I have the distributor of my L72 (transistor ignition) out and apart to clean and recondition it. I have a question about what are the proper centrifugal weights for this car.

First some background:
My distributor is a Delco #1111093 which seems to be the correct one for an L72 with Transistor Ignition. I found the stock advance curve published on Dave Fiedler's website. However the advance curve I measured on my car (before I removed and disassembled the disty) looks very different.


the pink curve is the one I measured on my car using my dial-back timing light and the dark blue points are stock spec from Dave Fiedler's website. Aside from the obvious (and easily modified) difference in spring stiffness, the curve on my car has an upward curvature, but the stock curve has a downward curvature. At first I suspected I may have the wrong shaft/advance cam. But then I started to suspect I might just have the wrong weights with a wrong profile on the weights' lever arm.




these are the centrifugal weights that were on the disty:


this is a closer view. The arm seems short and also rounded over. Most photos I've seem show a straighter lever arm on the weight where it contacts the football.


These are the weights from the Mr Gasket kit. I had to notch them slightly so it would close all the way. But otherwise the arms are longer and more straight.



I can see wear marks on the side of the football that extend farther than the lever on the weights that were in the car, so I suspect the weights I have are not the original ones. The lever on the Mr Gasket weights are about the same length as the wear marks on my cam.


My question is, what are the advance weights for this engine and disty supposed to look like? I've read that there were several different part numbers, but all the parts shops sell one weight set for all years of C2. Also, I've seen comments on this forum that the Mr Gasket weights are not hardened and will wear quickly. Are the ones for sale from the various parts houses (they all look like the same part) better? Or do I need to scour eBay for NOS ones?

If it is helpful for diagnosis or getting the right weights, the rotor arm is stamped "734" and the advance cam is stamped "94"




Finally, another related question:
what is the spec for side-to-side play at the lower bearing? I can find the end-play spec in the service manual but not a spec for side to side play. Forum posts and rebuild instructions i've found just say if wear is "excessive" the bushing should be replaced. Mine has about 0.005" (5 thou) of side to side play, which seems like it might be acceptable, even though the shaft shows some grooving. I suspect a P.O. may have re-bushed it already, because I also found the tach drive gear replaced (with a steel one, I am putting a bronze one back in).

 

I should add: I'm not looking for NCRS-perfection for the weights, just trying to find a profile close to what the factory intended, and a quality part that will last a reasonable time.

 

The lateral bushing movement figure I’m aware of is .006” max. So you are marginally OK. I think Mopar recommended a new bushing if wear reached .008”.

 

I cannot find the lateral shaft tolerance in the Chevrolet Chassis Service Manual. As the lateral movement affects the dwell on the point type distributors, and the air gap on the electronic distributors, I would assume the tolerance would be similar to the allowable tolerance for point adjustment with a feeler gauge to obtain optimum performance. However, I know of no way to test it. The lateral movement Dan provided seems reasonable. However, depending on the depth of the wear on the shaft, replacement of the shaft and bushings may be considered. Due to the limited usage of our 'Classics', replacement of just the bushings might be considered.

The GM specifications for the mechanical advance specifications are provided in both the Service and Overhaul Manuals. However, it only provides 0 deg at 900 rpm and 30 deg. at 3800 rpm. Your distributor 1111093 has 34 degrees of mechanical advance built in with the stock springs, weights, and advance bushing. These mechanical advance specifications don't follow your chart above. I would have expected the distributor to be marked '730' in lieu of '734', but don't find it in the specifications. The books only list a 1111258 for a 427 / 435 hp. I don't believe it to be a problem and GM data may not always provide everything. Your springs appear to be lighter than the stock GM springs which of course provides faster mechanical advance and the curve you found prior to removing the distributor. Of course this could only be a problem if you are encountering performance issues or detonation under load. I personally don't like using two different strength springs due to off balance when the weights expand due to centrifugal force, but doubt there is any short term problem created.

I have found the Mr. G weights are soft steel and will wear rather quickly, however, for a seldom driven show car, they will probably last for the life of the car. I have several GM distributors and they have hardened steel weights. I have never found them with significant wear after 100K miles. The suppliers only offer 'one size fits all' weights and shafts with little information on the mechanical advance specifications nor the football configuration. I think it is a 'crap shoot' as to what you receive.

Just my thoughts, they and a couple of bucks will get you a cheap cup of coffee

Ron

 

Your test proves you do not have the original factory advance springs so I suspect the weights were also changed due to color. Only two aftermarket kits I know of are Mr. Gasket and Moroso. The part number I found for OEM weights is 1881371.

 

Thanks all for your inputs!

(1) I realized that those of you who quoted part numbers are looking them up in a parts book. So to reduce my helplessness, I acquired one (to add to the growing mountain of manuals and documents I've accumulated for this car!). As some have noted, there seem to be some gaps in the parts listings, the '66 427 "w/ SP. H/PERF" and "w/ T.IGN." seem to be missing or excluded for many of the parts.

(2) I found some NOS 1881371 on eBay and ordered those so I have some durable weights.

(3) with some more careful measurements of side play, I found the non-worn axis only 0.002", and the worn direction 0.005-0.007" of play. I also decided that I'm not so happy with the amount of grooving on the mainshaft even though the diameter is only worn down by 0.001". In the interest of "doing it right", I ordered a new T.I. shaft from Chicago Corvette and new bushings. Once they arrive I'll have to hunt for a machine shop to do the align honing for me.

I considered slapping it back together and getting the car running, but decided instead to make it right and not have to do it all again.

thanks again!

 

Have you seen this document?


http://www.masacc.org/Documents/T_%2...%20Fiedler.htm

 

When I rebuild my 1967 327/350 HP distributor a few years back, the car mileage was about 20,000 miles. My distributor side play at this time was 0.002-0.003 inches. I chose to leave it alone and not try to improve it........which worked out well for me. I THINK the "as new" spec is 0.0015 to 0.002 inches for each end/bushing.



Larry

 

Yes, that's also published on Dave Fiedler's TI Specialty website. I got the stock advance curve data (blue dots in my original plot) from the same website. Very helpful resource.
That doc shows new bushings should have 0.0005-0.002" clearance after reaming, but didn't specify a service limit for wear of used bushings. (half a thou of clearance seems a bit risky, but I guess the heat will come from the outside in so the shaft may never be hotter than the housing)

 

The only way to answer your question is to get a Distributor Machine and then start changing weights and springs to your hearts content - then YOU WILL LEARN something : cause and effect instead of surmising
There are many many different factory weights - you need resources : old distributors for parts.
That is HOW I BUILD DISTRIBUTORS.
Nobody on here will be able to give you a definitive answer unless by some stroke of coincidence they had your EXACT setup - shaft , weights , springs etc - too many variables / combinations !
HAVE FUN LOL