Delco single point distributor blueprint overhaul
04-19-2018, 03:33 PM
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Delco single point distributor blueprint overhaul
Since we get occasional distributor questions I copied and pasted the following from a Dec. 2012 thread I started on the NCRS TDB after completing a blueprint/overhaul of a '65 L-79 distributor.
Delco single point distributors from '57 to '74 are assembled from a family of parts. There were many different mainshafts due to different cams ("football") brazed to the top of the shaft and this along with the weights and springs determine the shape of the centrifugal spark advance curve. Also, there are different breaker plate cam assemblies because the length of the "slot" is a determining factor in total centrifugal, and different VACs are used.
The disassembly/assembly procedure is in the 1963 Corvette Shop Manual and Chassis Overhaul Manuals beginning in 1966. I highly recommend reviewing the procedure before proceeding, but keep in mind that the write-ups don't mention proper indexing of the gear. If the camshaft is properly manufactured the dimple on the side of the gear MUST be inline with the rotor tip or you likely won't be able to achieve the proper initial timing before the VAC hits the inlet manifold or coil bracket, but some aftermarket cams may require the dimple to face opposite the rotor tip.
This is also a good time to optimize the spark advance map, and starting points for all OE engine configurations are in my tuning seminar that is on this forum (Search for threads started by me, there aren't many.), and also download the breaker points article to select the correct type for you engine configuration and driving habits.
A couple of issues I forgot to mention in the original NCRS article... The NAPA-purchased grease seal was just a flat nylon washer not the "saucer shaped" OE seal, so I carefully removed it for reuse. Similarly the new breaker plate retaining snap ring was flat, not "wavy" like the OE, so we reused the original.
The distributor is the most overlooked component on the engine, but proper function (along with an optimized spark advance map) is critical to achieving best across-the-rev-range output and maximum fuel economy.
-------
I did a "blueprint overhaul" on a '65 L-79 1111087 single point distributor, today, for a local chapter member. Jim purchased the car about a year ago - 90K miles, and it had been stored for quite sometime, but was unmolested.
One glitch was the breaker plate ground wire broke off the terminal that is secured with the forward VAC mount screw as I was removing the screw, and the wire was rock hard. We had to improvise a fix. I had some black 12 gauge wire and some generic terminals. Don made a new lead using the fresh wire with a generic lug on the end that's secured with the VAC mounting screw. He was able to loosen the crimp on the terminal that's soldered to the breaker plate and recrimp the new wire.
My advice: Buy a NAPA LW42 breaker plate ground repair wire and install it now. It costs about two bucks and has indexed terminals on both ends. It can be installed with the distributor in the car between the forward VAC mounting screw and a breaker point mounting screw. The ground wire is constantly flexed by VAC action, so it work hardens and will eventually fatigue and break, which can leave you stranded. In a pinch, a test lead with alligator clips can be secured to the breaker plate through the window to a ground. Disconnnect the VAC and plug the hose so the breaker plate doesn't move around and knock the test lead off. This will get you home.
There was virtually no detectable shaft side play, so the housing bushings were not worn. End play was only .043" (I typically recall .060-.090"), and there was some visible spark scatter. The football was "71", cam assembly "730 CCW". The weights had no numbers, but looked OE, not aftermarket. They had a hole drilled on the big ends as in the post #42 photo. ( http://www.ncrs.org/forums/showthrea...-a-Distributor )
There was a "56" on the inside base of the housing under the breaker plate, and an O or zero externally with what looked like a square symbol.
The breaker plate fit nice and snug with no wobble, and the advance bushing was okay. The upper bushing grease well had some wet solid flakes, but at least the tach drive gears still had grease and were in good shape.
Once we got everything cleaned up I packed GM 1960954 distributor and starter motor grease into the upper bushing grease well, and also used it on the tach drive gears. We replaced the OE 236 8" VAC with a B26 (12") VAC, and Jim had already installed the gold springs from the Mr. Gasket 928G kit that we believe are the lightest. (BTW, compared to these the OE springs look like they came off a dump truck.) A .030" and .010" shim from the shim kit brought end play down to the .004-.005" range. Jim had already installed the Echlin CS7860 32 oz. point set with the trick little lubricator wick, and I dabbed on some of the 1960954 grease. This is exactly how I recommended setting up a L-79 distrubutor in my San Diego presentation.
