Thread inserts in the Norton head (2019)

Jim Comstock wrote: "Regular helicoil inserts -not the solid ones- work fine for spark plugs. Just install them 2 or 3 threads deep to keep them from coming out."

This caught my eye because I know a couple of friends who had a problem with the helicoils unscrewing with the spark plug. I am not sure what it means to install them 2 or 3 threads deep, or why this would keep them from coming out with the spark plug?

Stephen Hill
 
Jim Comstock wrote: "Regular helicoil inserts -not the solid ones- work fine for spark plugs. Just install them 2 or 3 threads deep to keep them from coming out."

This caught my eye because I know a couple of friends who had a problem with the helicoils unscrewing with the spark plug. I am not sure what it means to install them 2 or 3 threads deep, or why this would keep them from coming out with the spark plug?

Stephen Hill

Screw the helicoil in a couple turns after it is flush with the plug seat.
Because the sharp end of the coil will usually dig in a bit so it doesn't come out with the plug.
To help I used to take a punch and mash the thread a little just above the helicoil and use a spot of antiseize on the plug.

Are they perfect, no, but at least they don't crack the head like the solid plug inserts often do.
[And that's why I started doing the stepped Al/bronze inserts.]
 
And to make it more embarrassing, I couldn't find a 5/8" length 3/8-16 helical wire insert from any other manufacturer (Recoil, Precision, E-zlock, etc.). It seems like they all mostly pick lengths as a multiple of the thread nominal diameter, i.e. 1.0x, 1.5x, 2.0x.

Ken

I just buy them all long. Then wire snippers will make them any length I need.
Thread inserts in the Norton head (2019)
 
The helicoil may measure 9/16" overall but it only measures 1/2" down to the tang. Removing the tang requires a deeper hole and its a messy job that doesn't always go smoothly (the tang can bend instead of breaking or jam into the bottom of the hole). Removing the tang gives you about .540" length but most people probably just leave the tang alone.


Thread inserts in the Norton head (2019)


Starting with a longer helicoil and cutting it to length isn't a good solution because you still have to cut it close to 1/2". That's because the helicoil kit comes with a plug tap and you have to drill a deeper hole than the helicoil to tap it deep enough and leave extra depth to be able punch downward to remove the tang. The photo below shows a helicoil plug tap compared to a bottoming timesert tap for a 5/8" long timesert. A helicoil bottoming tap would help but you still have to provide the extra depth for the tang and/or its removal.
Thread inserts in the Norton head (2019)


The studs in a Norton head are in blind holes and that limits your drilling depth. I still think the best option is to use the longer 5/8" timeserts instead of the shorter helicoil. Plus a timesert looks better, won't retract like a helicoil can and you don't have to fuss with removing a tang.

Vids like the "Thread insert deathmatch" do not really apply to a Norton head because you can use a longer timesert (than a helicoil) in a Norton head.

Here's a vid showing how the install the 5/8" deep timeserts in a Norton head.
 
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Preparing to refurbish several 750 heads, I want to replace the 3 head studs, so I examined the contributions in this thread.

There are at least 6 factors affecting hot temperature (392 deg F / 200 deg C) strength of these stud bolts connections:

* Combination of materials (Aluminum, carbon steel or alloyed steel, brass)
* Difference in CTE of those materials combined (at the elevated temperature!)
* Shear strength of each of these materials (at the elevated temperature!)
* Area of interaction, i.e., thread depth x helical length of interaction at the average action diameter
* Nonlinear load distribution along interaction length
* Plasticity (the ability to deform before rupture / shear failure).

It has proven very difficult to obtain reliable shear strength and CTE figures for the materials in question, including RR53B and A356. If anyone has access to data, please let me know and I will follow up in another posting.

CTE figures (0 to 200 'C):
RR53(B): 23.2 E-6
A356: 23.2 E-6
C-Steel as used in bolts (38Cr2): 12.2E-6
Stainless steel 303 as used in
inserts and helicoils 17.8E-6
Brass (as used in inserts: CuZnPb) 20.5E-6

Shear strength:
Although there are no reliable temperature-dependent data available, we can with some certainty expect the materials to perfom in decreasing order as follows:
Stainless steel (cold worked), C-Steel (cold worked), Aluminum alloy (heat treated), Brass.

