Early air box clamping bolt size?

[LCRJ]

Hi all,
Does anyone know an original early air box clamping bolt size?
The parts # 26S/019 It?s 5/16x3 UNF or UNC?
Please tell me.
I very much appreciate your cooperation.
Thank you,
Hiro

[Quart Meg Miles]

Hi Hiro,

All threads in bobbins are UNC.

My bolt measures 3?" x 5/16" with 18 tpi and its threads don't project beyond the box. Is your car much earlier than mine with a thinner box?

[LCRJ]

Hi Meg,
Thank you for your reply.
I just happened to saw a parts list of Lotus Cortina.
The parts list mention the parts number and specification of the bolt.
The parts # 26S/019 and 5/16x3 UNC.
It has Elan parts number and I thought UNF is suitable for the place with vibration.
also I noticed new air box(reproduction?) has UNF threads in the bobbin.
So I would like to check the difference.
Your air box must be original. Maybe your air box rubber too thick?
I would like to see some photos if possible.
Thank you.
Hiro

[Quart Meg Miles]

Hiro,
Here are a couple of pictures:

I can't be certain that my rubber is the original design and it has a large O section fitting into the metal back plate but that wouldn't be an extra ?". It didn't want to come loose (Wellseal used) otherwise I would have shown it open.

The metal back plate is 1?" deep, ignoring the bracket, and the outer cover projects another 2" at the midpoint where the ruler is. The bobbin is inset and is a zinc casting which normally would not be strong enough for a UNF thread. Perhaps modern ones use a stronger material. There is a shake-proof washer under the bolt head to cope with the vibration and it has never come loose even though you can't do it up solid.

I wonder if the Cortina had a thinner box and as Lotus designed and made them they would have given the bolt a Lotus number automatically. Perhaps someone else could measure their bolt?

[LCRJ]

Hi Meg,
Many thanks for the photos.
I think your air box is original.
Lotus Cortina metal back plate is looks same as yours. It?s 1?" deep.
However the parts number 28S/021.
Also air box looks same as yours. However the parts number 28S/022. I know it has different shape.
It?s 1.58? deep(4cm) at the midpoint.
I think your air box looks a little bit fat and your rubber is wrong or need change the direction of mounting?
I checked my parts list of lotus Cortina again. The early one mention the parts number and specification of the bolt.
But no a shake-proof washer under the bolt.
The later one no mention the parts number and specification of the bolt! Why??
How about Lotus Elan parts list?
If I have a chance, I?d like to see some photos of the bolt and rubber.
Thank you.
Hiro

[LCRJ]

I just did dig my garage and find Lotus Elan master parts list for S1,S2 in 1966.
From the list, the metal back plate same as Cortina parts number 28S/021.
The air box is parts number 26S/022.
The bolt is not mention of the parts number. However the specification printed 5/16?UNCx3/4?
It?s misprint?
The bolt with 5/16 washer and lock washer.
How about your bolt?
Thank you.
Hiro

[Quart Meg Miles]

Hiro,

It's just an ordinary 3?" bolt with about 1" thread (I measured it before my first post) and that "UNC3/4" is clearly a misprint; I'm not keen to remove my box as it is nicely sealed but it is probably not the original rubber seal strip as I had a lot of trouble with it decades ago. So my bolt is probably just to suit the seal.

Sorry if I've confused things, one forgets details after a long time. Forget the parts lists, use the length you need with your seal and check the thread in your box with another bolt.