It fired right up after having to tweek the oil pump shaft with my special tool (a Home Depot paint mixing stick
) to get the distributor to seat. Centrifugal starts at about 700 and is all in at 3000-3500. The L-79 centrifugal is 30 maximum (at 5000 OE), but it's non-linear, so most of it is in by about 4000. Of course, the gold springs quicken it so it's nearly all in at 3000.
I set the total WOT advance at 39 @ 3500 and the initial is 9. The timing mark is dead steady at any RPM. That's what tight end play does - gets rid of the spark scatter that the sloppy OE assembly produces, which can result in a noticeably smoother engine. Idle mixture screws are out one turn from the seat, and it pulls 14-15" at 750, so I have no reason to believe is doesn't have the correct L-79 cam.
Given the car's history I thought the heads may have never been off, but the gasket measured .036", so they have been off. In my brief road test I couldn't get it to detonate lugging up some steep hills. Start up from a dead stop with minimal clutch slippage was smooth with no stumble, and it was tractable down to less than 1000 revs. Low end torque was a little weak (as expected compared to the 300 HP engine), but it was strong and linear from 3500-6000 even though it might have lower than OE compression ratio. Jim's next step is to do a fuel test to see if it will operate detonation free on 87 PON regular or 89 PON mid-grade.
Jim and Don reported that the engine ran smooth and strong on the trip back up the world's busiest freeway to the San Fernando Valley.
Duke
Delco single point distributors from '57 to '74 are assembled from a family of parts. There were many different mainshafts due to different cams ("football") brazed to the top of the shaft and this along with the weights and springs determine the shape of the centrifugal spark advance curve. Also, there are different breaker plate cam assemblies because the length of the "slot" is a determining factor in total centrifugal, and different VACs are used.
The disassembly/assembly procedure is in the 1963 Corvette Shop Manual and Chassis Overhaul Manuals beginning in 1966. I highly recommend reviewing the procedure before proceeding, but keep in mind that the write-ups don't mention proper indexing of the gear. If the camshaft is properly manufactured the dimple on the side of the gear MUST be inline with the rotor tip or you likely won't be able to achieve the proper initial timing before the VAC hits the inlet manifold or coil bracket, but some aftermarket cams may require the dimple to face opposite the rotor tip.
This is also a good time to optimize the spark advance map, and starting points for all OE engine configurations are in my tuning seminar that is on this forum (Search for threads started by me, there aren't many.), and also download the breaker points article to select the correct type for you engine configuration and driving habits.
A couple of issues I forgot to mention in the original NCRS article... The NAPA-purchased grease seal was just a flat nylon washer not the "saucer shaped" OE seal, so I carefully removed it for reuse. Similarly the new breaker plate retaining snap ring was flat, not "wavy" like the OE, so we reused the original.
The distributor is the most overlooked component on the engine, but proper function (along with an optimized spark advance map) is critical to achieving best across-the-rev-range output and maximum fuel economy.
-------
I did a "blueprint overhaul" on a '65 L-79 1111087 single point distributor, today, for a local chapter member. Jim purchased the car about a year ago - 90K miles, and it had been stored for quite sometime, but was unmolested.
One glitch was the breaker plate ground wire broke off the terminal that is secured with the forward VAC mount screw as I was removing the screw, and the wire was rock hard. We had to improvise a fix. I had some black 12 gauge wire and some generic terminals. Don made a new lead using the fresh wire with a generic lug on the end that's secured with the VAC mounting screw. He was able to loosen the crimp on the terminal that's soldered to the breaker plate and recrimp the new wire.
My advice: Buy a NAPA LW42 breaker plate ground repair wire and install it now. It costs about two bucks and has indexed terminals on both ends. It can be installed with the distributor in the car between the forward VAC mounting screw and a breaker point mounting screw. The ground wire is constantly flexed by VAC action, so it work hardens and will eventually fatigue and break, which can leave you stranded. In a pinch, a test lead with alligator clips can be secured to the breaker plate through the window to a ground. Disconnnect the VAC and plug the hose so the breaker plate doesn't move around and knock the test lead off. This will get you home.
There was virtually no detectable shaft side play, so the housing bushings were not worn. End play was only .043" (I typically recall .060-.090"), and there was some visible spark scatter. The football was "71", cam assembly "730 CCW". The weights had no numbers, but looked OE, not aftermarket. They had a hole drilled on the big ends as in the post #42 photo. ( http://www.ncrs.org/forums/showthrea...-a-Distributor )
There was a "56" on the inside base of the housing under the breaker plate, and an O or zero externally with what looked like a square symbol.