Area of interaction:
External treads of these thread forms are compared: 3/8" BSC, BSF, BSW, UNF, UNC, and M10.
"h" is the effective height of the male thread.
"D" is the average action diameter between male and female threads, at "h"/2, slightly above the pitch diameter. Shear distribution across thread depth has been assumed to be constant (rectangular distribution). In reality it will be parabolic.
For unified and metric threads, D = Dmajor - 0.541266*p.
For BSF and BSW, D = Dmajor - 0.640327*p.

BSC 26 tpi h = 0.3660333*p = 0.358 mm D = 9.005 mm
BSF 20 tpi h = 5/8H = 0.6003*p = 0.7624 mm D = 8.712 mm
BSW 16 tpi h = 0.492492*p = 0.782 mm D = 8.509 mm
UNF 24 tpi h = 5/8H = 0.541266*p = 0.573 mm D = 8.952 mm
UNC 16 tpi h = 5/8H = 0.541266*p = 0.859 mm D = 8.666 mm
M10x1.5 16.93 tpi h = 5/8H = 0.541266*p = 0.812 mm D = 9.188 mm


Next: Effective helical length "S" along the cylinder "D" for an interaction length of 0.620" = 15.75 mm.

Discussion and conclusion to follow.
 
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Lots of info in the Helicoil catalog and data sheets available here

https://www.stanleyengineeredfasten...allanguages=[English]&f:brandlist=[Heli-Coil]

This is their installed strength chart for 3/8-16 inserts

View attachment 12666

For our case, the Aluminum 356-T6 spec is probably the one closest to the Commando head alloy, so you would want the insert length to be at least 1.5 times the diameter, or 9/16" to not pull out from torqueing a 170,000 psi bolt.
170,000 psi is a reasonable value to assume for high quality studs.
Norton heads are sand cast in RR53 aluminum alloy. Although no shear stress figure @ RT were published by High Duty Alloys Ltd, it's reasonable to assume ultimate shear stress ~ 0.6 * ultimate tensile stress.
This calculates to 149 MPa or 21.6 ksi which seems a likely figure when compared with sand cast A356 which was a much later development, cf. the graph in Ken's posting.

BS 1083:1965 lists fasteners with a minimum tensile strength 135 ksi by use of EN-16 , EN-20, and EN-25 steels, BS970:1955. The majority of bolts for low tensile stress applications would have been governed by BS 916:1953 and EN1A, EN 8 carbon steels, BS970:1955. However, a table from AMC's drawing office shows the highest general purpose bolts and studs in use for BSF threads (Z classs bolts, EN-25) had a tensile strength of 224 000 psi (1544 MPa).

- Knut
 
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Yikes, that is a lot of number crunching.

If the threads fail in the FA head I will simply Helicoil with 3/8 UNC but to the full depth of the hole.
 
Yikes, that is a lot of number crunching.

If the threads fail in the FA head I will simply Helicoil with 3/8 UNC but to the full depth of the hole.
Choice of 3/8 UNC is a good one as my numbers show it has the largest thread tensile and shear stress areas of thread forms available.

Of the various inserts available, Heli-Coil inserts win hands down, as shown below.

Cylinder head bolts are subjected to fatigue. Fatigue strength of a bolted connection in a semi-rigid aluminum block material (esp. 7075 alloy) has been shown to be essentially identical for assemblies with and without Heli-Coil inserts,
proving that the Heli-Coil inserts’ flexible coils do allow the excellent ductility characteristics of the aluminum parent material to be transferred to the bolt. Good ductility is one factor in improving fatigue life.
Rigid solid bushings, unlike the flexible Heli-Coil inserts, cannot distribute the load over the entire length of the engaged threads, so bolt fatigue life is reduced.

In semi-ductile aluminum alloys such as rolled 6061 and wrought 7075, the number ot effective furns (pitches) to achieve full strength of the parent material is 6, as for steel. This is seen in the figures of the report (see p.8). 6 * 16% ~ 96%. In ductile alloys such as A356 and RR53,
the number of turns required to achieve full parent material strength will be greater, but unlikely more than 10. For a 3/8" UNC threaded joint, this translates to an effective treaded hole length of 15.9 mm.

See enclosed extract of Stanley Technical Bulletin 71-2.

- Knut
 

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