I’m replacing a defective vacuum advance on my 69 427 390 HP (L36). The one that is on the distributor now is marked MS 360 12. Is this an original part number or a service replacement.
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Jeff------
GM number 1115360, stamped 360, is the original vacuum control used for the distributor originally installed for your L-36 application. It's impossible to say whether the vacuum control you described is actually the original unit installed on your distributor because the 1115360 was available in SERVICE from GM for many years and, thus, could be a replacement. Regardless, it's the correct vacuum control, originally installed or not.In Appreciation of John Hinckley -
There is no longer an exact replacement for the MS 360 12. In case you are not aware, 360 is the last three digits of the GM long part number and 12 is the maximum crankshaft advance, which is provided at 12 or more inches Hg manifold vacuum.
SMP used to provide an exact replacement, but it was discontinued at least 15 years ago. The closest is now the B26, NAPA VC-1765, which can be cross referenced to other "brands" like BWD, Standard, Delco, and most others.
This part starts to add advance at the same 6" Hg as the 360 12, but max advance is 16 @ 12" rather than 12 @ 12". This might cause some driveability issues since in my experience early closed chamber big blocks don't like as much low speed, low load advance as small blocks with similar valve overlap. Go ahead and install it and see what happens.
To install with the distributor in the engine, pump down the can as far as possible or manually force the pin to the limit. This will expose the forward mounting screw. Then remove the screws and carefully wiggle the pin out of the breaker plate. Pump down the new VAC to pull the pin to max advance and install.
Duke
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Thank you gentleman, very informative. I have the NAPA replacement on the way and it will be fun to check the advance at full idle vacuum. When the original was working I would get 12 degrees right on the nose at idle vacuum. I guess if I get 16 from the NAPA replacement and it becomes a problem I could fudge the initial back 2 degrees to split the difference. Also, I assume there is probably an adjustable advance available for this distributor. Either way, it’s going to be way better than the “no advance” I’m getting now😀. It failed slowly so the diagnosis was not apparent. The car would run and idle well, then it wouldn’t, with no rhyme or reason. I assume the diaphragm was slowly tearing, and would sometimes pull some advance and then other times not. I have the distributor out so replacement will be easy.Comment
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Duke,
For the L36 engine, the factory spec (factory service manual) for vacuum advance says 0@7”. So I assume the advance starts around 8”. The VC 1765 is in at 6”, whereas the VC 680 (B1) starts at 8”. Both provide a max advance of 16 deg, though the VC 680 requires a higher vacuum to reach max. However, the L36 should make plenty of vacuum to be 2” above that. It seems the VC 680 would be closer to the stock spec, but I assume there are reasons you prefer the 1765?Comment
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Zero at whatever inches Hg means it starts at or slightly above this, like a few tenths above, certainly no more than one inch. I don't have any '66 or '67 service manuals, but according to the Vehicle Information Kits from gmheritage.com the '66 L-36 VAC is start at 6", 15 @ 12" max and '67 is start at 7", 12 @ 12" max. I believe the '66 L-36 VAC ID is MS 360 12. I'm not sure of the ID number for the '67.
The VC 680, is a boat anchor, repeat BOAT ANCHOR! I don't recall the GM number but is was a totally inappropriate substitute as part of a parts consolidation effort decades ago and the aftermarket followed suit. It may not provide full advance until as much as 19" so It won't even pass The Two-Inch Rule for a base small block. NEVER order a VAC based on what some catalog says. Since a L-36 with OE equivalent cam and manual transmission has typical idle behavior in neutral of 600 @ 15", the VAC must provide full advance at no more than 13".
There are currently only three replacement VACs that are a reasonable fit for any OE small or big block from the era that is either OE full time vacuum advance or converted from ported that is typical of all emission controlled engines, but also includes '63 L-84, and all L-72, '67 L-71, and all '66 and '67 engines in cars delivered in California With K-19 Air Injection Reactor.
One of the following three available VACs, big or small blocks, single point or TI ignition will the least aggressive to meet the Two-Inch Rule for any OE cam or any aftermarket cam except radical "racing cams" that have no business in a road engine.
B-22: start @ 8", 16 @ 15"
B-26: start @ 6", 16 @ 12"
B-28: start @ 4" 16 @ 8"
These aren't real precision devices, so figure plus or minus 1 to 2" Hg and +/- 2 degrees max advance from spec, and in recent years many have been reported as significantly out of spec, which is why I recommend buying locally and bringing a vacuum pump to check that it's reasonably within spec before you accept the part and pay.
The Two-Inch Rule says the VAC should provide full vacuum advance at no less than 2" less than idle vacuum. You might have to think about this a little, and here are some examples of typical idle vacuum for OE engines with manual transmissions and no AC.
18" @ 500, base small blocks
15" @ 600, L-36/68
14-15" @ 750, L-79
14" @ 900 L-72/71
12" @ 900 small blocks with Duntov or LT-1 cam
10" @ 900 small blocks with 30-30 cam.
