correct size sill plate screws

[Gary B.]

Re: correct size sill plate screws

Gary
What is the head size dimension for the #8 screw?

AMK lists the diameter of their repro 9414766

[Garry B.]

Re: correct size sill plate screws

Gary, I have some interesting findings for you. Measuring the diameter of the #8 repro screw that I have (checked multiple angles) I get 0.29. I also checked the height of the "neck" from the threads to the top and got 0.13 for what that is worth. I then was able to measure the diameter of the known original screws in my Bowtie car (without removing) and got 0.29 as well. I also more closely inspected each sill plate screw on the bowtie car and several of them do stick up further than others...just like my other car. So, with my Bowtie car as my screw originality baseline (BTW-my interior scored 100%) measuring the same on the reproduction screws, I can only deduce a few things. 1. could the "neck" of the reproduction screws be slightly longer than the original screws causing them to ride higher? 2. could the reproduction sill plates countersunk holes be slightly off causing the screws to sit up higher? 3. Could the judge be mistaken in what his understanding of just how low these screws should sit down into the sill plate? Not having an available original screw or available original sill plate to measure, I can't answer the first two questions. As far as the third possibility, I know judges make mistakes and that's okay. For every point deduct I get, I probably get one back on something overlooked, so it's not about that. To me it's about sorting things out to get it right for educational purposes on these cars. If it affects the judging process and increases our knowledge about these cars in a positive way then I think everyone wins.

[Gary B.]

Re: correct size sill plate screws

Guys,

Here you go. These photos show an original sill plate screw from my ‘66 and a repro sill plate screw from LIC. The LIC screws are:

IMG_1264.jpg

In the following images, whenever you see two screws in the same photo, the original is on the left.

IMG_1261.jpg

The next two images show the two screws sitting in the holes in my original sill plate; again, original screw at left.

IMG_1268.jpg

IMG_1267.jpg

The next images show the head diameter, original first:

IMG_1262.jpg

Next is the repro:

IMG_1263.jpg

For reference, a sheet of 24-lb photocopy paper is 0.0045”. The two head diameters are pretty darn close.

My conclusion is that I don’t see a whole lot of difference between the original and repro screws. I can’t say that any of the modern repro screws are the same as the LIC screws. I bought the LIC screws at least 20 years ago.

Gary

[Gary B.]

Re: correct size sill plate screws

Garry,

I just measured 11 of my 12 original screws. (The head of the 12th screw was really buggered during removal.) Here are my measurements:

0.313
0.312
0.315
0.316
0.311
0.322
0.314
0.318
0.317
0.314
0.312

All measurements vary by a few thousands in either direction as I spin the screw head in the digital calipers, so my measurements are sort of an average for each screw head.

I just found one website that lists the mix and max allowable head diameter for a #8 screw as 0.306” to 0.322”. The 0.31” value that AMK gives for their screws is within that range.

Gary

[Gary B.]

Re: correct size sill plate screws

Gary, I have some interesting findings for you. Measuring the diameter of the #8 repro screw that I have (checked multiple angles) I get 0.29. I also checked the height of the "neck" from the threads to the top and got 0.13 for what that is worth. I then was able to measure the diameter of the known original screws in my Bowtie car (without removing) and got 0.29 as well. I also more closely inspected each sill plate screw on the bowtie car and several of them do stick up further than others...just like my other car. So, with my Bowtie car as my screw originality baseline (BTW-my interior scored 100%) measuring the same on the reproduction screws, I can only deduce a few things. 1. could the "neck" of the reproduction screws be slightly longer than the original screws causing them to ride higher? 2. could the reproduction sill plates countersunk holes be slightly off causing the screws to sit up higher? 3. Could the judge be mistaken in what his understanding of just how low these screws should sit down into the sill plate? Not having an available original screw or available original sill plate to measure, I can't answer the first two questions. As far as the third possibility, I know judges make mistakes and that's okay. For every point deduct I get, I probably get one back on something overlooked, so it's not about that. To me it's about sorting things out to get it right for educational purposes on these cars. If it affects the judging process and increases our knowledge about these cars in a positive way then I think everyone wins.

Garry,

I think that any of your deductions could be true.

Gary

[Garry B.]

Re: correct size sill plate screws

To me the most telling photo is #4 from the top. These screws are far from being flat to the surface of the sill plate. Thanks Gary for your insights, but I have to go find something less tedious to do now. The screws are what they are and the fit is what it is. Some judges will call you on it and some will not, like many things with these cars. If everything had a specific answer it would be a boring hobby. Thank to those who contributed to this thread. Garry

[Gary B.]

Re: correct size sill plate screws

Garry,

Maybe, just maybe, I can see the repro screw head has a slightly higher bulge. But given the sill plates and screws have a grand total of 4 originality point, if I were judging I would not think it reasonable to take a 1-point deduction based on that very subtle screw difference. Say the sill plates are worth 2 points, and the screws are worth 2 points, a 20% deduction for configuration of the screw heads is only 0.4 pts. Not enough to justify a 1 point deduction in my opinion.

Gary

[Robert P.]

Re: correct size sill plate screws

I had the same issue as Garry at the same event but different judges, my sill plates are NOS ones with original screws that i bought from Rich
that sells restored original hardware and received a deduction
Bob

[Gary B.]

Re: correct size sill plate screws

Bob,

Did you get a 1 point deduction for the screw heads being too proud or too large in diameter? Or for so e deviation in the sill plates themselves? What was the algorithm for coming up with the 1 point deduction? X% of what?

If you have original screws and got a deduction, does that mean there is some difference in the shape or size of the hole in the NOS and THE repro sill plates?

Gary

[Robert P.]

Re: correct size sill plate screws

The answer i was given was they didn't sit down enough in the sill plate , i believe the screws are correct maybe the over the counter NOS ones need a little more taper but i don't want to take a chance and mess one up
Bob

[Mark F.]

https://www.youtube.com/watch?v=y3y0tATB0lg

PS - you might not (or may not ?) believe how noisy these machines were back then - nor the huge mass of oil mist thrown into the machine shop air !

I'm gonna go look at my sill plate screws, too

[David B.]

Re: correct size sill plate screws

GM screw tolerances are very specific. Assuming 9414766 is the screw part number the head profile for this screw should be:
Head diameter: .332/.308 Total head height: .141/.124 Head profile taper degree: 80/82

[Mark F.]

Parameter	Upper	Lower	Tolerance Range	vs. Upper	vs. Lower	AVG
Head Diam.	0.332	0.308	0.024	7.2%	7.8%	8%
Head Height	0.141	0.124	0.017	12.1%	13.7%	13%
	82	80	2	2.4%	2.5%	2%

Sorry - Vbulletin doesn't import tables very nicely...

[Gary B.]

IMG_1292.jpg

[Steve A.]

Re: correct size sill plate screws

Just an observation, but the "height" of the cone on the repro appears longer than the original, which could prevent it from seating as deep into the sill plate as the original.