The distributor saga continues....

Reply

  #1  
Old 08-06-09, 10:27 AM
the_tow_guy's Avatar
Group Moderator
Thread Starter
Join Date: Feb 2001
Location: SW Fla USA
Posts: 11,513
The distributor saga continues....

Many of you will remember all my hair pulling over intermittent misfires. Was convinced it was all fixed after replacing distributor with one from a performance automotive supplier (and replacing plug wires).

Well, had a very bad time Monday night/Tuesday morning; truck died 130 miles from home in the middle of the night with a car on the hook. The tow in cost me $100 more than I made on the car I was towing. Oh, well. Investigation Tuesday p.m. (after I got some sleep) quickly revealed the distributor was a goner. Wobbly shaft and rotor and cap had damage. After some switching and swapping and head scratching here's what we discovered: poor machining, apparently, of the distributor housing. This is the case on BOTH the parts house unit AND the high performance unit. The poor quality is in two areas; (1) there is a "height" problem in that when the distributor is dropped down in the hole they do not seat all the way to the hole in the intake manifold, which means when the hold-down is tightened things are being stresssed in some manner they weren't designed to be. (2) The wide collars machined into the lower portion of the shaft housings (just above the drive gear) are much narrower than a stock GM/AC-Delco unit, confirmed by dropping an old used one in. The parts house unit and the HP unit both drop right down the hole with no resistance whatsoever and when all the way down can be wiggled back and forth. The old GM unit is a snug fit in the machined hole down in the block. Apparently the combination of sloppy fit in the block bore and the height issue combine to cause premature failure of the shaft bearing (3500 miles on the HP unit).

Going to monitor to see what happens. Right now I have the old used GM unit in it and running fine.
 

Last edited by the_tow_guy; 08-06-09 at 03:29 PM.
Sponsored Links
  #2  
Old 08-06-09, 10:38 AM
Forum Topic Moderator
Join Date: Oct 2002
Location: Hamilton County, Ohio
Posts: 4,287
Maybe the " we recommend that you use only genuine ( your brand of vehicle ) replacement parts" is more than just sales hype.
 
  #3  
Old 08-06-09, 10:53 AM
the_tow_guy's Avatar
Group Moderator
Thread Starter
Join Date: Feb 2001
Location: SW Fla USA
Posts: 11,513
Would seem so in this case. I'm having a hard time getting my brain around the fact that two unrelated units would exhibit the same fatal dimensional faults. My only guess is that somebody doing the engineering read the blueprints wrong. I didn't have a micrometer handy to measure the difference on the machined collars, but I may pull things apart later to make a definitive comparison in dimensions. I'm thinking something in the several thousandths error in size. I won't be able to mic the HP unit because it's sitting in my truck boxed up for Fedex to go back to the company for a refund, but the parts house unit I still have and the slop was similar so it should make a good comparison. Would be hard to accurately measure the unit "height" unfortunately, so I can only go on the basis of the gap remaining when the units are seated.
 
  #4  
Old 08-06-09, 10:55 AM
Gunguy45's Avatar
Super Moderator
Join Date: Dec 2007
Location: USA
Posts: 20,785
What brand was the HP unit...something like Accell or Mallory?

And hey...seems like I was kinda correct on my guess in the earlier thread...think I said worn cam gear didn't I? And part of the problem was down NEAR the cam gear..so I was close...lol
 
  #5  
Old 08-06-09, 11:10 AM
the_tow_guy's Avatar
Group Moderator
Thread Starter
Join Date: Feb 2001
Location: SW Fla USA
Posts: 11,513
Yeah, you were in the ballpark sorta.

My tow buddy who gives me needed technical support was opining that there was a problem at the junction of the oil pump shaft and the extension shaft. What he was thinking (and tech support at the HP supplier was maybe leaning that way) was that when the original failed distributor was pulled (the one with 300,000 miles on it) the two shaft halves had partially uncoupled and subesquently something was keeping them from re-seating. This idea went out the window when I dropped the used GM unit in and it went clear to the manifold surface.

Didn't want to bash either of the suppliers. The HP supplier is mentioned in my earlier thread.

