Firewall thickness and material

[Campbell Tellman]

Our touring car has a replacement firewall made of particleboard that is 3/4 inch thick. I am asking for advice as to thickness and material of the original.
Thanks for the input.
Campbell Tellman II
1915 or 1916 touring
Beaufort,NC

[CudaMan]

Somewhat helpful, but not a direct answer to your questions:

http://www.mtfca.com/discus/messages/50 ... 1227781080

viewtopic.php?f=2&t=23227&view=previous

[TRDxB2]

By John F. Regan on Saturday, December 26, 2009 - 01:31 pm:

Wooden fire walls were used through about February of 1923. The first metal firewalls appeared then and were of the "low" type but very soon replaced by the metal "high" type that was typical of 1924 and later. In early 1915 the wood firewall changed thickness from 3/4" to 11/16 but it didn't go away for a number of years and it was a source of constant issues of warping and swelling as Ford changed the method of making it from Horizontal core to Vertical core to laminated core with the veneers running horizontal and then a 3 ply design was used for a time. The final solution was the metal firewall but at first that had issues too with the steering column flopping around due to the lack of stiffness of the new metal dashes. That was solved by the addition of a bracket to tie the column to the dash panel. Then as now when designs change hopefully for the better to solve some issue - there are unforeseen pitfalls that then must be addressed.
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This is how they were made, now plywood note a 3/4” sheet is actually 23/32” thick

[Allan]

Campbell, as advised, 3/4" plywood is just a tad thicker than original firewalls. For your 15-16 hood shelf to fit it will require a little relief in the thickness of the firewall. If you carefully sand the back edge [ the side that goes to the cowl] that will allow the narrow back edge of the hood shelf to clear the plywood. This little operation is not noticeable when the firewall is assemble to the car.

Hope this helps.
Allan from down under

[Steve Jelf]

Allan is correct. The ¾" ply is slightly too thick. With a little sanding of the back edge it should fit right into the hood former.

[Ray Syverson]

Try to find some baltic birch plywood, as it is much stronger due to it's multiple core build up. It is also usually thinner than the regular plywoods, and I have sometimes found it at 11/16 thick, or slightly over.

[TRDxB2]

Dimensional lumber isn't the advertised size in the USA
3/4" plywood's nominal thickness is actually 23/32”
and if 11/16" is the desired thickness
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Then doing the math
11/16 Inches (fraction), 0.69 Inches, 17.46 Millimeters
23/32 Inches (fraction), 0.72 Inches, 18.26 Millimeters
Difference .03" (.8millimeters) is approximately the thickness of a Business Card - well within Model T tolerances.

[Steve Jelf]

I've found a reference that gives the actual thickness given on the Ford drawing. It's .677" to .697", so .6875" (¹¹⁄₁₆") is good. I think I may spend a few bucks and get a copy of the drawing.