CAVEATS: If an automatic transmission the idle speed/mixture should be set up in Drive (set parking brake securely and chock wheels), and if equipped with AC the compressor should be engaged, either transmission type. Measure and document vacuum. It will be lower than the same engine at the same speed with a manual transmission in neutral, and it may be best to use a higher idle speed. The AC compressor puts a considerable load on the engine at idle, so set idle speed as low as you find acceptable, and then with the compressor off the engine will likely idle 100-200 RPM higher. And if you drive to higher altitude idle speed and vacuum will fall and require adjustment. Always measure your idle vacuum for whatever configuration you have and use the Two-Inch Rule to choose the least aggressive from the three listed, which is the "best fit".
So, if anyone is game use The Two-Inch Rule to pick the best fit VAC from the three choices for any or all of the above cases with manual trans and no AC, and I'll let you know if you're correct or not.
Finally, OE specs are a good place to start, but who knows what camshaft or spark advance map your "original appearing" engine might have. Over the decades most were modified, so you have to measure the spark advance map and idle vacuum to understand YOUR engine. The above listed idle behaviors are typical. My recommendation is to set idle speed "as low as possible commensurate with acceptable idle quality". This is subjective, so it's a matter of personal choice. Especially with medium to high overlap cam engines some guys like a low, lumpy, idle speed where the engine feels like it'a about ready to stall. I prefer a higher idle speed that still has some lope, but isn't lumpy and rough. A higher idle speed also helps get the car rolling easier from a dead stop, especially with taller axle ratios. Your car, your choice!
DukeComment
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Thank you Duke. The engine makes 15” at idle, and I have both the 680 and 1765 units. I’ll use the former to secure my boat, and install the latter. I have a good vacuum pump with a gauge so I’ll compare the two for fun.
I also disassembled the distributor (OEM electronic) and cleaned things up. However, the drive gear to housing clearance was set very loose from the factory - about .048. There is nothing about that in the service manuals but from what I have read .007 would be a good target. But, since the action of the cam gear pushes the distributor main shaft upward, I am a bit concerned that changing the shimming might move the gear out of its established wear pattern.Comment
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The 1963 Corvette Shop Manual states in bold face type that end play should be two to seven thousandths, but most from the factory are several times that as you found. I assume from what you say that you have a TI system. I've shimmed both point-type and TI distributors down to within the spec range, and I like the lower end.
When checking timing, especially using the WOT advance method where you set it in the range or 36-40 at an engine speed above the speed of max centrifugal advance with the VAC hose disconnected and plugged the balancer notch often looks slightly blurred. This is caused by "spark scatter" due to the main shaft bouncing up and down at high frequency within the end play, and since the gears are helical, the timing varies a degree or two.
Shimming up the end play to the tight spec reduces this scatter, which reduces normal cyclic variation of the torque provided by each cylinder that makes the engine run smoother.
Since these gears are not highly loaded It's very unlikely that eliminating the end play will cause wear problems. GM used to sell shim kits with sizes down to five thou, but they're now only available in the aftermarket. There are a couple of brands and a web search should reveal available brands and sources.
Since the distributor is out I suggest you do a "blueprint overhaul", which is easy as long as the housing bushings don't need to be replaced (They are okay if there is virtually no side play.) The disassembly procedure is in the 1963 Corvette Shop Manual and '66 and later COMs. Be sure when you install the gear with the dimple on the side is pointing the same direction as the rotor tip. This is not stated in the procedure, but can lead to all kinds of issues if it's not installed correctly.
Carefully remove the tiny wavy c-clip that secures the stationary pole piece (breaker plate on a point distributor) and also carefully remove the grease well seal without damaging it so it can be reused.
While you're at it buy a Mr. Gasket 928G spring kit to bring the lazy OE centrifugal in earlier. I believe the black springs are lightest, but some say the gold are. Depending on your actual CR and available fuel octane you should be able to bring it all in by 3000-3500 without detonation.
Another issue is distributor installation, which usually begins with improper removal. Prior to removing, the balancer notch should be set at whatever initial timing number (NOT TDC) you use, BTC #1 compression stroke. If you did't do so, do it now. Also make sure the wires are indexed on the cap per the CSM. Proper distributor installation and static timing the engine have been described here and on the Corvette Forum by both me and Lars many times.
https://www.forums.ncrs.org/forum/te...print-overhaul
https://www.forums.ncrs.org/forum/te...tor-spring-kit
DukeComment
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Thanks Duke, great explanation about why shimming is important. I will shoot for .005”. I do see that spec in the 63 manuals but I’m working on a 69 and I could not find that in either the 69 service or overall manual… I may have missed it. The 69 service manual describes the distributor disassembly but has very little information. Reassembly is described as “reverse order of disassembly”, and that’s all you get. Earlier manuals were better!
