Thinking through making a new throttle shaft for a Kingston 5 ball. I can buy some phosphor bronze rod for the shaft and a 1 1/8" 18 gage brass disc for the parts I need. I am thinking I can use a slotting saw on a vertical mill, holding the shaft in a vise to cut the slot. I can drill the two holes in the shaft with the shaft and disc assembled in the carb. I am also considering opening up the top and bottom holes in the carb body a bit to eliminate any egg shape. I can turn the shaft to whatever diameter I wish. I've never done it before so I am looking for some suggestions.
One of my concerns is whether I can get a decent cut with the slotting saw. I can buy one with the right width but am concerned that the slot will be larger because of wobble.
If the saw is mounted on a mill and the shaft is in a vise, I would expect any wobble to be negligible.
I haven't made such a shaft for a Kingston 5 ball, but I have made them for 1907 Holley Bros. carburetors for Model N Fords, and a few other carbs as well. The process you describe works very well. If you've got a proper holder, (or make one), for your slitting saw, you'll be just fine. Since the saw will be a bigger diameter than the disc, you might have to make your slitting cut from each side to minimize the washout at either end of the slot. I sometimes cut the slot length a bit short for this reason and finish the length with a very tin file. It makes a slot with nice square ends and no overcut.
Your idea to enlarge the worn shaft holes is a good one. For extreme wear, I've bored them way oversize and installed small bushings to get back to original size.
I trust you know that a ordinary drill bit doesn't work well for opening up the holes. It will make kind of a "3 sided hole".
Your plans sound fine to me
Use a reamer on the hole. And as the shaft size get larger it will restrict the
air flow. Maybe you can reduce the shaft on the inside. Scott
Dean Yoder made oversize NH throttle shafts earlier this year: http://www.mtfca.com/discus/messages/599638/609313.html?1454041238
If the slitting saw will not work for you, I have seen shafts made where half the shaft is milled away to make a flat for the plate. One advantage of this is the flat can be exactly the width of the plate, so there is no runout. This will also remove a little restriction in the throat of the carb.
Hope this helps.
Allan from down under.
Thanks to all for the thoughtful responses.
Jerry, I hope to replicate your success with the 07 Holley using the technique.
Les, I've purchased a long reach 2 fluted end mill for enlarging the holes in the carb.
Scott, a reamer will work for the top hole but the bottom hole "bottoms" after 1/8" in the body of the carb. I'll use the end mill I think.
Roger, I am torn between using bushings or making the shaft larger at the two ends, same as what Dean did in the post you referenced.
Allen, if I can't get a good result with the slotting saw, I like your idea of milling one edge. I'll have to mill more than the width of the disc to keep the disc centered but that's not a problem.
Steve, thanks as always.
P.S. The discs measure 1 1/8" and the carb opening is 1.122. What's a good way to take off 4 or 5 thou from the disc uniformly? I cant figure a way to hold it for spinning on a lathe.
On a lathe rough turn down a piece of Aluminum or dense hardwood round stock to around 1 3/16 dia. and 3" or so long. Face and center drill the end. Cut a piece about 1" long from the faced and drilled end and face the other end of it. Re- chuck and face the end of the remaining 2" long piece. Position and clamp the face of the 1" piece against the 2" long piece with a live center in the center drilled end. Turn the two pieces to the diameter of the Brass disk across the parting line area. Now sandwich and clamp the disk between the 2 turned pieces and turn the assembly to the desired diameter.
What I find very difficult is to rebore the holes of the throttle shaft in the carb body when there are egg shape. The shaft must be very well centered so that the disk will close perfectly. If you do it simply with a reamer, it will go in the "medium" position including the wear
Thanks Art, I never would have thought of that.
My concern as well, Philippe. I'll open the holes in the carb body only if the new shaft has excessive movement. Hopefully most all the wear is in the old shaft.