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MG MGA - New carb shaft

I received a new carburetter butterfly shaft from Moss. It is not drilled for the pin which secures the throttle stop lever. The pin hole is in between 7/64 and 1/8. Anyone know the correct drill size? The old pin is about .117 dia, the new one tapered .122-.132
BTW, should the shaft not be pre-drilled?
Art Pearse

Art If I were you I would return it to Moss and get one from Burlen. You are taking a risk buying SU parts from Moss. Mike
m.j. moore

It is going to be easier to fix it myself this time. The shaft fits well otherwise. But point taken for future orders.
Another thing - I got new jets for the float chambers. The lever position is not quite square on the needle when adjusted using the 7/16 drill method. This will give a slight sideways force when operating. Should I correct this? I could, by making a bend close to the pivot.
Art Pearse

Looks like there is a #32 drill - 0.116 dia.
Art Pearse

Art,
When I installed a new shaft I made a fixture to drill a perpendicular hole in the new shaft. It is just a piece of 3/8" thick steel with a hole to accept the main shaft and a pilot hole for a .120" dia #31 drill perpendicular to the shaft. You have to be carefull to get the correct orientation of the small hole to the slot for the butterfly plate. My shaft had a .120" diameter hole,#31 drill, in it not .116 " diameter. I don't know if they were rebuilt before or not. This worked perfectly for me. You are welcome to borrow it if the .120" hole will be ok for you. Good luck and have a good day!

John
John Progess

The shafts come undrilled because it is a "fit to unit" deal. The shafts also have lots of different applications.

The hole is drilled through the lever and shaft in one go, with the butterfly assembled & held in closed position and the idle stop screw about 1.5 turn out (tip sticking through lever)in contact with the carb body.

The difficulty is in doing it with a pre-existing hole already in the lever - hard to line up. The holes are frequently not even on the shaft centerline. I think that these were drilled by eye at the factory.

Taper pins go in tapered holes! The hole is drilled with a couple of steps, then reamed with a "taper pin reamer", obtainable from machine shop supply places.

Before guys start telling me how theirs is "fine" with just drilling a straight hole:
After some use - a week or a year or whatever - the pins have a tendency to fall out, because the load is all taken on one side of the lever/shaft and the other side is loose.
If the pin is beat in tight enough, it will make its own taper in the hole, and distort the lever and bend the shaft.
If you put the pin in and peen the end over, it won't fall out but will wear and let the lever move on the shaft, causing wear and adjustments that adjust themselves, generally not to your liking. A common failure form is that the pin shears on the side that has been carrying the load, and the remaining bit of pin then beats up the parts until it falls out.

The compromise is to drill the hole in several steps, attempting to duplicate the pin dimension especially at the two points where the pin goes from shaft to lever, so there is no slop there. This gets to be quite a precision bit of work with an accurate and controllable drilling machine, and you begin to understand why they used tapered reamers, and why "Loctite is your friend" .

FRM

FR Millmore

FRM, job done, but I wish I had waited for your comments. I made a jig for the correct angle to the butterfly and used a 32 drill in a press, eyeballing the centre line. It turned out very much in line with the lever holes, but I ran it through both to be sure. Pretty tight fit of the new pin, but it went in no damage and no strain on the shaft. The WOT stop looks good. I can't believe it will ever work loose.
As for the float lever, it is OK now after making a slight bend at the hinge end.
Art Pearse

Art-
Like many things, common sense and careful work will do the job. I can't tell you how aggravating it is to try to set carbs when somebody has messed all this up, and used a bent nail for a pin!
Good job on the float lever, It used to be that you could change real SU needle valves and not even have to reset the levels, but with the random parts today, all bets are off.

FRM
FR Millmore

I bought HS6 shafts from Moss and when they arrived I found that they were made by Joe Curto. Joe has a good reputation for rebuilding and making SU parts, so if that's where Moss is sourcing their parts I wouldn't worry about the quality.
Mark J Michalak

Mark, (and Art)
My reason for saying there is a risk buying SU parts from Moss (or anyone else except Burlen) is that they don't have the original SU drawings with specified dimensions,tolerances and materials. Before SU folded they did a deal with Burlen to pass on these drawings as well as tooling for parts made in-house. So in effect there is zero risk when buying Burlen parts of getting good quality.
The time I used Moss to replace a pair of the cast brass jet levers for my carbs I found they were dreadful quality with oversized holes and a twist along their length. I returned them and bought Burlen - they were beautiful!
There are not many parts suppliers using the original engineering drawings and this is why so many reproduction parts are poor. When you do come across one it's madness not to use them. Mike
m.j. moore

This thread was discussed between 20/07/2009 and 22/07/2009

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