The bushings are good with no side play, and all the other parts look good. One more question, do you have a favorite lube for the grease well? A moly assembly lube maybe?Comment
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Jeff----
I'll mention something I've mentioned before:
The Delco-Remy spec for end clearance of all tach drive distributors is 0.030"-0.056". That spec is found on the original drawings for the distributors. This is why clearance in that range (usually in the middle of that range) is found on original, long-serving distributors removed from Corvette engines. It's NOT because they have worn to that clearance.
I have several NOS Corvette tach drive distributors [none for sale]. All have clearance just about in the middle of the aforementioned range.
Note that the specified range has both an upper and LOWER limit so it's not as if this range was established primarily to provide manufacturing "flexibility". If that were the intention, they could have provided even more manufacturing flexibility by specifying the range at 0.001"-0.056". But, they did not.
Personally, I follow the Delco-Remy spec. I have no idea why the 1963 CSM specifies 0.002"-0.007". However, as you have found (and as I have also found from prior research) that spec is not found in later editions of the CSM. The fact that it was apparently dropped may, itself, imply something.In Appreciation of John HinckleyComment
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This is a very good point Joe, and it’s been stuck in the back of my mind as I approach this. My distributor is a tach drive and TI and was factory shimmed at .048”. Almost right in the middle of the Delco-Remy spec. I would also think that at this point in production the factory would have it right. And as you say, it’s definitely not wear, it was built that way. I too have had multiple tach drive distributors with this larger tolerance, and have never seen an original tach drive distributors shimmed in the .002-.007 range. I remember making the decision on my 63 and 64 restorations to leave the clearance at the factory delivered tolerance of roughly .05. My intention was to do that this time also… this thread started about the vacuum advance and worked its way into the other aspects of a distributor rebuild, which is thought provoking.
A closer look at the CSM for 69 (which covers multiple cars) reveals that the .002-.007 spec IS in the 69 Manual, but only for the standard non-tach drive breaker point type distributor. When you get to the “Corvette Distributor” where the tach drive is mentioned, then there is no mention of a shim spec. However, the 1963 manual, which is Corvette specific, does specify the closer shim tolerance. With the spec sheet from Delco-Remy being very clear, it makes you wonder if the 63 CSM has this language left over from other manuals that are not Corvette specific, an artifact as it were.
This distributor is from a very original 69 L36, the engine has never been out of the car. The distributor looks to be original, and short of the vacuum advance failure, has no indication of wear or any other failures. The main shaft and cross shaft look perfect as do the TI components. I’m thinking I may reassemble it the way it came. It worked fine for the first 93,000 miles.
Thanks to everyone who responded, a lot of good information here.Comment
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The 1963 Corvette Shop Manual contains ALL "service" and "overhaul" information including the distributor, but beginning in '66 service and overhaul procedures were broken up to two different books each including the information for all passenger car model lines. If these manuals include a spec for distributor end play, it's probably in the overhaul manuals.
It's interesting that the 1963 Corvette Shop Manual has the two to seven thou spec, but all that I have seen have had the typical sloppy end play. I've heard that FI distributors have much tighter end play, but I've never had one out for overhaul to check.
I'm sure we've all heard the term "blueprinting", which means we tighten up the OE tolerance range or perhaps even establish a different tolerance range based on experience. The '63 Corvette Shop manual specifies main bearing clearance tolerance in the range of .0008-.0034. I'm sure no precision engine builder, amateur or pro would accept less than one thou on the mains. ( I recommend 1.5 to 2.0 thou.) Likewise, I don't accept sloppy end play on the distributors due to the spark scatter it causes, and it's interesting that GMPD offered shim kits that included five thou shims that you might need to get into that five thou tolerance range.
I've been setting up distributors for decades, always shooting for the lower end of the range, and I've never had a problem with this on my SWC or heard of a problem from any other's whose distributors I have set up.
All I can figure is that the sloppy tolerance on most original distributors was simply a matter of "manufacturing convenience" and saving time/cost in the manufacturing process. 25 thou is quite a bit of tolerance stack up for the range of distributor housing bore and mainshaft length, and I'm sure distributors were assembled without checking end play. I've never seen a shim on an OE assembled distributor, just the thrust washer.
One other thing I'll mention is that since Corvette tach drive distributors all have cast iron housings, and cast iron has only a slightly different thermal expansion coefficient than, steel the difference between cold and "hot" clearance is virutally nil, so the two thou minimum is okay, but I've never set up one with less than three.
I don't recall when Chevrolet went to aluminum distributor housings for non-tach drive distributors, but in this case I would adjust this tolerance range by adding the length of the housing bore times the difference in thermal expansion coefficients between steel and cast aluminum, which would likely add a few thou to the tolerance range.
DukeComment
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Duke
All good stuff! You would think the specs would be in the Overhaul manual, but in the 69 manuals the distributor tear down is in the service manual and the Overhaul manual is silent on the distributor. I did restore a 63 fuel Injector and distributor for my SWC (see glamor photo) and I remember it having about 035” to start with. Thanks again for your help.
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Jeff,
With 93000 miles I would inspect carefully the area where the tach gear rides against the inside housing for wear. If the gear gets out of line with the main shaft gear you will be replacing both parts.Comment
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