1964 303 rear cover
How do I repair this? I understand its a common problem. Found it yesterday when I pulled my spring bolts and oil came out. I would like to save this cover as I believe its original to the car. I think this explains the spacer in the spring....wrong length bolts
O
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Re: 1964 303 rear cover
Pat, cast iron is a tough fix, I try some JB weld and then coat the bolt with some Teflon pipe dope, and fill the diff and let set for a few days before re-install to make sure its sealed up.New England chapter member, 63 Convert. 327/340- Chapter/Regional/national Top Flight, 72 coupe- chapter and regional Top Flight. -
Re: 1964 303 rear cover
I suggest looking for an excellent replacement. They are out there. Try eBay, Bairs, Lonestar.
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Re: 1964 303 rear cover
They can be repaired, and we just did one. Required pre and post weld heat treatment and a VERY GOOD cast iron welder. You can shop around where you live and see if you can locate a good industrial repair shop for cast iron repairs and welding. But repair will not be cheap.
Hemmings used to have a list of a few shops that repaired cast iron. You can check this.........or Google "cast iron repair".
Ikerd's in Indiana should have what you need. Also David S. in California.
I have stated this over a dozen times in various forum posts, but you need to measure and then cut the spring bolts to fit, and leave about 3/32 inch free clearance for the enclosed bolts after tightening. Also, these bolts are not to be torqued/tightened until car is back on its wheels with full weight on the spring.
LarryComment
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Re: 1964 303 rear cover
Second what Larry says about the bolt lengths, the so called "correct" bolts most of the parts houses sell are too long and will break the housing on the two forward bolts as has apparently happened here. Even original bolts with replacement springs that are not as thick as OEM can do this. You really do need to measure and shorten if required. The best fix is to find a correct original cover (right casting number & date) and replace. Or, you can use a thread sealant on the bolt. The usual PFTE teflon pipe thread sealants and silicone won't work well on this. You should also not use any JB weld type epoxy. I have used Hylomar which is very oil resistant on some of these with good results. But once again, replace if possible.Comment
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Re: 1964 303 rear cover
A friend suggested putting a small threaded plug in and then brazing the housing to it. I like the idea. Or forget the brazing and use JB weld or hylomar on the threads to lock it in?Comment
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Re: 1964 303 rear cover
All------
Here's the deal: all 1963-74 Corvette used spring plate bolt GM #3833486. This bolt was 9/16-12 X 3-7/32". In addition, it was a special bolt with 2-7/32" shank length and 1" thread length. This bolt was designed so that the shank of the bolt, not the threads, contacted the side of the spring leaves. In addition, the 1" thread length was the same as the blind tappings in the differential cover. So, there is no way that the bolt could break into the casting above the threads.
The GM #3833486 was available in SERVICE until March, 1988 when it was replaced by the GM #9428598. However, this bolt is 3-3/4" in length with a thread length of 1-3/8". This was a poor choice by GM. This bolt can be threaded too deeply into the cover casting and cause exactly the problem seen in the post above.
The reproductions of the GM #3833486 that I've seen are too long, too. They are 3-1/2" long. However, by shortening the threaded end by 9/32" will result in a bolt virtually identical to the original 3833486 in all respects.
The photos below depict an NOS GM #3833486 and a reproduction of the 3833486:
In Appreciation of John HinckleyComment
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Re: 1964 303 rear cover
The other "variables" in this are the individual spring leaf thickness and the liner thickness. Reproduction springs can have a different thickness than original, and this must be considered. Also both GM replacement liners (no longer available), DR REBUILD liners, and Quanta liners are all thinner than originals. When 6-9 liners are used in the spring assembly this total difference in thickness makes a difference.
So even if original bolts are used, there may be a need to adjust its threaded portion to get a tight fit on the spring and also not break the rear cover. Therefore measure everything and adjust accordingly.
FWIW.
LarryComment
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Re: 1964 303 rear cover
Thanks Larry,
You can bet I am going to be very careful with the bolt length!Comment
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Re: 1964 303 rear cover
Repairing cast-iron by brazing is the only method I would use.
It seems to be less prone to cracking after the welding is completed.
It also is more forgiving with the different alloys used in cast-iron.
If it is stick welded, with high nickle rod, it will require preheating.
Both procedures required someone that knows what they are doing.
RickComment
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Re: 1964 303 rear cover
Rick-----
I agree; brazing is the only repair method I would recommend for this. Cast iron welding is fraught with too many potential problems and unpredictable results and reliability.
It would be one thing if we were talking about a rare and irreplaceable casting that could only be repaired by welding. In a case like that, you have to take a chance. In a case like this cover where there are several options available, including a brazed repair, there's no reason to try welding.Last edited by Joe L.; March 12, 2019, 07:07 PM.In Appreciation of John HinckleyComment
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Re: 1964 303 rear cover
Larry-------
Yes, caution must be exercised when installing the spring assembly. However, bolt length adjustment might not be the best route to take in all cases. For example, in a case in which original length/configuration bolts are used and the spring is, say, 1/2" less in height, too much of the length of threads would have to be removed. A better approach would be to add a plate to the top of the spring which compensates for a spring with lesser height than stock (which is the usual condition with aftermarket springs). Of course, if the spring height of the installed spring were greater than original, then longer bolts would need to be used. These bolts would need to be exactly as much longer than original as the spring is higher than original.
If a plate is installed, it should be the thickness of the difference between the original spring height and the replacement spring height. It should be the length and width of the mounting flange on the differential cover. It should be installed by removing the spring center bolt, installing the plate, and re-installing a center bolt through the plate and spring. It may be necessary to use a new and longer center bolt.
Here's the original specs:
original 64-74 9 leaf spring height (GM #3850839)-------2.29"
original bolts (GM #3833486)-------------------------------3.22"
original washers (GM #103330)-----------------------------0.15"
original spring plate (GM #3817582)------------------------0.23"
The above allows for 0.55" thread engagement in the cover which is correct for a 9/16" thread size and will prevent any chance of the problem seen in the original post photos.In Appreciation of John HinckleyComment
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Re: 1964 303 rear cover
I have decided on a fix for my rear cover. A friend milled off the inside bosses, one was broken the other was cracked. He then made a couple of 1/4 inch hex plugs. I will install the plugs with JB weld. The thread depth is .820 deep to the plug when the plug is fully seated. I like this mechanical fix. Just need to make sure I get the bolts the right length......I hope all that engineering education still works as my dad paid a lot for it
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Re: 1964 303 rear cover
Patrick
I like this fix also- actually like it better than brazing -Comment
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