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MG MGB Technical - Throttle Adjustment Screw

The idle adjustment screw in the 64's HS4 front carb has stripped out. Actually not the screw but the machined part of the carb body is shot. This was determined by simple substitution of an extra screw and unfortunatly, it's the carb body. I was thinking of mixing a little JB Weld, filling the hole and then retapping.

Does anyone know the thread size of the carb idle adjustment screw? My simple set of taps and dies doesn't have the size. Perhaps any other ideas for repair. Must keep the original screw though, so tapping to the next bigger size is out. And no, the car isn't green.

Paul

Paul Hanley

Paul - The thread size is 2BA, a British thread form. That said, take heart, a 10-32 will work fine. I use 10-32 helicoils to repair the 2BA threads in fuel pumps when I find one stripped and have never had a problem with them. In fact, I would recommend getting a 10-32 Heli-Coil kit to repair the casting with, as it will hold up better than the JB weld fix, although as an interium fix, the JB weld should work all right. Good luck - Dave
David DuBois

Thanks Dave. I was afraid it might be a British thread. What you suggest with the Heli-Coil will certainly work but I would then need a 10-32 screw for adjustment, right? What I was hoping was to keep the original hex head slotted screw--a restore if you will rather than a strict repair. Would 8-36 work if I ran the original screw through the die? It looks prety close but I don't have an 8-36 set, just a thread gauge. Thanks.

Paul
Paul Hanley

There's a product on the market called Form-A-Thread you may want to try. It's like JB Weld, but there's a release agent you apply to the screw, and then install it into the wet epoxy. I've used it many times on tranny pan bolts and it works very well!!! I suspect it would be a good fix for your carb
Jeff Schlemmer

Paul - The thread form and pitch of the 2BA and a 10-32 are so close that they are interchangable. As I remember the 2BA is slightly loose in the 10-32 thread but not so loose as to cause any problems. I too have used the Permatex Form-a Thread, but am a bit reluctant to use it where there is continual adjustment of the screw. I think that a helicoil would be a better fix in this case.
Good luck - Dave
David DuBois

Helicoil taps and inserts are available in 2BA.
Chris at Octarine Services

Chris. I have not seen 2BA available here in the US. It may be available as a special order, but that would take a while to arrive. Mostly from trying to find someone who would be willing to order the special size.

Jeff. As David notes, the 10-32 and 2BA are so close that you cannot tell the difference in use. I have turned a 2BA screw into a 10-32 threaded plate and used a 10-32 screw with a 2BA nut. They interchange so well, you need to use precision measuring equipment to tell which is which. I would get the 10-32 Heli-coil set, which may have to be special ordered. This is not a commonly used size. Les
Les Bengtson

Thanks guys but the adjusting screw falls through my 10-32 die. Its more like an 8-36. I have an 8-32 die but the screw will only go in two/three turns. The die is too coarse, but seems to be right diameter.

Paul
Paul Hanley

Paul --

Couldn't you just go up to a 10-32 thread?

Don't have a carb to see if there is room, but just another lazy thought.
glg

I'm trying to save the original screw and frankly was being thrifty not wanting to buy taps and dies that were the wrong sizes. But wait, now I have an excuse to buy more tools!! I'm almost sure it's 8-36 so its off to the hardware store for me under the guise that I'm Christmas shopping!

Paul
Paul Hanley

I am following this thread with interest. The screw on my front carb slowly turns on a long fast drive - I found out the first time as I was exiting off a major divided highway and wondered why the idle was at at around 2000 rpm!

Barry
73B
B.J. Quartermaine

Paul. I just checked a current production set of HS-4s I am getting ready to install. As you have guessed, the screw is an 8-36 UNF with a hex head, a slot across the top and a spring under it to keep it from turning while moving.

Barry. You might want to make sure the spring is in place under each of the throttle adjustment screws. If they are, a drop of linseed oil on the screws will make them more resistant to turning and may help with your problem. Les
Les Bengtson

I think the idle screw is 4 BA not 2 BA - helicoils also available in the UK for this size.
Chris at Octarine Services

Thanks Les

In hindsight I'm probably looking at a different screw. The one I am referring to is the one that sets the slow idle by providing a stop for throttle spindle rotation. It has a lock nut on it, no spring, and either the screw, locknut or thread in the bracket is stripped. Also, mine are HIF carbs.

Barry
73B
B.J. Quartermaine

Barry. Just dug out a set of old HIF carbs. The screws, having a stop nut, seem to be size 4BA. They measure .140" and the 38 tpi leaf of my thread pitch gauge is the best fit. Porter shows the 4BA as being .1417" with 38.5 TPI. You should be able to simply tap the hole out to a number 8 machine screw, either the 8-32 or the 8-40 would work. The coarser thread is a deeper thread and less likely to strip out, but there will not be as many threads engaging the stamped piece. I would probably try the 8-32 myself. Les
Les Bengtson

Here's my solution. After trips to the local machine shop, Home Depot and the local hardware store, I learned that 8-36 is special order tap and die. 8-32 was all that's readily available so, in a moment of what the heck I put the spare idle screw in a vise and ran the 6-32 die down the shaft and presto, wiped the 8-38.5 threads right off leaving brand new 6-32 in its wake. Now the carb body can be 6-32 heli-coiled or epoxied. Job done--original screw intact. Was a little concerned that the fine threads are needed for fine adjustment of the carb, but I think it would be splitting hairs. Thanks to all for the help

Paul
Paul Hanley

Many thanks Les - you are a mine of information!

Regards to all

Barry
73B
B.J. Quartermaine

Barry. More than happy to be of help.

Paul. I am not convinced that the thead count is of sigificance in this application. Either a 6-32 or a 6-40 size would work fine if you wish to go through the trouble of fitting a heli-coil. I would go up to the 8 series screws myself, but I have the taps and dies in both the 8-32 UNC, 8-36 UNF and the 8-40 sizes. Standard and special taps and dies may be ordered from Brownells, Inc. who have a website. They carry all of the standard UNC/UNF as well as a number of specialty sizes. Les
Les Bengtson

Paul,
You know that once you fix this something else will go wrong with the carb. Why not save yourself some time and just buy a new set under the guise of "Christmas Shopping"? As it just so happens we at University Motors have a set we just rebuilt. About 600 bucks with the core charge. Just a thought. My dad said that the easiest and quickest way to fix it is "in citu". Just simply place a dolley underneth it and put a ballpeen hammer on top. Strike the hammer therefore squishing the hole down and in. Then just chase it with a 4BA tap and you don't even need to use a different screw. That is the easy way out, or you could get a nice new shiny set. Makes excellent christmas presents. Hope this helps.

Brooks
Brooks Twist

Brooks,

You one funny man!! Humm..six hundred bucks or a ballpeen hammer? I need another ballpeen hammer. And I get to buy a dolley. Been wanting one. You guys are good!

Paul
Paul Hanley

Well, if you've ever seen Paul's car you'll understand why he is wanting to retain that same screw - his car is damn near perfect! One of the most exacting restorations I've seen.

Robert
Robert Rushing

Brooks,

Fixed!! Did a variation on your dad's technique. Took a 3/16 punch and gently tapped with old ballpeen three times (I heard the screw say, "I wanna go home").
Screw snugged right in--tight! I was very gentle on the hammer since I wasn't backing the job up with a dolley hence risking a snap off of the threaded ear piece.

Something to be said for the KISS method fix. Thanks to University once again.

And cheers Robert. Look forward to seeing you and everyone else at NAMBGR 2004. Good piece on the luggage rack install.

Paul


Paul Hanley

This thread was discussed between 07/12/2003 and 09/12/2003

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