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MG MGB Technical - Dizzy Bush material

Im about to replace the dizzy bush in my lucas 25D and since the part is going to cost between $35 and $45, i figure I can make one for a fraction of the cost (dimensions and specs from Les Bensingtons site).

I am assuming the Bush is made from brass and not some special combination of metals not redily available???

ps.. yes, I enjoy tinkering and I would rather spend a few hours building something rather than going and buying a new part.
Innes Bint

Innes. I contacted my local metals supplier who recommended a local specialist company for the "oil impregnated bronze bushing stock"--i.e. a piece of the basic material which was about 6" long and just over 3/4" in outside diameter. Seemed to be a pressure molded product rather than a heater and rolled product. Was somewhat circular in cross section, but not perfectly rounded such as drill rod or hot/cold rolled steel rod is.

Thus, you need to have it sufficiently over sized that you can turn the external diameter down to 3/4" and have it perfectly round when you reach that size. The bushing stock is very easy to machine and makes a good bushing. I have made up a couple of them for my own use and for friends. I still do when I am doing a rebuild on a car that will be a daily driver.

However, for general use, at least here in the US, the ready made bushings available from World Wide Auto Parts of Madison, Wisconsin are a drop in replacement, of reasonable quality, for less than half the price of the Lucas bushing. (Last time I checked.)

Thus, it is possible to get a good quality bushing, and one that will work without extra machining, for a reasonable price. You also learn a lot about how to set up a dizzy and about how parts can be fitted and adjusted, when you make your own bushings. Good fun if you have a lathe.

Les
Les Bengtson

Les,

i checked out the WWAP site and peter quoted me $35 for a bush which i thought was a bit steep. thats why i thought i would have a go at machining one on a friends lathe. if the bush material is difficult to obtain or expensive, i guess im stuck with buying one.

do you recall what an 'ingot' of the bronz costs?

(incidently i emailed you via your website about these and other issues las tnight)

cheers
Innes Bint

Innes. Have answered your e-mail.

The ingot cost $6 US last time I purchased them. Price of every raw material has gone up significantly though. One ingot produces two bushings with a small piece left over for other uses.

Much will depend on how many distributors you expect to work on over the years. A piloted bushing driver is a must for this task as it allows the old bushing to be removed and the new bushing to be installed without damage. Some of the old bushings have been in place for 40 years and can be hard to remove. "Field expedient" tools can damage the distributor body in such cases. I use a special jig to support the distributor body when removing/replacing the bushing. This minimizes the chances of damage to the body.

The bushings need to be outside turned to about 3/4". Leave a little extra material on the outside when roughing them out. Then, they need to be drilled, then reamed, to shaft diameter plus about .0005". The drill bits are common items. The reamer is a specialty item and costs about $25. After drilling and reaming, the bushing is mounted between centers on the lathe (so the outside is perfectly in coincidence with the hole inside) and turned to final diameters. The bushing is turned to .750" over its entire length, then, to .742" over about 3/4ths of its length. The full size section is the bottom of the bushing.

For me, the cost of making the tools and purchasing the reamer(s) was worth it. I have rebuild several distributors for my own use and for friends. Plus, I enjoyed inventing and making the special tooling.

For a single project, however, the costs in raw materials, time, and the purchase of the reamer might be excessive. Thus, the "drop in" bushing makes sense for a single project by someone who only wants to replace a bushing. The costs, for someone who wants to learn about machining, and expects to do several rebuilds, are not excessive.

Les
Les Bengtson

I make mine from Oilite bronze that I buy from McMaster Carr, pretty much along the same lines that Les does but have never found the need for a pilioted reamer. I press the bushings into the body then ream them to fit while turning the body in my lathe using adjustable reamers.
gerry masterman

Gerry. Nice to see you back. Have missed not seeing you around.

Please note that my comment was about a "piloted bushing driver", not a "piloted reamer".

A piloted reamer, like a chambering reamer used to cut the chamber of a firearm, has a stub of metal, about 1/4" long, which is very slightly under the size of the hole it fits into. Something in the nature of .0005" under the size of the hole, or about one quarter the diameter of a very fine human hair. The purpose of such a pilot is to align the reamer with the center line of the bore being reamed, allowing the most accurate reaming of the hole to be accomplished. A piloted reamer is not needed, in the process I describe, because the hole itself will form the precise centerline of the bushing when done in the manner I describe.

A piloted bushing driver is another tool. It has a pilot that is just under the diameter of the hole, by about .001" and a major diameter of slightly less than the outside diameter of the piece being driven by the driver. The various distributor shafts I have measured have been very consistent, 0.490". Thus, the bushing driver should be a length of 3/4" cold rolled steel (CRS) of the nominal diameter of .750". A four inch length of this should be turned down to .742". From one end, the piece should be turned down to 0.488" so it will fit into the hole in the distributor bushing and pilot on the centerline of the bushing/distributor housing. Should the upper portion of the bushing be worn, as they most often are, the .008" of clearance, .004" per side, will allow the bushing to be pressed out/driven out without damage to the distributor body. Ditto for installing the new bushing.

When a new, exact diameter, bushing is installed, it is pressed into place. Thus, the 1/4-1/3rd portion of the bushing which is of "major diameter"--i.e. 0.750" diameter, squeezes down into an internal diameter of something less than 0.4905" and the shaft does not fit properly into the lower portion of the bushing.

In such cases, the distributor, with its new bushing, is secured in a special jig which is then dialed into exact relationship (less than .0005") and the new bushing again reamed using the 0.4905" reamer which makes the compressed area the exact same size as the non-compressed area. This allows the majority of distributor shafts, if they are perfectly straight, to fit into the reamed hole.

The bushings provided by World Wide Auto Parts are, as close as I can measure with my equipment, .0492" in internal diameter. This allows them to be fitted to most of the available distributor bodies/shafts and work fine with no further treatment.

Not a perfect solution. But, in my opinion, one which works well for the majority of today's owners.

Les
Les Bengtson

I have bushings similar to the ones Peter at WWAP has, but with an ID of .4910"s. I had a local bushing manufacturer make up a bunch for me. They're a tight fit and require VERY light reaming (in most cases) in the press-fit end. They're the military spec self-lubricating bronze alloy too. I sell them outright for $20 plus shipping. I could potentially install, ream, and uninstall one for a "prefit" delivery as well.


I also have the bronze "blanks" on hand which now cost about $9 each, and they're big enough to have plenty of "chucking" room! I also have smaller blanks from which you can make upper/lower bushings as well. Those run about $4 each and are predrilled with a 3/8" center hole (less work, less weight to ship).
Jeff Schlemmer

This thread was discussed between 12/03/2007 and 17/03/2007

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