The 'E' Type Forum

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

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

Thanks,
Anyone know the correct BA tap number?

Google is your friend....Steve

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

Regards

4BA.

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

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

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.

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.

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

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.

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.