1963 340HP distributor bushing (timing limiting bushing)

[Richard G.]

I recently rebuilt my distributor and notice there was not any busing on the small pin that limits the advance.
I know one can purchase kits with different diameter bushing to adjust the mechanical advance.
What i don't know is which diameter to use if I want the stock advance. I do suspect this distributor had one installed.
Pictured below is the bushing i am referencing. Picture is from another distributor not the 340HP one I am working on.


Looks to be rubber, some of the aftermarket ones seem to be brass or other metal.


I just read this online;
The GM distributor cams are numbered based on the timing slot in the cam , a 524 would use the last two digits "24" and be 24 degrees of spark advance with the rubber bushing.

Pictured is the backside of the stock 340HP weights.

If this is true GM was setting the spark advance at 37 degrees for the 340HP?
I should be able to confirm this with my dial back timing light.


Any advice is appreciated.
Rick

[Timothy B.]

Re: 1963 340HP distributor bushing (timing limiting bushing)

Richard,

The 524 weight base and #37 stamped weights are correct for all 1963 distributors regardless of hp rating. Go get a Mr gasket spring kit and the brass bushing will be included. swap in the light springs and they will get the centrifugal curve in by 3000 rpm's. I would not change anything else except the vacuum advance, get the control that's pulled full at approx 8" vacuum.

I don't think the stamped 37 on the weights has anything to do with the amount of advance.

[Joe L.]

Re: 1963 340HP distributor bushing (timing limiting bushing)

I recently rebuilt my distributor and notice there was not any busing on the small pin that limits the advance.
I know one can purchase kits with different diameter bushing to adjust the mechanical advance.
What i don't know is which diameter to use if I want the stock advance. I do suspect this distributor had one installed.
Pictured below is the bushing i am referencing. Picture is from another distributor not the 340HP one I am working on.


Looks to be rubber, some of the aftermarket ones seem to be brass or other metal.


I just read this online;
The GM distributor cams are numbered based on the timing slot in the cam , a 524 would use the last two digits "24" and be 24 degrees of spark advance with the rubber bushing.

Pictured is the backside of the stock 340HP weights.

If this is true GM was setting the spark advance at 37 degrees for the 340HP?
I should be able to confirm this with my dial back timing light.


Any advice is appreciated.
Rick

Rick-----

GM only used one sleeve size. It was GM #1961234 and it had an OD of 1/4". Any other sizes are aftermarket only.

The "37" on the weight represents the last 2 digits of the weight's part number----GM #1924737.

[Richard G.]

Re: 1963 340HP distributor bushing (timing limiting bushing)

Thanks for the assistance.
I have ordered the Mr. Gasket kit.
Now I know the OD could have manufactured one easily enough.
But my springs were what I though as slightly loose. Not enough to come off but the weights obviously would move some before any applying any tension to the weights. Thanks for the information on the part number suffix. I don't believe anything I read on the internet till it is qualified!
You guys are great....
Rick

[Duke W.]

Re: 1963 340HP distributor bushing (timing limiting bushing)

The '63 340 HP spark advance map was the one of the absolute worst ever configured by GM. What in the hell were they doing? The same dist. was used in the 250/300 HP engines and was somewhat okay for those configs., but totally inappropriate for the 340 HP engine.

For starters the OE 15.5" 201 15 VAC does not pass the Two-Inch Rule and causes idle stability and stalling. It was the first year they installed a VAC on Duntov-cam engines, and they really screwed the pooch. AFAIK they never wrote a TSB on the issue, but must have realized there was a problem in testing.

Assuming it still has a Duntov cam (or any other OE mechanical lifter cam) it needs a 8" 236 16 VAC, the modern replacement being B28. It is available in the Airtex brand, 4V1053. The centrifugal curve is listed in your shop manual, 24 @ 4600, which is very lazy.

I don't believe the dist. originally had a limit bushing.

For starters use your dial-back timing light to determine the centrifugal start and stop points. It may have been changed over the years.

