Differential Covers

[Patrick H.]

Gentlemen (and Reba),

While "restoring" the 72, I decided to remove the very dirty and somewhat corroded rear spring, and re-line and repaint it. Not a big deal, as I have done it to the 71.

Of course, in this case one of the differential cover "ears" that holds a spring bolt decided to part ways with the cover, and a replacement is in order. As I do not know of 100% correct replacements, I have a fellow club member who has a cover for the later (2 1/2 inch wide) spring, which he would sell me.

Now, I know mine came with a 2 1/4 inch spring, but I thought that the newer covers were supposed to come with some type of "spacer" to allow there use with the narrower spring. Is this true? Or must the spacers be purchased separately? Or am I best off with a heavy-duty aftermarket cover? The new cover that I am inspecting is still in the GM box, and includes the spring retaining plate. The cover has 464910 cast into it, and 464909 on the box. Any idea what it really is? My friend bought it many years ago as a future replacement (if needed) for his 72 LT-1 AC Convertible, but now (nearly 20 years later) we believe the GM parts guys got the wrong units.

Thanks in advance for your assistance. Patrick Hulst

[ken girven]

Re: Differential Covers

Patrick,

I was restoring the rear suspension on my 69 and as I was tightening the thick spring bolts that attache to the rear end "ears(with a torque wrench no less!) I heard this loud snap.....sure enough the "ear" broke off where the bolt goes through.This was my original rear end cover for the car so i was pissed to say the least . I looked through a few of the catalogues and was thinking about the 'heavy duty" aftermarket type rear end covers but wanted an original type one. I gave Paragon a call and it turns out they had a used original correct cover for my rear!! It was a bit expensive$$ (100-125 or so) but amazingly this cover had a near perfect casting date of april 9 , 1969 and my original cracked cover had a casting date of apr 10,1969. I would suggest going with an original rear cover if possible but thats just my opinion! good luck.....

Kenny #30871

[ken girven]

Re: Differential Covers

Patrick,

I was restoring the rear suspension on my 69 and as I was tightening the thick spring bolts that attache to the rear end "ears(with a torque wrench no less!) I heard this loud snap.....sure enough the "ear" broke off where the bolt goes through.This was my original rear end cover for the car so i was pissed to say the least . I looked through a few of the catalogues and was thinking about the 'heavy duty" aftermarket type rear end covers but wanted an original type one. I gave Paragon a call and it turns out they had a used original correct cover for my rear!! It was a bit expensive$$ (100-125 or so) but amazingly this cover had a near perfect casting date of april 9 , 1969 and my original cracked cover had a casting date of apr 10,1969. I would suggest going with an original rear cover if possible but thats just my opinion! good luck.....

Kenny #30871

[Joe L.]

Re: Differential Covers

Patrick----

First of all, breakage of the rear cover spring retainer bolt bosses is usually caused by failure to de-arch the spring prior to torquing the bolts---sometimes you get away with it, but most times you don't. The spring must be de-arched or "flattened" before applying significant torque to the bolts. This is done by CAREFULLY AND SECURELY jacking up both ends of the spring with two floor jacks until the spring is "flat" as it is in the normal ride configuration with wheels on the ground. The vast majority of cover breakage is caused by this mistake; covers rarely break on the car under ANY in-service loading conditions, as some folks would have you believe.

The cover you are considering is the 78-79 cover which replaced in service your car's original GM part #3871374 cover. This cover has been the only cover available from GM for all 63-79 Corvettes since about 1980. However, this cover uses four 7/16" spring retainer bolts instead of the 9/16" bolts which all 63-77 Corvettes originally used and, consequently, uses a different spring retainer plate, GM #457977. As far as I know, this plate does not come with the 464909 cover; it must be purchased seperately. Someone may have earlier purchased it and put it in the box with the cover.

In any event, in order to install the GM #464909 cover on 1963-77 Corvettes originally equipped with the 2-1/4" wide spring, you have to use the GM #457977 plate, two GM #459032 spacers, and four GM #25525860 bolts. Unfortunately, the GM #459032 spacers have been discontinued from service.

[Joe L.]

Re: Differential Covers

Patrick----

First of all, breakage of the rear cover spring retainer bolt bosses is usually caused by failure to de-arch the spring prior to torquing the bolts---sometimes you get away with it, but most times you don't. The spring must be de-arched or "flattened" before applying significant torque to the bolts. This is done by CAREFULLY AND SECURELY jacking up both ends of the spring with two floor jacks until the spring is "flat" as it is in the normal ride configuration with wheels on the ground. The vast majority of cover breakage is caused by this mistake; covers rarely break on the car under ANY in-service loading conditions, as some folks would have you believe.

The cover you are considering is the 78-79 cover which replaced in service your car's original GM part #3871374 cover. This cover has been the only cover available from GM for all 63-79 Corvettes since about 1980. However, this cover uses four 7/16" spring retainer bolts instead of the 9/16" bolts which all 63-77 Corvettes originally used and, consequently, uses a different spring retainer plate, GM #457977. As far as I know, this plate does not come with the 464909 cover; it must be purchased seperately. Someone may have earlier purchased it and put it in the box with the cover.

In any event, in order to install the GM #464909 cover on 1963-77 Corvettes originally equipped with the 2-1/4" wide spring, you have to use the GM #457977 plate, two GM #459032 spacers, and four GM #25525860 bolts. Unfortunately, the GM #459032 spacers have been discontinued from service.

[Patrick H.]

Re: Differential Covers

Joe,

Thanks for the tip on spring removal. I assume that the spring should be compressed "flat" also in order to install it later? I can't even find this tip in a Repair Manual, and I haven't done this step before. Where did you get this info (not that I doubt you)? And once you have started to loosen the spring bolts you can then decompress the spring? On installation, do you then compress (flatten) the spring as you get near to drawing the spring tight against the differential?

