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Replacing the rear main seal in a 350 small block Chevy

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The idea of replacing my rear main seal was very uncomfortable for me. Something about the idea of pulling main bearing caps had me feeling very skittish. However I guess my hatred of oil leaks evoked stronger feelings than my nervousness about replacing the rear main seal.

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Replacing the rear main seal in a 350 small block Chevy Image 1

First off if you post up on any GM board about an oil leak that presents as oil running down the back of the block and over the starter, most people will tell you it’s a leaking valve cover gasket, intake manifold gasket or oil pressure sender. These are very common and should definitely be checked first. Believe me; given my feelings about changing the rear main seal, I had hoped like hell this was my problem. No matter how many times I checked or how hard I wished, it just wasn’t the case.

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Just to be sure, I had taken my truck to the mechanic for some exhaust work and I asked him to take a look at it. His first response was that it was probably coming from the valve cover gasket. They checked it out and it wasn’t the valve cover gasket or any of the other usual suspects. They believed it was the oil pan gasket and rear main seal. So I knew I had to knuckle down and just do it.

While I’m no treehugger, I do like to do my part to keep the environment clean. Shoring up any fluid leaks in your truck is a good way to do this.

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Based on the recommendation of the mechanic, I purchased a one-piece rubber oil pan gasket made by Fel-Pro, part number FEL-OS34509T. I was definitely worth the extra money for doing this job with the engine in the truck. I purchased the most expensive rear main seal I could find on Summit Racing in hopes that I was getting what I paid for; it was also made by Fel-Pro part number FPP-2912 and it’s made with a material called fluoroelastomer.

An alternative would be the Peterson Fluid Systems SM86625 Rear Main Seal for Small Block Chevrolet 350
Those of you like me that wondered what fluoroelastomer is, here you go:

  • Fluoroelastomers are a class of synthetic rubber which provide extraordinary levels of resistance to chemicals, oil and heat, while providing useful service life above 200°C. The outstanding heat stability and excellent oil resistance of these materials are due to the high ratio of fluorine to hydrogen, the strength of the carbon-fluorine bond, and the absence of unsaturation. Fluoroelastomers are referred to generically as FKM polymers per the nomenclature noted in ASTM D1418. In the SAE J200 / ASTM D2000 classification system for rubber materials, fluoroelastomers are documented as a HK material, and can be found in the HK section of this specification.

The procedure for the rear main seal replacement is pretty straight forward. As long as you are careful and take your time, it shouldn’t be a problem. I can comfortably say that now that I’ve done it.

First step is to drain your oil. I can’t imagine the mess you’d have if you didn’t!
Most procedures I read have you remove the starter, I didn’t. I thought I might regret it, but it was never an issue for me.

Next step would be to remove the oil filter.

Then take off the dust cover. Now you can remove all 18 bolts holding the oil pan on. There are four 5/16″ bolts and the rest are ¼” bolts. My one-piece gasket kit included new bolts so I just threw the old ones away.

Lucky for me, my engine was rebuilt a few years ago so just a little prying with a screw driver was necessary to get the pan to break loose. With the pan removed, this is what you’ll see:

Replacing the rear main seal in a 350 small block Chevy Image 2

The next thing I did was removed the oil pump. There is just one bolt holding it in place and it uses a 5/8″ socket. The drive shaft that connects the oil pump and distributor will come out with it.

Next step is to remove the main bearing cap. On my motor there were only two bolts to removed, also require the 5/8″ socket. Tap the cap gently a couple of times and it’ll come loose. You can see it below with the blue rear main seal, this would be the bottom half of the seal.

Replacing the rear main seal in a 350 small block Chevy Image 1

Be careful not to damage the bearing material you can see here at the bottom. A bad gouge or scratch and you’ll get an oil leak that’ll never end.

The bottom half of the rear main seal simply pops out of the main bearing cap. The top half that’s still in the motor requires a bit more effort. I used a blunt ended punch and a hammer to tap one end of the seal. It didn’t take much to move the seal around enough to where I could get a hold of the other end with a pair of needle nose pliers. Just pull it the rest of the way out.

At this point I went ahead a removed all of the old gasket material from the engine and the oil pan. It came of surprisingly easy. One nice thing about doing this in the truck is gravity helps to keep any debris out of the motor.

