testpros

Does anyone have the torque specs that bolts the leaf spring mounting plate to the differential cover? I need to start investigating this slow oil leak that appears to be coming from my diff cover. Thought I would just check the torque to start out with.

Powershift

I would venture a "guess" that you have a hole punched in the cover at one of the two mounting bolts with blind (not visible) threads.

But the factory torque spec is 65 ft-lbs. I would go to around 55-60 ft-lbs and check again......the blind threads are soft and known to have issues if overtightened. Torque with all four wheels on the ground.

Larry

silver837

Is it indeed dif. oil ?

testpros

Thanks Larry. I agree with you that it's very likely a hole punched in the cover. It's a very slow leak. I was thinking I might try liquid teflon an see how that goes before I take the trouble of replacing the entire cover.. I heard someone else did that with success. Worth a try anyway.

testpros

Indeed it is diff oil. When I removed one of the bolts about a cup of oil ran out. The other bolt was fine.

C2car

start with 35 to 45lbs no load and when on ground with load final 55 to 65lb or chance of breaking off ear tabs on diff. cover. put liquid thread sealer diff. bolts

Powershift

That is the "smoking gun". No doubt anymore. I just went thru this helping a friend with his 69 Corvette.

We repaired the crack using a certified cast iron industrial welder with pre and post heat treatment. Repair looked good, but ultimately still had a very small drip. We then used thread sealant on final assembly and so far it has been holding.

Larry

Nowhere Man

I can see that they are hardware store bolts and someone did not measure and make sure they had the correct length for the blind holes.

testpros

What is the correct length of bolt needed for my 67? I'll make sure to put the correct length back in.

Nowhere Man

put a hard piece of wire in there and measure and then remove some

testpros

Great idea. I'll do just that.

buns

Factory bolts were 9/16"-12 x 3 1/4". Interested to see what you have in there.

testpros

Crap. Mine looks like 3 1/2". No wonder it cracked the differential cover. All the bolts I see online are all 3 1/2". Where the heck can I get 3 1/4" bolts (in case I have trouble cutting mine down)?

silver837

Put a correct nut (thread size) on the bolt and screw it down to expose the length you want and cut off. remove the nut and dress the cut.

67L88Vette

Fastenal lists both 3-1/2" and 3-1/4" hex bolts in various grades and finishes ... Do a search in Fasteners to see what all they have, and if there's a store near you ... It's worth a visit or call, and make sure to check if their catalog lists total length, or underhead length ... In this size thread, the difference is about 1/4" ... I get many of my automotive hardware from Fastenal these days ...
John A, 67L88Vette

Nowhere Man

That length is only good if it’s a GM spring Who knows what a aftermarket spring width is

rich5962

And reproduction liners are different thickness from originals as well. Learned that years ago when I restored a 67 F41 set.

Needed to take very accurate measurements to use the correct length bolts. This means adding/subtracting plate thickness, and lock washer thickness (using the 2 method technique) after measuring hole depths to the machined face. I had to cut bolts 1/8" to leave apx 1/4" bolt tip depth voids.

Also note if rebuilding a set, make sure the repro center round alignment bolt head is ground lower to fit in the head recess of the case.

Triple check the numbers, do your homework, and make sure final torque is done with car on ground fully comfortable after roll up/roll down and ride height adjusted to spec(AIM) by adjusting outer spring attach bolts.

BTW, I always run up torque in stages alternating diagonally and always the lower number of the spec range. Old stuff doesn't need max imo.

Rich

Powershift

Run a threaded bolt in the blind holes (each one) until finger tight. Then use some tape to mark the bolt inserted length and remove bolt. Measure the amount of bolt thread you threaded into the blind hole. Subtract about 3/32 inch from this number................that is what you ultimately need.

Now accurately measure the thickness of you leaf spring stack with liners installed. Measure in the exact center of the stack. Also measure the thickness of the mounting plate and also the lock washer thickness you will use. Add bolt length -3/32 from first measurement + spring stack thickness + mounting plate thickness + lock washer thickness. That is the length that you need. Cut to fit and bevel the cut edge.

Make absolutely sure car is on its wheels before any torquing is done.

Add some sealer to the bolts for the blind holes after trying to clean thing up a bit to get clean surfaces. You can use whatever length works for the outer bolts with nuts.

I believe my final length for bolts into blind holes was 3-1/8 inches.

Larry

testpros

Thanks Larry. So basically you are recommending a 3/32"gap between the bolt and diff cover when all said and done, correct?

Powershift

That is correct..........it is equal to about 1 bolt thread. Bolt = 9/16 x 12 thread per inch or 0.083 inch per thread. 3/32 gap = 0.093 inch gap.

I believe Rich recommended 1/4 inch (8/32 inch)......that may be a bit much in my opinion, but should also be fine. I would not exceed 1/4 inch, as you will begin losing more of your holding threads within the rear housing..........which is soft cast iron and prone to wear. A 1/4 inch = three threads.

Larry

EDIT: The threads in the blind holes in my own car's rear cover were noticeably worn, so I did not want to give up any extra threads used for holding. That is why I chose the 3/32 inch.........which worked out well for me. If you want a bit more you can add it..........just don't go to far . I also used the very low end on the specified bolt torque range when tightening.