Bolt length for leaf spring center plate?

[Patrick N.]

Thought I would ask for help before I destroy something Can someone provide me the length for the four bolts used to anchor the dog bone shaped plate which holds the compressed leaf spring to the differential? (GM 3833486). Two of the bolts are a good 1/8" from seating and very difficult to turn at this point. The bolts I am using are aftermarket and they were unmarked in my parts bin-I may not be using the correct ones.

My leaf is installed on both ends so it is compressed prior to torquing these four bolts, I also followed the advice on other threads, and shaved down the height of the aftermarket center bolt to fit in the pocket under the differential.

Thanks for your help,
Pat

[Stephen L.]

Re: Bolt length for leaf spring center plate?

DON'T TORQUE those bolts if the weight of a completed car isn't on the spring... in other words the completed car needs to be sitting on the wheels. If you don't do this you risk breaking the ears off the carrier. They are not designed to be able to compress a non-loaded spring.
If the bolts you are using are the correct length (measure the depth of the hole + the thickness of the spring and the "dog bone plate" and subtract at least 1/8" so the bolt isn't bottoming out) and the correct grade AND they have the correct threads, then insure the bolts will screw into the holes easily. You should be able to install the bolts finger tight, lower the car onto its full weight and then finish the tightening to the torque spec, and should be good to go.

[Patrick N.]

Re: Bolt length for leaf spring center plate?

Wow, Thanks Steve! It did not feel right so I stopped where I was , glad I asked. I will double check the bolt length etc, and wait to final torque after the body drop. You guys are great, thanks again Steve for saving me the headache and the dollars.

Best,
Pat

[Terry M.]

Re: Bolt length for leaf spring center plate?

Some replacement bolts are too long. Careless installation will result in piercing into the differential cavity and a perpetual differential fluid leak.

[Patrick N.]

Re: Bolt length for leaf spring center plate?

Some replacement bolts are too long. Careless installation will result in piercing into the differential cavity and a perpetual differential fluid leak.

Thanks Terry, I am suspecting these bolts may qualify for that. they go in easy enough to start, the two that have through-holes on the ears stick up through the ears by an 1/8 or so and the two in that i mentioned are down by an 1/8". Any experience on who sells accurate length bolts for this application?

Thanks,
Pat

[Keith L.]

Re: Bolt length for leaf spring center plate?

And make sure you grind the head down on the center spring bolt to match the original.

[Patrick N.]

Re: Bolt length for leaf spring center plate?

And make sure you grind the head down on the center spring bolt to match the original.

I did catch that tip. Sure enough, when I matched original and new, the new was taller. I ground it down per the advice given and it nested nicely.

[Mark Z.]

Re: Bolt length for leaf spring center plate?

Tom Keen used to have the correct length bolt Grade 8.

[Mark H.]

Re: Bolt length for leaf spring center plate?

Recently completed this on our '66 coupe. I bought four Grade 8 bolts at Fastenal, at about 1/3 the price of what many of the Corvette parts vendors are charging. Just a thought.

[Larry M.]

Re: Bolt length for leaf spring center plate?

Thanks Terry, I am suspecting these bolts may qualify for that. they go in easy enough to start, the two that have through-holes on the ears stick up through the ears by an 1/8 or so and the two in that i mentioned are down by an 1/8". Any experience on who sells accurate length bolts for this application?

Thanks,
Pat

You need to measure as Steve L stated earlier, and then cut your bolts to fit. That is what I did. Not certain if any vendor has the "correct" length, since spring leaf thickness and liner thickness vary considerably.

Steve didn't mention that these bolts use lock washers, so include the lockwasher thickness in your measurements. I believe that the lockwasher thickness is about 3/32 to 1/8 inch in this particular bolt size. Adding the lockwashers will provide additional clearance, but you NEED about 1-2 threads of clearance (or 1/8 inch) in the blind threaded holes.

Larry

[Wayne M.]

Re: Bolt length for leaf spring center plate?

.... Any experience on who sells accurate length bolts for this application? ..

Pat -- original bolts were 3_7/32" shank length, with 11 or 12 threads. The split washer is 0.17" thick. Don't know if anyone sells these dimensions.

Thread count is critical, as you don't want to break into the inside of the cover on the forward bolts.

[John L.]

Re: Bolt length for leaf spring center plate?

I faced the same issue last year with my '66 and purchased the correct sized bolts from Paragon.

[Scott S.]

Re: Bolt length for leaf spring center plate?

I can't say for certain whether the four bolts that attach my anchor plate are original, but they are old and well-corroded

They are also two different lengths. Two bolts measure 9/16-12 x 3-1/4". They have 11-12 threads each. They do not appear to have been cut, unless someone went to the trouble to smooth the edges and hollow out a slight depression in the otherwise flat tip. The "WB" headmark is barely visible, heads are too corroded to make out any Grade lines (if there are any).

The other two rear spring mount bolts appear generally the same, but measure 9/16-12 x 3-1/2". They have 16-17 threads each (the unthreaded portion of the shank is a little shorter than on the first two bolts). These also do not appear to have been cut, but they might have been, they are fairly corroded. The tips are flat, with no depression. The regular hex heads are too corroded to make out any headmark or grade lines.

The parts books say 9/16-12 x 3-1/4", but beginning in 1968 this part number is listed twice. First in Group 7.529 with the 3-1/4" length. Then GM 3833486 is listed again in the Standard Parts Section of the same P&A. Here it is part of Group 8.900 and has a measurement of 9/16-12 x 3-7/32". It is listed there as being 260-M steel (Grade 2).

Thoughts?

[Larry M.]

Re: Bolt length for leaf spring center plate?

Pat -- original bolts were 3_7/32" shank length, with 11 or 12 threads. The split washer is 0.17" thick. Don't know if anyone sells these dimensions.

Thread count is critical, as you don't want to break into the inside of the cover on the forward bolts.

Even if you find the original, correct length bolts, they may still be too long...........since all the GM service replacement and reproduction liners I know of are thinner than the originals. The only way the original bolts will work correctly is that the springs and liners are 1960's originals.

Measure everything and then cut to fit.

Larry

[Don L.]

Re: Bolt length for leaf spring center plate?

Even if you find the original, correct length bolts, they may still be too long...........since all the GM service replacement and reproduction liners I know of are thinner than the originals. The only way the original bolts will work correctly is that the springs and liners are 1960's originals.

Measure everything and then cut to fit.

Larry

A good point regarding bolt length and varying thickness springs.

So, if we know what the original bolt length was, I think all we need to know is the overall thickness (9 leafs plus 7 liners) of an original spring, and from that, we can make a fair/proportionate decision on needed bolt length when using a replacement spring. Anyone?