Topic: Wrong Size Helicoil???

[sammermpc]

So, I managed to remove all of the cylinder studs on my '72 CB500 save one — and had a local mechanic/machinist go to work on it. He couldn't get it out w/ a welded bolt, so ended up drilling it out and installing a helicoil. Bummer, but OK.

Now the question — I got the motor back and started threading in my new M8x1.25 APE studs, and realized that the stud in the helicoiled thread was way looser than the others. This is just finger tightened down, but probably like 1-2mm play either way. Definitely more than the flex of the stud, and anyhow, it "feels" loose in the stud.

Of course, I'm going to go back and ask him and of course, ask him to repair it if something's amiss, but he's an experienced guy, so I wanted a few more thoughts. What do you think? Possible? Simple install a wrong-sized helicoil? How difficult is it to replace?

[Trad]

Is the stud still loose when you turn it all the way in and snug it down?

[calj737]

Probably he used an SAE helicoil if it's loose. Maybe a 5/16 or 3/8? These are close, but not close enough to 8mmx1.25. If this does prove to be true, remove the helicoil and replace with the proper one.

[sammermpc]

Is the stud still loose when you turn it all the way in and snug it down?

Yeah, definitely. The others 'start' loose, but snug-up when you tighten em'.

These are close, but not close enough to 8mmx1.25. If this does prove to be true, remove the helicoil and replace with the proper one.

Ok. I know he typically works on Harleys, so maybe that's what happened. Pretty surprising though. Even though I didn't leave him with any new studs, it would have been easy enough to check on the other threads. I guess mistakes just happen.

[Garage_guy_chris]

The thread fit on the original stud holes is a tight fit, they are meant to partially bind when the studs are installed. the helicoil repair will be a looser thread fit by design of the repair tap and the helicoil insert. Its not ideal but will probably be fine provided it will hold the recommended torque spec after the top end is assembled.

[sammermpc]

The thread fit on the original stud holes is a tight fit, they are meant to partially bind when the studs are installed. the helicoil repair will be a looser thread fit by design of the repair tap and the helicoil insert.

Is there a way that I can verify that it will, other than torque it down and pray?

When I say loose, I mean even threaded all the way down, there is a significant amount of freeplay. It definitely does not feel like a correctly-threaded stud, interference-fit or not.

I've course I'll give my guy a call, but I'm concerned that he'll tell me he did put in the correct helicoil, and I honestly don't feel good about assembling the head like this.

[Davez134]

The thread fit on the original stud holes is a tight fit, they are meant to partially bind when the studs are installed. the helicoil repair will be a looser thread fit by design of the repair tap and the helicoil insert.

Is there a way that I can verify that it will, other than torque it down and pray?

When I say loose, I mean even threaded all the way down, there is a significant amount of freeplay. It definitely does not feel like a correctly-threaded stud, interference-fit or not.

I've course I'll give my guy a call, but I'm concerned that he'll tell me he did put in the correct helicoil, and I honestly don't feel good about assembling the head like this.

If it doesn't seem right, don't do it. I would not feel comfortable with a "significant amount" of play. Ask him what size he used, not "did you use M8x1.25" Also, the coils are cheap and easy to install if you want to give the right size a try yourself for peace of mind.

If it still feels loose, there are other, a little more pricey options. I have used a product called time sert. Same concept, but a solid steel piece instead of coil.

[madmtnmotors]

I would go with your gut here. At least contact the machinist and ask him if he remembers what size Heli-coil was used. Also, remove the stud, bust out the caliper and at least measure the I.D. of the threads. Maybe try some other bolt sizes, both SAE and metric to see if something else seems to fit better. The proper size Heli-coil should fit well. I've had very good results with Heli-coils (and even knock off thread repair coils for that matter) that always fit well. If the Helicoil that was used is too big then you may have to opt for the "Time Sert" which I believe requires a larger pilot hole and repair tap. They are more expensive, but they also look like a stronger repair, at least from what I can tell by the repair process, although I don't have any "hands on" with these.

[sammermpc]

Just spoke w/ my guy — he knew right off the top of his head that'd he'd put an M8x1.25 helicoil in there, and that he'd tested it with a bolt, as well as chased all of the other threads. He's definitely thorough. 

He also said that it would feel significantly different, because the shape of the helicoil threads is different — rolled vs. cut, or perhaps vice-versa? And that it was moving to "60%" coverage, or something to that effect, which I took to mean that the contact area was smaller, so it would feel like the stud was 'floating' more in the hole — and that studs should float, so they don't go askew, etc., when you're torqueing them down. With the stainless steel threads and the 10mm thread with of the helicoil insert, he was pretty confident that it would hold more torque than the others, and that I shouldn't overthink it. Granted, I usually do overthink it, but it still makes me nervous.

Forgive me if this is gobbley-gook, just trying to get down what he told me.

At any rate, it does sound like you're right, Chris, about the threads being different. I'm going to go ahead and assemble and see what happens...

[calj737]

Before you do- it is well worth the effort to test fit bolts of different pitch and measure to insure the helicoil installed was truly an M8. Even if, the machinist intended to use M8, its also possible that someone misplaced a 5/16 or 3/8 into the M8 bin.

Assembling your head and testing is lots more work than test fitting a few loose bolts.

[tlbranth]

What you describe doesn't sound right. I would get my hands on the correct size helicoil, holding it in your hand, thread your stud into it and see if it feels the same.

[madmtnmotors]

What you describe doesn't sound right. I would get my hands on the correct size helicoil, holding it in your hand, thread your stud into it and see if it feels the same.

+1

[754]

An uninstalled helicoil is even looser than when installed.
You may be able to take up some slack with tape or paint it, or powder coat one end of stud.

[tlbranth]

An uninstalled helicoil is even looser than when installed.
You may be able to take up some slack with tape or paint it, or powder coat one end of stud.

An uninstalled helicoil is even looser than when installed.
You may be able to take up some slack with tape or paint it, or powder coat one end of stud.

Yeah I thought about the 'tighten on install' thing as I wrote. The looseness doesn't sound right though. I've helicoiled my studs and bolt hole the same thread size on my GL1000 water pump and they didn't feel suspect at all.