Throttle Plate Chuck from Bolt

[hemmjo]

Bolts are a good source of round stock. I never throw away a bolt, even old rusty ones. They are not rusty on the inside. Be sure to note the grade of the bolt if you are making something strength critical.

Today I found this tool I figured I would never need again in the drawer where I keep old bolts, etc.

I made this about 4 years ago to repair an old Gravely walk behind garden tractor. It had been sitting under a tarp for a while. A hole got worn in the tarp and let water get into the carburetor. After sitting in there over a number of years, the throttle plate corroded so it would no longer close the carburetor throat enough to allow the engine to idle properly (fig 1)

The only way to get a new throttle plate was to buy a whole new carburetor… or make a new plate. The plate was not round but rather like two 1/2 circles stretched apart just a little. (fig 2)

I used this bolt and made a special chuck. Faced the head. Turned part of the head round to the match the 1.076 dimension of the original throttle plate. Drilled two holes to match the hole spacing in the throttle plate. Pins are from a broken 1/8 drill bit.(fig 3) The pins drove the new throttle plate (original is shown in fig 4). In use, an appropriate size plate was drilled to match the original holes then pressed against the chuck by live center in the tail stock.(fig 5) A hex nut was used to cover the point in the center to hold the plate firmly against the chuck.

I turned the new plate to 1.093 diameter matching the dimension of the original. Then hand filed the sides, using the turned portion of the “chuck” as guide.

Can’t show the new throttle plate, it is in the tractor.

Click to enlarge

[metric_taper]

My moto cross friend had me enlarge a carburetor (throttle body as it's fuel injected) bore throat diameter, so I had to make a new plate. What I noticed was the original plate was manufactured so the top and bottom edges were parallel to the throat top and bottom (when this butterfly valve is closed). When closed this butterfly valve is at an angle to the bore relative to the throttle shaft that holds this plate. I made a guestimate on what that angle was, and took a round bar that could be chucked in the lathe 3 jaw on my mill and set that to the angle (I recall it being 8 degrees, where the throttle was closed), machined that round bar end to that angle. Then did a similar thing using the tail stock to hold the new plate in place and machined it into the ellipse shape with the top and bottom edges now able to seal the air flow when installed. It took a few tries to get this right. I used a piece of brass for the throttle butterfly plate, as the original one was made from that material.
Seems my description is hard to read.

[hemmjo]

Make perfect sense to me Metric. I think most butterflies are as you describe, that is why they are not really round. I did put some bevel on the edges on my plate with the file. But it is not a racing tractor, so not as critical.

[DIYer]

Thanks hemmjo! We've added your Throttle Plate Chuck to our Carburetor category,
as well as to your builder page: hemmjo's Homemade Tools. Your receipt:

Throttle Plate Chuck by hemmjo

tags: carburetor

[metric_taper]

I did record some of the making of the butterfly valve and machining of the throttle body. I used double side tape to machine it, and then figured out to use a tailstock center to hold it in place while machining it.

[DIYer]

Thanks metric_taper! We've added your Throttle Body Plate to our Carburetor category,
as well as to your builder page: metric_taper's Homemade Tools. Your receipt:

Throttle Body Plate by metric_taper

tags: carburetor