tire tub bolts 1966

[Richard S.]

So I removed my spare tire assembly to do a little work on my leaking differential. When I removed the 4 fasteners that secure the tub to the frame. I found 5/16-18 bolts not 5/16-24. I did read other articles that indicated that others also found 18 thread, and yes some previous owner may have changed them before my ownership (have owned this car about 40 yrs). The odd thing is 2 of these bolts have anchor head markings, which I believe is period correct for a 1966. Also the tips are a chisel design with a shoulder under the head. If not correct for the tub where else could these been used? Anyone else found these on their '66? Pics included

Thanks for any comments
Ric

[Edward J.]

Re: tire tub bolts 1966

Rich, I have a couple of set of tire tub bolts and they are 5/16 index head and fine threads with a slight tapper and a thin flat washer. I do not recall seeing that style bolt before, the anchor head mark was used through the mid year cars as you suspect.

[Gary B.]

Re: tire tub bolts 1966

Ric,

The “screw” called out in the ‘66 AIM for the tire tub attachment is GM 9420825. The same screws are called out in the ‘66 AIM for the fuel line and brake line to frame attachment. (I think the same is true for ‘65 and ‘67.)

Rich Pasqualone has reported that the most common headmarks for that screw are D inside 3 circles, E, RBW, anchor, and plain (no headmark). Plus a few less common ones. Also, there are no grade lines.

The ‘66 AIM calls for a loose, plain washer to be used with the tire tub screws. So, your screws with an integral washer do not match up with the AIM. I would advise you not to over-tighten the screws, as the holes can easily be stripped.

David Bartush gives the specs below for GM 9420825. In spite of the GM specs a number of people have reported finding 5/16-24 screws that are otherwise the same as what David reports, but fine thread screws where the coarse thread 9420825 is called for in the AIM. Fine threads makes sense to me for something that screws into something that is 1/8” thick, such as the frame and the gas tank cross member.

As to other locations on a ‘66 where your 5/16-18 flange head screws might be called for, I can’t say.

Gary

[David B.]

Re: tire tub bolts 1966

Gary,
Not sure where you got info? PN 9420825 is a type C Hexagon head tapping screw 5/16 - 18 x 9/16 phosphate coated GM-4435-M code A

[Gary B.]

Re: tire tub bolts 1966

David,

Thanks. Man, I thought I had done my homework on that one, but I seem to have gotten a grade of F. I just edited my posting above to hopefully make it correct.

Gary

[Gary B.]

Re: tire tub bolts 1966



Gary

[Mike B.]

Re: tire tub bolts 1966

Gary,

I'm curious as to what size screws were originally installed on your '66? Are your spare tire tub's screws 5/16 - 24 x 1/2 or are they 5/16 - 18 x 9/16? It would be great to hear from you and other '66 owners what screw size are installed on their cars.

The '65-'67 AIMs do call for the 9420825 screws for this application. My '66 was missing it's spare tire carrier when I purchased the car in '88 so I can't accurately report what screws were there. But I am the second owner of my '65 and I found the fine thread screws 171110 there. The head marks found were "3 Dots."

Mike

[Richard S.]

Re: tire tub bolts 1966

Morning Gents.....So I took the 5/16-18 for the tub and checked to make sure the threaded part of the frame was good and all bolts tighten as they should. Odd that these 4 bolts meet the AIM requirements for securing the tub other than the flange as they are 5/16-18 x 9/16 in length. So with that being said when I return the tub to the car I plan on using these 4 fasteners and not try to us 24 thread bolts. I could look for 5/16-18 x 9/16 at a later date and swap them out. Just think of the time I will save putting the tub back and not needing to fight with a bolt and washer. The only way to know for sure if these are correct is if others report the same configuration.
IMG_3372.jpgIMG_3373.jpg
While I was under the car I removed a brake line bolt and it is 5/16-18 x 9/16 (see attached). This is a Dec. 7/65 build.

[Gary B.]

Re: tire tub bolts 1966

Gary,

I'm curious as to what size screws were originally installed on your '66? Are your spare tire tub's screws 5/16 - 24 x 1/2 or are they 5/16 - 18 x 9/16? It would be great to hear from you and other '66 owners what screw size are installed on their cars.

The '65-'67 AIMs do call for the 9420825 screws for this application. My '66 was missing it's spare tire carrier when I purchased the car in '88 so I can't accurately report what screws were there. But I am the second owner of my '65 and I found the fine thread screws 171110 there. The head marks found were "3 Dots."

Mike

[Gary B.]

Re: tire tub bolts 1966

Morning Gents.....So I took the 5/16-18 for the tub and checked to make sure the threaded part of the frame was good and all bolts tighten as they should. Odd that these 4 bolts meet the AIM requirements for securing the tub other than the flange as they are 5/16-18 x 9/16 in length. So with that being said when I return the tub to the car I plan on using these 4 fasteners and not try to us 24 thread bolts. I could look for 5/16-18 x 9/16 at a later date and swap them out. Just think of the time I will save putting the tub back and not needing to fight with a bolt and washer. The only way to know for sure if these are correct is if others report the same configuration.
[ATTACH=CONFIG]121247[/ATTACH][ATTACH=CONFIG]121248[/ATTACH]
While I was under the car I removed a brake line bolt and it is 5/16-18 x 9/16 (see attached). This is a Dec. 7/65 build.

The RBW screws from your brake line clamps are spot on for GM 9420825.

Gary

[Gary B.]

IMG_8689.jpg

[Mike B.]

Re: tire tub bolts 1966

Hi Gary,

Yes, Mark always does his homework. Excellent examples of correct spare tire tub screws and washers on his beautiful L72. I'm attaching 2 file photos from the archives that Wayne Midkiff shared with us from his early '65 from the 2008 archives. These are the exact screws that Mark shows. I've decided to use these same fine thread screws on my '66. Ric Sauve is a good buddy of mine and we often share info about our two '66 convertibles. They are just 600 cars apart. Both December '65 built.

I've shared with Ric complete stats from Don Olson's '63-'67 AIM Database along with GM Engineering Standards info pertaining to both tub screws 171110 and 9420825. Now we should discuss the tub washers. I know that this plain washer 3748358 was used in various places on the C2 Corvette. The '66 saw this washer used in 5 locations. But the standard 2 locations were the spare tire tub and the centered upper bolt of the fan shroud.

To the best of my knowledge this plain washer measures: 11/32" ID x 7/8" OD x .070" thick. Black oxide? Maybe folks with vast GM libraries can look this one up for us all.

Mike
Spare Tire Tub Screw & Washer Example (4).jpgSpare Tire Tub Screw & Washer Example (5).jpg

[Gary B.]

[Mike B.]

Re: tire tub bolts 1966

Gary,

I used to hang out way back when in a privately owned Corvette shop in Upper Marlboro, MD where a circle of friends would restore their "plastic fantastics." One such guy was a big '67 enthusiast who had compiled vast notes pertaining to original '67 Corvettes he had disassembled. He always took detailed notes and measurements of the parts he removed and restored. Very meticulous fellow.

The dimensions I show for plain washer 3748358 in my previous post are from his notes that he shared with me. These notes have served me well through the years. You are probably right about the washers being black phosphate. The Wayne Midkiff washers appear dark. I will use these dimensions to hunt for the washers I'll need for my car. Unless someone can post exact manufacturer dimensions here.

Mike

[Gary B.]

IMG_4012.jpg

Gary