I'm thinking I might unbox the HP unit and mic it before shipping it off. I would like to do a three-way comparison with the parts house unit and the old GM.

Stay tuned.
 
  #6  
Old 08-06-09, 11:36 AM
Gunguy45's Avatar
Super Moderator
Join Date: Dec 2007
Location: USA
Posts: 20,785
Well, if the GM unit isn't moving..maybe you'll be good for another 300K.

Didn't realize Jegs had their name on units...thought they were just a supplier. No bashing..same unit is probably running in thousand of other vehicles with no problem.

Its just you tow guys abuse the poor things...lol.
 
  #7  
Old 08-06-09, 01:36 PM
Member
Join Date: May 2006
Location: Iowa!!!!!
Posts: 3,670
To venture a guess, I would say you have something other than the distributors at fault - probably cam, cam gear, or cam bushings.

You have had too many distributors mess up.
 
  #8  
Old 08-06-09, 01:56 PM
mattison's Avatar
Member
Join Date: Mar 2003
Location: Cinti, OH
Posts: 5,549
I think it's internal also. I've used Jegs branded stuff many times without an issue one.
 
  #9  
Old 08-06-09, 02:18 PM
pugsl's Avatar
Forum Topic Moderator
Join Date: Mar 2008
Location: Raleigh, NC
Posts: 7,631
Back to basics, Did you line up the oil pump drive??? If not limed up the dist. will not drop all the way down. Been many years since I did one but think it looks like a screwdriver slot in bottom of distributor.
 
  #10  
Old 08-06-09, 03:28 PM
the_tow_guy's Avatar
Group Moderator
Thread Starter
Join Date: Feb 2001
Location: SW Fla USA
Posts: 11,513
Yes, pump drive is fully engaged. I'm afraid I'm not buying the internal problem angle. For instance why would two separate aftermarket units wiggle back and forth and have a lot of tolerance down in the bore in the block, while the old, used GM unit be a nice snug fit? And what would stop the aftermarkets from fully seating against the intake manifold, but the old, used part slide right down flush? Doesn't make any sense.

The real test is going to be practical application. I wore out the first parts house replacement in 8,000 miles; the Jegs went 3,500. Depending on how busy I am I can do 8,000 miles in about 3 months.

BTW I mic'd the two aftermarkets in the area of the machined collars near the drive gear and both were identical, 1.245". I'm thinking the bore down in the block could be 1.25" (nice round number - 1 1/4"). If so, the wiggle room would be .005. Sometime this weekend I'm going to yank the installed old used one out and mic that dimension; I'm really curious what it's going to show.
 
  #11  
Old 08-06-09, 05:35 PM
Member
Join Date: May 2006
Location: Iowa!!!!!
Posts: 3,670
You're saying the bore tolerance inside the block is .005 or this is what you would like for it to be with the 1.245 collar size?

Did you check the upper casing diameter against the bore in the intake?

The question is how do you have two distributors from two different aftermarket sources with the same problem, measuring identically and both failing prematurely?

I'm wondering if the pump drive isn't holding the distributor up - either the drive is too long for the distributor or the bore in the distributor is too shallow? If that was the case, when you tighten the distributor, you'd have a vertical thrust on the distributor bearings which is taking them out ahead of time. That would account for just about all your headaches.
 

Last edited by marbobj; 08-06-09 at 06:16 PM.
  #12  
Old 08-07-09, 05:24 AM
the_tow_guy's Avatar
Group Moderator
Thread Starter
Join Date: Feb 2001
Location: SW Fla USA
Posts: 11,513
The protruding oil pump shaft was probably numero uno on the list until I dropped in the old, used GM unit and it went right down to the manifold. The vertical thrust is at the head of the causes list. I have no way of measuring the bore down in the block; I was just opining that 1.25 would not be an unusual dimension. Any Chevy experts with an idea on that?

Definitely going to pull the old used one this weekend and mic it for comparison.
 
  #13  
Old 08-07-09, 06:05 AM
Member
Join Date: May 2006
Location: Iowa!!!!!
Posts: 3,670
It sounds like we're on the same page.

I don't believe the drive is too long if everything is OEM, but the matching of it to the aftermarket distributors is the issue. Measuring the bottom of the bore of the drive in the aftermarket job to the top of its mounting base and comparing that to the same on your OEM will probably produce a difference.

