Mixture screw-HD8 SU carb

Talk about the E-Type Series 2

Topic author
TexasJagMan
Posts: 9
Joined: Mon Jan 13, 2020 4:31 am
Location: Texas
United States of America

#1 Mixture screw-HD8 SU carb

Post by TexasJagMan » Wed Jul 13, 2022 5:41 pm

Anyone know the thread pitch for the mixture screw on an HD8 SU carb?
I had to drill one out and need to re-tap for the new screw and am having a hard time determining the thread pitch so that I can get the correct tap. # 27 seems close, but not quite right. :?:
Thanks

Link:
BBcode:
HTML:
Hide post links
Show post links

User avatar

mgcjag
Moderator
Posts: 7285
Joined: Fri Jul 09, 2010 6:15 pm
Location: Ludlow Shropshire
Great Britain

#2 Re: Mixture screw-HD8 SU carb

Post by mgcjag » Wed Jul 13, 2022 6:00 pm

Think i remember them as 3BA......the thumb nuts on the back of the instruments are 3BA so you could remove one to check.....Steve
Steve
69 S2 2+2 (just sold) & Building a C type replica, 1960 xk150fhc

Link:
BBcode:
HTML:
Hide post links
Show post links


Topic author
TexasJagMan
Posts: 9
Joined: Mon Jan 13, 2020 4:31 am
Location: Texas
United States of America

#3 Re: Mixture screw-HD8 SU carb

Post by TexasJagMan » Wed Jul 13, 2022 6:46 pm

Thanks,
Anyone know the correct BA tap number?

Link:
BBcode:
HTML:
Hide post links
Show post links

User avatar

mgcjag
Moderator
Posts: 7285
Joined: Fri Jul 09, 2010 6:15 pm
Location: Ludlow Shropshire
Great Britain

#4 Re: Mixture screw-HD8 SU carb

Post by mgcjag » Wed Jul 13, 2022 7:01 pm

Google is your friend....Steve
Steve
69 S2 2+2 (just sold) & Building a C type replica, 1960 xk150fhc

Link:
BBcode:
HTML:
Hide post links
Show post links

User avatar

Series1 Stu
Posts: 1483
Joined: Thu Feb 02, 2017 12:26 pm
Location: Shropshire
Great Britain

#5 Re: Mixture screw-HD8 SU carb

Post by Series1 Stu » Wed Jul 13, 2022 7:10 pm

If you mean tapping drill size then its 3.45mm or number 29 drill.

Regards
Stuart

If you can't make it work, make it complicated!

'62 FHC - Work In (slow) Progress
'69 Daimler 420 Sovereign
'78 Land Rover Series 3 109
'94 X300 XJR basket case

Link:
BBcode:
HTML:
Hide post links
Show post links

User avatar

abowie
Posts: 3645
Joined: Wed Sep 07, 2011 9:15 pm
Location: Australia
Contact:
Australia

#6 Re: Mixture screw-HD8 SU carb

Post by abowie » Wed Jul 13, 2022 11:48 pm

4BA.
Andrew.
881824, 1E21538. 889457. 1961 3.4l Mk2. 1975 XJS.
http://www.projectetype.com/index.php/the-blog.html
Adelaide, Australia

Link:
BBcode:
HTML:
Hide post links
Show post links

User avatar

mgcjag
Moderator
Posts: 7285
Joined: Fri Jul 09, 2010 6:15 pm
Location: Ludlow Shropshire
Great Britain

#7 Re: Mixture screw-HD8 SU carb

Post by mgcjag » Thu Jul 14, 2022 8:04 am

:bigrin: :bigrin: thats great Andrew....i got the idea it was 3BA and to check with a guage thumb nut from your old post viewtopic.php?t=13633
Steve
69 S2 2+2 (just sold) & Building a C type replica, 1960 xk150fhc

Link:
BBcode:
HTML:
Hide post links
Show post links


Topic author
TexasJagMan
Posts: 9
Joined: Mon Jan 13, 2020 4:31 am
Location: Texas
United States of America

#8 Re: Mixture screw-HD8 SU carb

Post by TexasJagMan » Thu Jul 14, 2022 4:04 pm

Thanks to all for your help.
I'll get the tap ordered.

Link:
BBcode:
HTML:
Hide post links
Show post links

User avatar

abowie
Posts: 3645
Joined: Wed Sep 07, 2011 9:15 pm
Location: Australia
Contact:
Australia

#9 Re: Mixture screw-HD8 SU carb

Post by abowie » Thu Jul 14, 2022 11:24 pm

TexasJagMan wrote:
Thu Jul 14, 2022 4:04 pm
Thanks to all for your help.
I'll get the tap ordered.
Having seen my old post wait until I can measure one properly for you!