Now it is time to start putting things back together starting with the new rear main seal. This is what my new seal looked like:

Replacing the rear main seal in a 350 small block Chevy Image 4

There are the two seal halves and the plastic shoehorn we’ll discuss later.
The first thing I was uncertain about was in regards to the “seal lip.” I was pretty certain I knew what they were referring to, but as you’ll recall I felt skittish about this whole thing so I wanted to be certain I had it right. You must be certain to install the seal with the seal lip facing the front of the engine, so it is very important to get it right. I have an arrow below indicating the seal lip on my new seal:

Replacing the rear main seal in a 350 small block Chevy Image 5

In the picture below you can also see the seal lip and how it rides on the crank. The oil pressure actually helps to further seat the lip against the crank.

Replacing the rear main seal in a 350 small block Chevy Image 6

Starting with the upper half, liberally coat the seal with fresh engine oil but keep the ends free of oil. This is so they will seal better once butted together. Now is when the plastic shoehorn comes into play, the little fella pictured below:

Replacing the rear main seal in a 350 small block Chevy Image 7

You will insert this into the upper seal channel so that the little tab will ride in the seal’s groove. It keeps the seal from getting nicked up by the edge of the block. Here’s a good picture from someone else’s install:

Replacing the rear main seal in a 350 small block Chevy Image 8

I thought it was going to be difficult to push the seal in, but it wasn’t that bad at all. Here’s where the directions included with my seal part from the typical directions. They recommend offsetting the seal 3/8″ from flush with the block. In other words, one side will be sticking 3/8″ out of the block and the other side will be 3/8″ inside the block. Fel-Pro states that not doing this is one cause for leaks after install.

Now install the bottom seal half in the bearing cap. Again liberally apply fresh engine oil keeping the ends dry. Also be sure to offset the seal 3/8″ to match the upper half in the motor.

Next apply some RTV sealer to the bearing cap where it mates with the engine block. As illustrated here:

Replacing the rear main seal in a 350 small block Chevy Image 9

Again this is another potential source for leaks.

Now install the main bearing cap back into the block. I thought it was going to be difficult to get the portion of the lower half that was sticking 3/8″ above the cap into the seal channel, but it wasn’t. Once you have the cap pressed into place, bolt it in. These bolts torque to 65ft-lbs on my motor.

The oil pump goes in next. Guide the driveshaft up into the hole to engage the distributor. You may need to rotate it some to get it to fully engage. You’ll know if it isn’t because the pump mounting flange won’t fully seat against the main bearing cap. Torque this bolt to 65ft-lbs.

Now it’s time to install the oil pan. I like to wipe down all of the mating surfaces with lacquer thinner, but not the gasket. Also make sure the flange on the pan is flat; you’ll usually find it is dimpled where the bolt holes are. I used a block of wood and a hammer to flatten the dimples out.

The instructions for the one piece gasket specifically say not to use gasket sealer, except in 2 places;

  1. Where timing chain cover meets the block
  2. Where rear main bearing cap meets the block

The one piece gasket set makes installing the pan a breeze. The kit comes with what they call snap-ins, little plastic dowels pictured below.

Replacing the rear main seal in a 350 small block Chevy Image 10

One end threads into each of the 4 large corner bolts. There’s a slot for a flat blade screw driver that makes it a breeze to thread in. Then you pop the gasket on over the snap-ins by the oil pan. You can see the flared parts of the snap-ins that hold the gasket and pan up. Now start the other 14 smaller 1/4″ bolts with the star washers. Once the bolts are snug you can remove the snap-ins and thread in the last four 5/16″ bolts with star washers. Again the Fel-Pro gasket kit included all new bolts.

Torque the bolts to no more than 100in-lbs. I went around the pan 3 times before they were all consistent. The gasket also has sleeves around the bolt holes to prevent over torquing.

Install the inspection cover.

Install oil filter. I like to add as much oil as I can to the filter before installing.
Refill the oil pan and you’re done.

Hopefully you will hit the road leak free!

Reader’s Tips

Tim Says:

Oil pan gasket surface should be cleaned while main cap and oil pump are installed (either  before removal of components or after installation of new parts) as this will help to keep debris off bearing and journal surfaces.  Also fill up oil filters by outer holes only, DO NOT pour oil in the center hole.  Oil flows in the small holes and out center hole, any dirt in center hole will go directly to bearing surfaces and, depending on hardness of bearing will either destroy bearing or imbed itself in the bearing’s surface.

 

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