How much up from flush is the distributor on the aftermarket jobs?

Obviously the best and easiest fix is to put an OEM back in it. If I was going to use an aftermarket, for whatever reason, though, I'd consider modifying the pump drive bore (if that's the culprit) to get the fit I needed.

*** The rounded off 1.25 = engineers aren't inclined to round anything off - the bigger the number the important it sounds.
 

Last edited by marbobj; 08-07-09 at 06:23 AM.
  #14  
Old 08-07-09, 10:35 AM
nightowlpunk's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Nov 2004
Location: Connectitax
Posts: 456
Any yards in your area specializing in commercial vehicles like yours? I was thinking you might want to see if you can find a OEM distributor that way depending on how many miles were on it.
 
  #15  
Old 08-07-09, 12:08 PM
Member
Join Date: Dec 2003
Location: Indiana
Posts: 312
TG - As an engineer, my guess would be that your aftermarket parts came from the same (overseas) manufacturer regardless of the distributor. I would also guess that they were reverse engineered as opposed to being built from O.E prints. Much harder to get accurate mating tolerances that way.

marbobj - I design machines for a living, and we absolutely LOVE nice round dimensions. It makes stack-up calculations much easier, and anything off nominal just adds cost.
 
  #16  
Old 08-07-09, 12:19 PM
Member
Join Date: Jun 2008
Location: Eastern Georgia
Posts: 486
An aftermarket dist. could have a taller deck height to accomadate different dimensions in aftermarket blocks, intakes,oil pumps, oil pump shafts or what have you. If I recall correctly I have seen a notice in the box with a replacement dist to check the flange to intake dimension( installed height) and shim accordingly to avoid downward pressure on the oil pump drive caused by the dist. hold down. Some Mallory distributors even have an adjustable flange to accomadate this adjustment. Just my 2 cents.
 
  #17  
Old 08-07-09, 03:16 PM
Member
Join Date: May 2006
Location: Iowa!!!!!
Posts: 3,670
With everything on the table now, TG, it really sounds like the problem is the pressure coming up from the pump drive. The thing that sounds little odd is the fit of the upper and lower parts of the distributors in their respective bores. Is there a chance those fits just seemed loose because the distributor was raised out of position by the drive?
 
  #18  
Old 08-08-09, 06:57 AM
the_tow_guy's Avatar
Group Moderator
Thread Starter
Join Date: Feb 2001
Location: SW Fla USA
Posts: 11,513
Okay, some more info. Got back from my morning walk this morning sweating like crazy and figured good a time as any to pop out the old used one and take the mic to it.

And the answer is (drum roll please) apparently the GM engineers do like round numbers; the machined collar down by the drive gear mic's right at 1.249-1.250 on the old used dist just as I was surmising. Don't have the tools to mic the bore of the hole, but it is such a nice fit it would have to be right at 1.250. This means that both of the aftermarket units have roughly .004" more wiggle room in that area. That's not a LOT of slop, but I would have expected it to be a little closer to what is apparently the factory spec. That and the installed height have to be getting together to do bad things. For the height issue I'm thinking the location of the roll pin hole in the shaft must be off slightly, but I really have no great desire to dismantle old and new to compare.

There was no reference to checking installed height with either unit; the Jegs unit came with a rather long instruction but was all generic dist installation stuff (I just dug around on my workbench to find and re-read it). Hadn't thought about it until shimming was mentioned that it could be a dimensional problem on the remann'ed engine, BUT the dist that originally failed had about 300,000 miles on it - 100,000 or so on that engine - and it did not have a height problem AND neither does this old used one I now have bolted in. Crazy...... Beer 4U2Beer 4U2Beer 4U2
 

Last edited by the_tow_guy; 08-08-09 at 07:24 AM.
  #19  
Old 08-08-09, 08:13 AM
nightowlpunk's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Nov 2004
Location: Connectitax
Posts: 456
Isn't techonogy wonderful? Almost makes you yearn for the horse and buggie days doesn't it?
 
Reply


Thread Tools
Search this Thread
Display Modes
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On



All times are GMT -7. The time now is 04:12 AM.