If it's still OE install lighter springs, determine where the centrifugal is all in, then set total WOT advance a few hundred RPM above that point as high in the 36-40 range as the engine will tolerate without detonation. Keep in mind that the timing tab is not accurate, so don't be concerned if initial timing reads "high" as long as you properly set total WOT advance (with the VAC disconnected and plugged, of course) and it doesn't detonate. The ideal spark advance map for the 340 HP engine is the same as the 365/375 HP engines.

My 340 HP distributor has been set up as such since 1965. The dealer was clueless on how to correct the idle instability and stalling, and so were the so-called performance tuning "experts" I talked to. I finally figured out that if the engine only pulls 12" at idle it needs a VAC that is all in at less vacuum so the VAC is "locked" at full advance at idle. This is the rationale behind The Two-Inch Rule for selecting an appropriate VAC for any engine configuration.

That's how a 19-year old sophomore ME undergrad figured it out. The 8" VAC solved the idle stability issue, and the lighter springs that brought the 24 deg. centrifugal all in by 2350 made for noticeably better low end torque, throttle response, and fuel economy.

If you want all the whys and wherefores search the Web for my 2012 San Diego National Convention tuning seminar.

Duke

[John D.]

Re: 1963 340HP distributor bushing (timing limiting bushing)

Richard,

The 524 weight base and #37 stamped weights are correct for all 1963 distributors regardless of hp rating. Go get a Mr gasket spring kit and the brass bushing will be included. swap in the light springs and they will get the centrifugal curve in by 3000 rpm's. I would not change anything else except the vacuum advance, get the control that's pulled full at approx 8" vacuum.

I don't think the stamped 37 on the weights has anything to do with the amount of advance.

Tim, The 524 is a service replacement distributor cam. Yes it's listed in the famous Chevy parts book but that info is incorrect.
Correct number is 724. The weights are what you said. #37.
I see the 524 on Ebay as listed for a '63 but it isn't. But the owner saw it listed in the parts book- I bet.

Just like the VA's on the '63s. Later parts manuals were pushing the 163 VA's. I think that's the number. Once again GM was selling parts that would fit. Correct VA on a '63 is a two piece 201.

Over the years I have seen a ton of '63 Distributors. All the originals had 724's. The service replacement distributors had most anything.
Distributor shafts. For years all we had were generic FI distributor shafts. They were service replacements. The football on the top of the shaft is incorrect.


NOS Delco Remy distributors are typically built with whatever parts were only the shelf. One is better off restoring a good used ST. Louis/Flint distributor and using it. Thanks, John D.

[Joe L.]

Re: 1963 340HP distributor bushing (timing limiting bushing)

Tim, The 524 is a service replacement distributor cam. Yes it's listed in the famous Chevy parts book but that info is incorrect.
Correct number is 724. The weights are what you said. #37.
I see the 524 on Ebay as listed for a '63 but it isn't. But the owner saw it listed in the parts book- I bet.

Just like the VA's on the '63s. Later parts manuals were pushing the 163 VA's. I think that's the number. Once again GM was selling parts that would fit. Correct VA on a '63 is a two piece 201.

Over the years I have seen a ton of '63 Distributors. All the originals had 724's. The service replacement distributors had most anything.
Distributor shafts. For years all we had were generic FI distributor shafts. They were service replacements. The football on the top of the shaft is incorrect.


NOS Delco Remy distributors are typically built with whatever parts were only the shelf. One is better off restoring a good used ST. Louis/Flint distributor and using it. Thanks, John D.

John------

For whatever reason, the 3 digit numbers stamped on these cams is not a derivative of the part number which was, otherwise, a common Delco-Remy practice.

The original part number for the 1963 distributor cam/weight plate was GM #1958990. This part was used in PRODUCTION and available in SERVICE. I believe this was the cam stamped '724'.

The above cam was discontinued from SERVICE in May, 1965 and replaced by GM #1942360. I do not know what the stamped number was on this cam. It may also have been '724' or it may have been some other number.

The GM #1942360 was discontinued in June, 1973 and replaced by GM #1966686. The latter cam carried the stamped number '524'.

So, while the 1966686 was not the original cam used for the 1963 distributor, it was the "direct descendant" of the original part and therefore, according to GM, functionally correct for the application. I have no doubt whatsoever that it will perform as well or better than the original cam.