The breaker plate fit nice and snug with no wobble, and the advance bushing was okay. The upper bushing grease well had some wet solid flakes, but at least the tach drive gears still had grease and were in good shape.
Once we got everything cleaned up I packed GM 1960954 distributor and starter motor grease into the upper bushing grease well, and also used it on the tach drive gears. We replaced the OE 236 8" VAC with a B26 (12") VAC, and Jim had already installed the gold springs from the Mr. Gasket 928G kit that we believe are the lightest. (BTW, compared to these the OE springs look like they came off a dump truck.) A .030" and .010" shim from the shim kit brought end play down to the .004-.005" range. Jim had already installed the Echlin CS7860 32 oz. point set with the trick little lubricator wick, and I dabbed on some of the 1960954 grease. This is exactly how I recommended setting up a L-79 distrubutor in my San Diego presentation.
It fired right up after having to tweek the oil pump shaft with my special tool (a Home Depot paint mixing stick
) to get the distributor to seat. Centrifugal starts at about 700 and is all in at 3000-3500. The L-79 centrifugal is 30 maximum (at 5000 OE), but it's non-linear, so most of it is in by about 4000. Of course, the gold springs quicken it so it's nearly all in at 3000.I set the total WOT advance at 39 @ 3500 and the initial is 9. The timing mark is dead steady at any RPM. That's what tight end play does - gets rid of the spark scatter that the sloppy OE assembly produces, which can result in a noticeably smoother engine. Idle mixture screws are out one turn from the seat, and it pulls 14-15" at 750, so I have no reason to believe is doesn't have the correct L-79 cam.
Given the car's history I thought the heads may have never been off, but the gasket measured .036", so they have been off. In my brief road test I couldn't get it to detonate lugging up some steep hills. Start up from a dead stop with minimal clutch slippage was smooth with no stumble, and it was tractable down to less than 1000 revs. Low end torque was a little weak (as expected compared to the 300 HP engine), but it was strong and linear from 3500-6000 even though it might have lower than OE compression ratio. Jim's next step is to do a fuel test to see if it will operate detonation free on 87 PON regular or 89 PON mid-grade.
Jim and Don reported that the engine ran smooth and strong on the trip back up the world's busiest freeway to the San Fernando Valley.
Duke
Last edited by SWCDuke; 04-19-2018 at 04:31 PM.
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SWC Tim (04-19-2018)
04-19-2018, 04:48 PM
#2
Melting Slicks
Nice write-up, Duke
I was in an old Chev dealer decades ago rootin around and found a tiny Delco pull string box of 5 of those breaker plate ground straps. I still have them around here somewhere. I suppose it was a somewhat common problem but I've only seen about three of those broken over the decades.
I was in an old Chev dealer decades ago rootin around and found a tiny Delco pull string box of 5 of those breaker plate ground straps. I still have them around here somewhere. I suppose it was a somewhat common problem but I've only seen about three of those broken over the decades.
04-20-2018, 08:43 AM
#4
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Member Since: May 2000
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Nice write-up, Duke
I was in an old Chev dealer decades ago rootin around and found a tiny Delco pull string box of 5 of those breaker plate ground straps. I still have them around here somewhere. I suppose it was a somewhat common problem but I've only seen about three of those broken over the decades.
I was in an old Chev dealer decades ago rootin around and found a tiny Delco pull string box of 5 of those breaker plate ground straps. I still have them around here somewhere. I suppose it was a somewhat common problem but I've only seen about three of those broken over the decades.
Whether you can get a LW42 repair wire or fab your own, it's an easy job to do now, which may save a breakdown down the road.
Duke
07-02-2019, 05:10 PM
#5
Melting Slicks
I'm surprised it doesn't happen more often. I'm sure many vintage GM cars still have the original breaker plate ground wires, which are now up to 60 years old.
Whether you can get a LW42 repair wire or fab your own, it's an easy job to do now, which may save a breakdown down the road.
Duke
Whether you can get a LW42 repair wire or fab your own, it's an easy job to do now, which may save a breakdown down the road.
Duke
Last edited by 74 LS4-454; 07-02-2019 at 05:15 PM.