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MG MGB Technical - Hood frame screw threads

Does anyone know if the 1/4" screws that are used to screw the folding hood frame to the body has ordinary UNF threads? When I put a 1/4" UNF nut on my screws just to test the threads it is strangely loose.

Tore
Tore

Tore, Yes, the screws are 1/4"-28 tpi which is the UNF thread for that size. The screw I just measured had an external diameter of .248".

Les
Les Bengtson

Les,

Would help me to know the length of those. If you have one to hand could you measure for me?
Regards
Roger
Roger T

Thanks, Les. I wonder if there is any other thread or bolt dimension that comes so close to 1/4" UNF that it is almost compatible with it, but not quite?

As I said above, when I put a 1/4" UNF nut on the hood frame screws, the nut is very loose on the screw, but runs easily all the way. The same nut on an ordinary 1/4" UNF bolt is just as tight as it should be. The screws are also very loose when I enter them in the captive nuts in the body, but when I try an ordinary 1/4" UNF bolt, it is very tight. And there is no indication that the threads on the hood frame screws are in any way worn.

Think I have to order some new screws.

Tore
Tore

They might be 1BA threads - 28.2 TPI but a root diameter of .1665 versus .2113 for the 1/4 UNF
Chris at Octarine Services

I think that explains it. I measured the screws yesterday and found they han an external diameter a little less than 6 mm, and when I convert .248" that is 6.299 mm.

Definitely need new screws!

Tore
Tore

Roger. All of the screws are of the same overall length. The head is designed to fit into a countersunk hole while the other end is pointed to allow easier fitting. The screw is threaded all the way to the where the head begins to flare outwards.

Diameter/Pitch: 1/4"-18 UNF
Over all length" 1.845"
Length of threaded area: 1.565"
Type of screwdriver used: Cross-point

Let me know if you need anything else.


Tore. I have pulled five top mechanism screws out of my parts collection, all removed from a single vehicle. The five have a diameter of:

.245", .248", .246", .246", and .248".

For comparison purposes, a grade 8 bolt in that size measures .244" while a socket headed cap screw in that size measures .246". So the factory screws seem to be within acceptable tolerances. The fact that you have a nut which seems "a little loose" when turned onto the screw means nothing. It is the match of the screws with the captive nuts inside the body that matters.

There are tolerances in all things mechanical. I believe, based on the fact that the nut seems loose on the factory screw and that a bolt seems tight when inserted into the captive nuts, that the nut and bolt you are using as your gauges are made to a slightly larger diameter than the original, factory hardware was. Unless you can see that the threads on your original parts are damaged, I would not bother to replace the screws if they are in good condition. They should be capable of being assembled by hand and should draw up fully when tightened. If you find that is the case, they are working as intended.

Les
Les Bengtson

Thats great, thank you Les.
Regards
Roger
Roger T

This BBS is unique! I will have a closer look.

Tore
Tore

Got the new hood frame screws yesterday. Chris was right - the old screws definitely must have 1BA threads. The new screws suited the captive nuts pefectly.

So now my new Michelotti type folding hood frame is in place. Not correct for my ´63 MGB, but so much easier to use than the old one. Next I have to find out how the new hood should be folded with this type of frame...

Tore
Tore

Paul Hunt can help you with that:

http://www.mgb-stuff.org.uk/

Go to Spanners, and then Body, and then Roadster Hood/top. The instructions for folding the Michelotti frame hood are there.

warmly,
dave
Dave Braun

This thread was discussed between 26/06/2007 and 03/07/2007

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