Of course, with respect to the stamped number it could be considered not correct with respect to originality. However, I don't think too many folks will be in a position to discern this discrepancy.

[Timothy B.]

Re: 1963 340HP distributor bushing (timing limiting bushing)

John,

I stand corrected, the weight base cam # is 724. My 1963 300hp distributor had the 724, I guess I got confused because the distributor in my 1967 300hp has a 530 number.

[Joe L.]

Re: 1963 340HP distributor bushing (timing limiting bushing)

John,

I stand corrected, the weight base cam # is 724. My 1963 300hp distributor had the 724, I guess I got confused because the distributor in my 1967 300hp has a 530 number.

Timothy-----

The '530' stamped cam/breaker plate is that used for cam/breaker plate GM #1967002. The 1967002 was not in use as early as 1967. However, it's possible '530' was also used for a predecessor piece. That would be unusual but it's possible.

[David Z.]

Re: 1963 340HP distributor bushing (timing limiting bushing)

Duke's seminar is attached.
Dave Z

[Joe R.]

Re: 1963 340HP distributor bushing (timing limiting bushing)

My advise would be to find someone that has a Sun Distributor Machine and knows how to use it.

Took all my distributors to a man in Memphis named Arnold Bonner. Some of you may have seen Arnold at an NHRA track with his Henry J.

Expert on distributors but he has passed from the distributor world!

JR

[Duke W.]

Re: 1963 340HP distributor bushing (timing limiting bushing)

The problem is the guy may know how to use the machine (It's easy to run.), but in my experience very few "mechanics" understand how to select a spark advance map for a specific engine configuration.

With the advent of inexpensive dial-back timing lights, it's easy to set up the map without removing the distributor, and with some basic engine data like idle speed/vacuum and studying my tuning seminar, any DIYer should be able to set up a near optimum map.

Duke

[Joe R.]

Re: 1963 340HP distributor bushing (timing limiting bushing)

The problem is the guy may know how to use the machine (It's easy to run.), but in my experience very few "mechanics" understand how to select a spark advance map for a specific engine configuration.

With the advent of inexpensive dial-back timing lights, it's easy to set up the map without removing the distributor, and with some basic engine data like idle speed/vacuum and studying my tuning seminar, any DIYer should be able to set up a near optimum map.

Duke

The key statement in my response would be (Expert on Distributors)!

You know one?

JR

[Duke W.]

Re: 1963 340HP distributor bushing (timing limiting bushing)

Me, for one and anyone who studies my tuning seminar. A "distributor expert" is one who can set up a near optimum spark advance map for any engine configuration given some basic data like manifold vacuum at idle speed, apply the simple Two-Inch Rule, and understand that the optimum total WOT advance for both Chevy small and big blocks is in the range of 36-40, the higher the better and get it in as quickly as possible, detonation being the limiting factor.

Other engine designs have different optimum total WOT advance. For example the Chevy Vortec is about 32 degrees, and typical DOHC 4-valve designs with a centrally located spark plug are in the same ballpark or maybe a little less.

In a nutshell, that's it.

Duke

[John D.]

Re: 1963 340HP distributor bushing (timing limiting bushing)

John,

I stand corrected, the weight base cam # is 724. My 1963 300hp distributor had the 724, I guess I got confused because the distributor in my 1967 300hp has a 530 number.

That is goo news Tim.
By the way Joe, the 524 is not functionally the same as a 724. My information comes from Don Baker, the FI and Carb distributor restored.
Also at the same time Delco Remy was pushing the 524 cams they were also putting out a generic FI distributor shaft advertised as '62-'65 I believe. It has the incorrect football on top. Says 54W. Once again DR just was using up parts. Like the B1 Vacuum advances in the box for the also service replacements 163's. The 163 was supposed to be used in lieu of a 201. So confusion big time and it's not our fault.
All that superceeding puts a wrench into correct restorations. Like the time I bought a ton of exhaust tips. Found out none were correct for my 63. Thanks everyone. Hope I didn't confuse you. John D.