Distributor shaft bushings
Thread Starter
Racer
Joined: Aug 2013
Posts: 358
Likes: 23
From: Brigantine NJ USA
Distributor shaft bushings
1969 Corvette SB stock distributor.
About 2 years ago I had a local engine shop press new shaft bushings into my distributor. He decided the top pushing was in good condition and did not replace it. So he only replaced the bottom bushing. I remove the distributor yesterday and disassembled it to check its condition. The bottom bushing has failed and is out of alignment, you can see that it chewed up the bottom of the shaft. It is noticeable that the bottom bushing has become oblong and wore excessively on one side. The upper bushing still appears to be in very good condition based on the upper shaft. I don't know what went wrong but I'm going to buy new bushings and a new shaft and start over.
1. Can anyone explain what went wrong?
2. Can shaft replacements be done without high tech machine shop equipment?
3. How do I go about removing and pressing in new bushings in a square manner?
See pictures below
Rick
About 2 years ago I had a local engine shop press new shaft bushings into my distributor. He decided the top pushing was in good condition and did not replace it. So he only replaced the bottom bushing. I remove the distributor yesterday and disassembled it to check its condition. The bottom bushing has failed and is out of alignment, you can see that it chewed up the bottom of the shaft. It is noticeable that the bottom bushing has become oblong and wore excessively on one side. The upper bushing still appears to be in very good condition based on the upper shaft. I don't know what went wrong but I'm going to buy new bushings and a new shaft and start over.
1. Can anyone explain what went wrong?
2. Can shaft replacements be done without high tech machine shop equipment?
3. How do I go about removing and pressing in new bushings in a square manner?
See pictures below
Rick
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
That bottom bushing was most likely out of round when installed.
When a bushing like this is installed.......it really should have a ream run through it afterward to confirm concentricity.
The clue to this being out of round is that it is burnished on one half only......that half would have been tight.
Have a machine shop that does many different things give it a shot next time rather than an engine machine shop......
There are some brilliant guys out there who do not care what the part is....they care about the art of machining....that is the guy you want.
Jebby
When a bushing like this is installed.......it really should have a ream run through it afterward to confirm concentricity.
The clue to this being out of round is that it is burnished on one half only......that half would have been tight.
Have a machine shop that does many different things give it a shot next time rather than an engine machine shop......
There are some brilliant guys out there who do not care what the part is....they care about the art of machining....that is the guy you want.
Jebby
Thread Starter
Racer
Joined: Aug 2013
Posts: 358
Likes: 23
From: Brigantine NJ USA
Jebby,
I will try a machine shop as you suggest, I have one in mind that may help with the reaming portion. I guess it's fair to say if I just install new bushings I may end up in the same place. I will keep looking for a bushing removal and install method, I think I remember reading something somewhere.
Rick
I will try a machine shop as you suggest, I have one in mind that may help with the reaming portion. I guess it's fair to say if I just install new bushings I may end up in the same place. I will keep looking for a bushing removal and install method, I think I remember reading something somewhere.
Rick
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
Jebby,
I will try a machine shop as you suggest, I have one in mind that may help with the reaming portion. I guess it's fair to say if I just install new bushings I may end up in the same place. I will keep looking for a bushing removal and install method, I think I remember reading something somewhere.
Rick
I will try a machine shop as you suggest, I have one in mind that may help with the reaming portion. I guess it's fair to say if I just install new bushings I may end up in the same place. I will keep looking for a bushing removal and install method, I think I remember reading something somewhere.
Rick
Buy a new upper bushing and have it cut down, it gives more surface area. Removing and installing is a snap. Align reaming is the key.
Keep in mind, about 1 in 10 housings had core shift in the casting. Nothing solves this well.
Thread Starter
Racer
Joined: Aug 2013
Posts: 358
Likes: 23
From: Brigantine NJ USA
I snapped nothing happen, I'll keep looking.
I bet. Any one know what the tolerance should be between the bushings and shaft so I can pass that on to the machinist?
Rick
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
Make a slot in the lower oil galley boss about .005" deep to spray on the gears, which will help splash up to the bushing.
Last edited by Big2Bird; May 8, 2018 at 08:08 AM.
Thread Starter
Racer
Joined: Aug 2013
Posts: 358
Likes: 23
From: Brigantine NJ USA
Okay so you just reminded me that I have that upper bushing that was did not use. It mikes out at .496" and my old specs on the shaft where .490 diff of .006 which was from Paragon. So including reaming for concentricity how much total tolerance would be too much?
Rick
Rick
Thread Starter
Racer
Joined: Aug 2013
Posts: 358
Likes: 23
From: Brigantine NJ USA
I've done some reading on older posts, bushing removal sounds straight forward. I feel my upper bushing is in a condition that it does not warrant replacing it. Opinions? I've heard that the lower bushing always goes in straight, this was likely not the case for my lower bushing. I'll replace the lower bushing, will I be able to feel by hand if it's off centered and if not how can I determine if it is aligned properly?
Rick
Rick
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Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
The distributor bushings must be reamed after you install them to a measurement of 0005” to .002” larger than the main shaft diameter. Even if you are using the correct installation tool, you can cause the bushing to compress during installation. As advised above, find a good machine shop and they can handle this for you.
There is another issue that in all my years I've only seen three times...
(twice in one year, which I thought was odd).
Dave Fielder turned me onto this years ago and I believe he's published a paper on this and possibly that's where I saw it... I don't remember and I can't find it (you might check out his articles on his website and I'll keep looking too)...
A small fraction of the factory distributors were not bored inline correctly so it won't matter how many bushings you put in the car it'll still flub up.
Willcox
Edit: Here is what Dave posted about the issue above.
"Main shaft centerline to tach drive gear centerline problem- This is not a wear problem, but it is a condition that exists in an estimated ten percent of tach drive distributor housings. The problem is that the centerlines were machined too closely by Delco, causing abnormal wear between the main shaft gear and the tach drive gear."
There is another issue that in all my years I've only seen three times...
(twice in one year, which I thought was odd).
Dave Fielder turned me onto this years ago and I believe he's published a paper on this and possibly that's where I saw it... I don't remember and I can't find it (you might check out his articles on his website and I'll keep looking too)...
A small fraction of the factory distributors were not bored inline correctly so it won't matter how many bushings you put in the car it'll still flub up.
Willcox
Edit: Here is what Dave posted about the issue above.
"Main shaft centerline to tach drive gear centerline problem- This is not a wear problem, but it is a condition that exists in an estimated ten percent of tach drive distributor housings. The problem is that the centerlines were machined too closely by Delco, causing abnormal wear between the main shaft gear and the tach drive gear."
Last edited by Willcox Corvette; May 7, 2018 at 01:22 PM.