The 4BA screws are on the HS8. I'll need to find an HD8 to check for you.
Andrew.
881824, 1E21538. 889457. 1961 3.4l Mk2. 1975 XJS.
http://www.projectetype.com/index.php/the-blog.html
Adelaide, Australia

Link:
BBcode:
HTML:
Hide post links
Show post links

User avatar

abowie
Posts: 3645
Joined: Wed Sep 07, 2011 9:15 pm
Location: Australia
Contact:
Australia

#10 Re: Mixture screw-HD8 SU carb

Post by abowie » Fri Jul 15, 2022 12:06 am

abowie wrote:
Thu Jul 14, 2022 11:24 pm
TexasJagMan wrote:
Thu Jul 14, 2022 4:04 pm
Thanks to all for your help.
I'll get the tap ordered.
Having seen my old post wait until I can measure one properly for you!

The 4BA screws are on the HS8. I'll need to find an HD8 to check for you.
I don't have an E Type HD8 at home to check.

I DO have a box of HD8s from Mk10 etc. Both the threads on the jet housing and the screws are DEFINITELY 4BA.

BUT, you can screw a 4BA screw into a 3BA die nut because they have very similar TPI. This might explain why I got the thumb nuts to go onto the throttle screws in my old post.

If you have a vernier caliper, the diameter of the throttle screws I have is 0.136". The studs on the back of a tacho are 0.156".

Here's a picture of the two side by side, with dies on them.

So I'm fairly confident that 4BA is correct, but I don't have an E type at hand to measure, sorry.

For safety's sake maybe order a 3BA tap as well. Then you can make thumb nuts for the instruments; always a plus.



Image
Andrew.
881824, 1E21538. 889457. 1961 3.4l Mk2. 1975 XJS.
http://www.projectetype.com/index.php/the-blog.html
Adelaide, Australia

Link:
BBcode:
HTML:
Hide post links
Show post links


Topic author
TexasJagMan
Posts: 9
Joined: Mon Jan 13, 2020 4:31 am
Location: Texas
United States of America

#11 Re: Mixture screw-HD8 SU carb

Post by TexasJagMan » Fri Jul 15, 2022 2:50 am

I am posting this pic to be sure we are talking about the same thing.
Thanks
Image

Link:
BBcode:
HTML:
Hide post links
Show post links

User avatar

abowie
Posts: 3645
Joined: Wed Sep 07, 2011 9:15 pm
Location: Australia
Contact:
Australia

#12 Re: Mixture screw-HD8 SU carb

Post by abowie » Fri Jul 15, 2022 4:02 am

Good thing you put that up.

That is a slow idle screw. Not a mixture screw.

I have just checked a couple against my extensive collection of taps and dies and I am not certain what the thread is.

It is 0.307" major diameter and 26tpi. The closest I have to that is a 5/16 British Standard Bicycle thread, which is 5/16" (0.312") and 26tpi (most of BSB is 26tpi).

The screw will not go freely into the die, although it has been freshly zinc plated, which might interfere.

A tap will not go easily (finger tight) into a carb body, although the slow idle screw does.

There are some weird fine and extra fine Unified threads, but none of them are 26tpi as far as I can see.

So, I'm not sure what you have.

I have rung SU Midel, the Australian SU distributors. They don't know either.

Bill Angel might know.
Andrew.
881824, 1E21538. 889457. 1961 3.4l Mk2. 1975 XJS.
http://www.projectetype.com/index.php/the-blog.html
Adelaide, Australia

Link:
BBcode:
HTML:
Hide post links
Show post links


Topic author
TexasJagMan
Posts: 9
Joined: Mon Jan 13, 2020 4:31 am
Location: Texas
United States of America

#13 Re: Mixture screw-HD8 SU carb

Post by TexasJagMan » Sat Jul 23, 2022 7:46 pm

The correct size is: 5/16 inch- 26TPI- BCSY (British Cycle Standard)
So, "abowie" was correct and, per his post on the US Jag forum, drilled a piece of aluminum stock, tapped it with the proper tap and the screw fit perfectly! Surprisingly, a company in the U.S. had the tap in stock.
Go Figure!
I will now have to make an insert to repair the carb body using instructions and pictures graciously sent to me from "philp" on this forum. He had this same problem and arrived at an elegant solution, making my job much simpler. That's the beauty of these forums. So many people willing to share their knowledge.

Link:
BBcode:
HTML:
Hide post links
Show post links

Post Reply Previous topicNext topic