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US FlagWednesday, July 24, 2024
Norton 3/8" Head Stud Study

While working on a customer bike I was installing the head when a terrible thing happened. You can read the details starting at August 22, 2023 here but the short story is the rear stud pulled out of the head at low torque. This problem will finally be solved in about three weeks (around September 22, 2023) by Jim Comstock. Considering that I planned to test ride the bike by August 26, 2023 so I could give it back to the owner, you can imagine how upset I am. Not only the delays but a lot of money wasted. This is the second time a stud pulled in the last year. I've put many heads on without problem so I started investigating what is causing this.  Of course, the heads are old - but so are the ones that don't give me a headache!

After some study I noticed the three 3/8" BSF stud holes are threaded about 5/8" and drilled even deeper. However, the new studs from Andover Norton (AN) are threaded only about .449". I checked with them and they say they are made to the factory drawings that specify .453" on the head end. The particular head that I was working on had two good original studs and one with a frozen nut, so it had to be replaced - that's the stud that pulled. I checked the other head with a pulled stud, and it was also a new AN stud. We're talking about part number 06.7885. The other two studs in that head and the studs in three more heads I checked all appeared to be original and all had .515" of thread on the head end. Therefore, it is clear that Norton was not following their drawings!

A little more study and I found that RGM sells studs that are overall 1/8" longer and have 9/16" of threads on both ends so I ordered some for testing and they arrived today.

First a comment an impression and then the tests. I am a fan of AN and most of their parts are perfect. The original studs and the ones that AN is having made are inferior to those RGM has made. The RGM threads are cleaner, smoother, and sharper - it pains me to say that - I'm not used to things at RGM being better and I've certainly not used to them costing much less.

Update 9/2/2023: See the RGM stud update below the picture.
Update 9/6/2023: See another update RGM update at the bottom

I tested with a RH10 head and a Combat head. The treads in both heads appear to be very good.

The picture below shows three studs numbered 1,2, and 3 in the Combat head used for testing. Note that studs 2 and 3 appear very crooked - they are straight, it is a camera illusion!

In both heads, 1 is the RGM stud, 2 is the AN stud, and 3 is the original stud. First, I measured the depth of each hole (middle row) and then how far out of the hole the stud projected (bottom row).

RH10 Head   Combat Head
1 2 3   1 2 3
.777" .746" .760"   .788" .724" .697"
1.453" 1.464" 1.333"   1.462" 1.444" 1.355"

Then I just started each stud in it's hole. I tried very hard to just catch the very beginning of the thread, marked the stud, and then counted the turns to seat it. The table shows the number of full turns to seat each stud in each head (middle row) and for the RGM and original studs the percentage of improvement over the AN studs (bottom row).

RH10 Head   Combat Head
1 2 3   1 2 3
10.5 8 9.5   11 7 9.5
31%   19%   51%   36%

This shows pretty clearly that when using the AN studs, that there's a very good chance that they will pull. It also shows a significantly lower chance with original studs and a WAY lower chance with RGM studs.

And that's not the half of it! In the next test, I screwed each stud into its hole four turns and then carefully measured the movement of the top of the stud. This took many tries especially since I was trying to record it (need a tripod for such things). The recording below the next table is the only time I got one usable (just barely).

The table shows the deflection in one direction (so they need to be doubled) for each stud in each head.

RH10 Head   Combat Head
1 2 3   1 2 3
0" .010" .020"   0" .028" .010"

I was shocked that the RGM studs were rock solid at four turns so I tried them at two turns. In the RH10 head the deflection at two turns was .005" and in the Combat .007". Then I tried the RGM stud in all three holes in both heads, at four turns in - rock solid in all!


September 2, 2023 update:

The RGM threads do look a lot better than the AN or original studs, and I have 12 of them to test with. I did a test fit with a 850 head with original or AN studs that drops on two sets of cylinders today with no problems. Here are the results with the RGM studs:
  • The head end (BSF) of each fits the threads in the head much better. However, once screwed in every stud but two were slightly to a little crooked. Two are crooked in the body, the rest are crooked where the head-end threads stop or at the end of the nut-end threads.
  • The other end (BSC) varied from the sleeve nuts screwing on easily, to jamming when barely started.  Of the 12, I only have two that require nothing and I have six that must be chased for sure.  The others get stuck part way on so maybe a thread file will correct them
  • I measured the diameter of the middle of all the loose original studs I have, the three new AN studs I have, and the 12 RGM studs. All rounded to the nearest thousandth:
    • Original: Largest is .374", smallest is .372", average is .373.
    • AN: All exactly .375"
    • RGM: Largest is .379", smallest is .376, average is .378
  • In other words, the RGM studs are about .005" larger in diameter.
What does all that mean? The head won't go on! After three hours of fiddling with it I resolved the problem.  Each thing helped, but all were required for the head to go on fully:
  • At first I didn't try the test head with the original or AN studs and only tried with one set of cylinders. I thought the two smaller holes for the studs screwed into the cylinder were the problem and slightly counter sunk their holes in the head. Helped very slightly. Got out another set of cylinders that didn't have those studs installed and the head still didn't come close to going on.
  • It appeared that the rear stud was the problem so I removed it and the head dropped on easily.
  • I tried all the RGM studs in the rear and the head would go on part way with some and not at all with others.
  • I did the measurement above and since there is almost no room for error, realized the everything must be even more perfect for the RGM studs.
  • I ran a 13/32" drill through the cylinder holes.  That is their original size so I was just cleaning up the holes. The head still would only go on about 1/4".
  • I used a tapered reamer to slightly open the top 1/16" of the three holes.  That got the head on about 3/8".
  • I finally started checking the studs for being vertical in the head. They weren't. the rear stud was straight, and perfectly vertical side-to-side but leaning back about two degrees. I put a sleeve nut on it and gave it a few light taps with a hammer. I was shocked to see that the stud was now perfectly vertical, but the nut end was bent backwards. Installed another stud and had to tap it backward and towards the timing side, but the whole stud was straight.  Dropped the head on - bad idea. It when down about 1/2 way and stuck hard! A LOT of wiggling and it was finally off.
  • Remember I said it dropped on without problem with the rear stud removed. I thought that meant that the front studs were fine.  No, both happened to be perfectly vertical side-to-side but one leaned forward and one leaned backward! More tapping and checking. Now the head went on about 90% of the way. The timing side stud was bent at the nut end. Changed that stud, more tapping, and the head finally dropped on with some resistance. Then I tried it on the other cylinders and it dropped on there too.
So, I'm now convinced that the RGM studs won't pull out, but it takes serious effort to get them to align properly and it is important to buy extras if you're going to use them!

September 6, 2023 update:

I ordered some studs from British Tools & Fasteners and they arrived today. At first glance, they look identical to the RGM studs. After measuring, they are very slightly longer and the thread length on both ends is the same as RGM. I almost didn't test them since I thought they were probably made by the same people. Still, I wanted to know so I removed my carefully straightened RGM studs and put these three in and tried the head. It dropped on completely with no issue! With all my straightening, that head using the RGM studs would just go on.

I also received a set of precision machinist squares today. The first picture shows the best RGM stud from the best angle. The second picture shows what ALL the British Tools studs look like. Also, the treads going into the head went in smoothly and the original sleeve nuts screw onto the nut ends properly.

I have no relationship with British Tools & Fasteners other than being a customer. I have trade accounts with Andover Norton and RGM Norton. The studs cost more at British Tools & Fasteners but considering that 100% of theirs were good and 25% of the RGM studs were just OK, and 100% of the Andover Norton studs are not usable, it's a good price!

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