Also, I assume that since the spacers are no longer in service, the owner of the differential cover (not me, yet) is SOL since they are needed? He would like to know, as then he may sell these.

Thanks. Sorry for all the questions, but again you seem to have given us info I haven't seen elsewhere. You may consider putting it in the Restorer for more to learn.

Patrick

[Patrick H.]

Re: Differential Covers

Joe,

Thanks for the tip on spring removal. I assume that the spring should be compressed "flat" also in order to install it later? I can't even find this tip in a Repair Manual, and I haven't done this step before. Where did you get this info (not that I doubt you)? And once you have started to loosen the spring bolts you can then decompress the spring? On installation, do you then compress (flatten) the spring as you get near to drawing the spring tight against the differential?

Also, I assume that since the spacers are no longer in service, the owner of the differential cover (not me, yet) is SOL since they are needed? He would like to know, as then he may sell these.

Thanks. Sorry for all the questions, but again you seem to have given us info I haven't seen elsewhere. You may consider putting it in the Restorer for more to learn.

Patrick

[Gary Schisler]

Re: Differential Covers

Patrick, most of the articles, including the Restorer, recommend de-arcing the spring for installation and removal. If you have access to the entire library of past issues, Ron Dembrowski (sp?) had a very good article a few years ago. He did have a procedure to de-arc the spring before removing. I have followed this procedure three times during various spring removal and installation with great success.

I would certainly check with Paragon, because they advertise a large inventory of used parts. There is also a gent in Indiana who deals in a lot of used parts. I think the name is J.T. Piper (Pipen?). He advertises a lot in the Driveline. You might have good luck looking through past issues of the Driveline for parts, too.

[Gary Schisler]

Re: Differential Covers

Patrick, most of the articles, including the Restorer, recommend de-arcing the spring for installation and removal. If you have access to the entire library of past issues, Ron Dembrowski (sp?) had a very good article a few years ago. He did have a procedure to de-arc the spring before removing. I have followed this procedure three times during various spring removal and installation with great success.

I would certainly check with Paragon, because they advertise a large inventory of used parts. There is also a gent in Indiana who deals in a lot of used parts. I think the name is J.T. Piper (Pipen?). He advertises a lot in the Driveline. You might have good luck looking through past issues of the Driveline for parts, too.

[Tom Buddie]

Important Note

As Joe indicated, the spring MUST be de-arched before tightening the bolts. I recently supervised a "new" enthusiast while he replaced the spring on his '72. Even after de-arching the spring as Joe described (and as I insisted), the tabs snapped on his original cover (also using a torque wrench). I suspect fatigue played a part. I was flabbergasted. I have replaced many, many springs, and never seen this happen. He then took his torque wrench to have it calibrated... but it didn't need calibration... it was on-the-money. This stresses the importance of following Joes advice... if the tabs will break even with the spring de-arched, your chances of replacing the bolts successfully without de-arching the spring are next to nothing. Good Luck!

[Tom Buddie]

Important Note

As Joe indicated, the spring MUST be de-arched before tightening the bolts. I recently supervised a "new" enthusiast while he replaced the spring on his '72. Even after de-arching the spring as Joe described (and as I insisted), the tabs snapped on his original cover (also using a torque wrench). I suspect fatigue played a part. I was flabbergasted. I have replaced many, many springs, and never seen this happen. He then took his torque wrench to have it calibrated... but it didn't need calibration... it was on-the-money. This stresses the importance of following Joes advice... if the tabs will break even with the spring de-arched, your chances of replacing the bolts successfully without de-arching the spring are next to nothing. Good Luck!

[Joe L.]

Re: Important Note

Tom----

One other possible cause for the "tabs" breaking in the case you described is that they were cracked during a previous "non-de-arch" spring removal and/or installation. When the spring was installed during the episode that you describe, even though the spring was de-arched, the cracked tabs were "finished off".

[Joe L.]

Re: Important Note

Tom----

One other possible cause for the "tabs" breaking in the case you described is that they were cracked during a previous "non-de-arch" spring removal and/or installation. When the spring was installed during the episode that you describe, even though the spring was de-arched, the cracked tabs were "finished off".

[Patrick H.]

Spring DeArcing: the big unknown

All,

Thanks for the info so far, but I am still very curious as to where this idea of de-arcing originated. It is not in the 1971 Chassis Service Manual (which Joe admonishes us all to follow), nor the Haynes or Chilton Manuals. I have never seen this in the monthly (non-NCRS) Corvette magazines either, and I have a 15 year library. Heck, I even wrote an article on rear suspension restoration myself for Vette Magazine about 4 years ago, and never came upon this tip in extensive research before writing the article. In talking with other Corvette types today, they hadn't heard of it either.

Might I submit that this de-arcing process is not nearly as well known as you may believe? After all, NCRS membership has nearly doubled in the last 10 years, and the articles were from the mid-80's.

Thanks again, and I look forward to your input

[Patrick H.]

Spring DeArcing: the big unknown

All,

Thanks for the info so far, but I am still very curious as to where this idea of de-arcing originated. It is not in the 1971 Chassis Service Manual (which Joe admonishes us all to follow), nor the Haynes or Chilton Manuals. I have never seen this in the monthly (non-NCRS) Corvette magazines either, and I have a 15 year library. Heck, I even wrote an article on rear suspension restoration myself for Vette Magazine about 4 years ago, and never came upon this tip in extensive research before writing the article. In talking with other Corvette types today, they hadn't heard of it either.

Might I submit that this de-arcing process is not nearly as well known as you may believe? After all, NCRS membership has nearly doubled in the last 10 years, and the articles were from the mid-80's.

Thanks again, and I look forward to your input