger fitting than the original threaded fitting was.

I've always used the timesert. I wouldn't use a big sert yet because you can save that one in case it ever strips again (since its a larger diameter). The advantage of a time sert over a helicoil is that its one piece so you pull on the entire engagement of threads when you torque down whereas with a helicoil you only pull on the threads that your bolt is deep enough to reach. The timeserts also look better.

jseng1 said:

I've always used the timesert. I wouldn't use a big sert yet because you can save that one in case it ever strips again (since its a larger diameter). The advantage of a time sert over a helicoil is that its one piece so you pull on the entire engagement of threads when you torque down whereas with a helicoil you only pull on the threads that your bolt is deep enough to reach. The timeserts also look better.

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Thanks for the reply. Since it was already helicoiled I was thinking I had to use the big sert, but have you had luck using a time sert after a helicoil? I would much rather do that for all the reasons you said and the limited material around the studs

Are the threads in the barrel that support the helicoil damaged or is it only the helicoil itself that has come adrift ?
Corrction here - should read threads in crankcase if for barrel studs - my bad !

MexicoMike said:

Been using Helicoils for a LOONG time and never had any trouble with them stripping with the appropriate torque for the fitting. 1/4" is not much of an engagement for a 5/16" bolt/stud. Not saying that the serts you are referring to won't work well but I have installed a lot of helicoils in aluminum/cast iron/steel without any difficulty - a helicoil produces a stronger fitting than the original threaded fitting was.

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One of my helicoiled studs has started to pull out. It lasted about 24,000 miles, 10 years. There is a lot of heat. The thread is to fine. I think I should have used a thread insert with a 14mm thread on the outside. That is what Jim Comstock uses.

Metisse said:

Thanks for the reply. Since it was already helicoiled I was thinking I had to use the big sert, but have you had luck using a time sert after a helicoil? I would much rather do that for all the reasons you said and the limited material around the studs

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According to the Time-Sert FAQs, no. This is their response to the question.

Jim Comstock did a comparison test of the above mentioned 3 different sorts and the helicoil surprised him as it came out on top. I'll go with what Mexico Mike said. Have never had problems with helicoils, when the correct one is used in conjunction with the correct bolt or stud, even spark plugs.

kerinorton said:

Jim Comstock did a comparison test of the above mentioned 3 different sorts and the helicoil surprised him as it came out on top. I'll go with what Mexico Mike said. Have never had problems with helicoils, when the correct one is used in conjunction with the correct bolt or stud, even spark plugs.

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Well, unfortunately, as Metisse said in his first post, his cases had already been Helicoiled, and the Helicoil had failed, stripping the threads. So now he has to drill and tap to a larger size for some other sort of insert. If I was in his situation, I'd go with a BIG-SERT. I've used them to successfully repair similar pulled out Helicoils.

But, if he really wanted to stick with the helical type of insert, he could probably recover the threads by using a Twinsert, which requires drilling and tapping for a larger size helical insert with the same thread pitch, and then inserting a second helical insert inside the oversize one. I've never used them, but they are approved by the Air Force for salvage and repair operations, so they can't be too bad.


Ken

lcrken said:

Well, unfortunately, as Metisse said in his first post, his cases had already been Helicoiled, and the Helicoil had failed, stripping the threads. So now he has to drill and tap to a larger size for some other sort of insert. If I was in his situation, I'd go with a BIG-SERT. I've used them to successfully repair similar pulled out Helicoils.

But, if he really wanted to stick with the helical type of insert, he could probably recover the threads by using a Twinsert, which requires drilling and tapping for a larger size helical insert with the same thread pitch, and then inserting a second helical insert inside the oversize one. I've never used them, but they are approved by the Air Force for salvage and repair operations, so they can't be too bad.

Helicoil Oversize & Twinsert Screw Thread Inserts Kit | STANLEY

STANLEY offers Helicoil Oversize & Twinsert Screw Thread Inserts, approved for use by U.S. Government agencies in maintenance & salvage operations. Learn more!

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Ken

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Wow - a helicoiled helicoil !

I would go for the big serts (if there's enough meat left). Thats what its designed for - to replace a stripped out helicoil. There are problems with heilicoils - they can unscrew from the threads. I've seen them fail on spark plugs and I've seen them unscrew from stud mounts and stick out proud of the surface. The time sert or big sert locks in and stays there.

Big-Sert inserts will require drilling bore to .406" size. It looks feasible. I'd use this, but ensure sufficient thread length. Compare with original studs (e.g., 067571).

-Knut

One advantage time or big serts have over helicoils is that you can install a longer time or big sert that you can with a helicoil. The helicoil has a tang at the bottom that limits your bolt depth but a time sert does not - so you can use a longer bolt with a longer time sert and engage more threads. Look at the first post in this thread. The helicoil pulled out because there was only 1/4" of thread engagement and stripped out at only 20lbs torque. This happens because the helicoil is a spring and when the bolt is short it only engages a few threads and strips out the top few threads. A time sert is full length and one piece (no tang). Of course you should use the longest timesert you can fit. They make them in special order lengths. Note the longer time sert in the vid below which is longer and stronger.



I got my tip about using timeserts from Ron Wood who developed one of the highest HP 750 shortstrokes on the planet (84 HP as measured on Axtells dyno). He used timeserts in the cases - otherwise the cylinder through bolts would pull out the threads.