I have used just about every type of rear main rope seal there is at one time or another. I have found that the key to effective sealing in any rope style seal is how well you prepare and pack the seal into place. You can't just use a "chinese finger" to pull a new one in and call it done.
Ideally, if you have the motor and crankshaft out, you can really do a good job of packing the seal into the pocket. If working on a motor in the truck, pull the tranny so you can loosen the bearing caps and drop the crank a bit to be able to pull the the seal in without elongating it leaving the ends at least a couple inches long initially. Once in place, put the crank back up softly but not all the way home and start pushing the ends of the rope seal (leave enough to pack) into the seal pocket. I used a small flat screwdriver to coax the seal up and into the pocket. You want to fatten the rope up by compressing the ends together through the half-moon pocket as much as possible.
My 302 project was plagued with a pesky rear main leak. I had the engine apart FIVE times (counting today) in an attempt to seal up this leak. I've used every seal known to man including the NOS asbestos rope seal; all without success until now.
I emailed Best Gaskets, and got a nice response from the owner who asked that I call to discuss the problem. After a few calls back and forth, Best sent along some different gasket material to try. We tried that a couple months ago and unfortunately the leak continued unabated. Back to the drawing board.
Best sent some new gasket material, and, some various thickness shim stock to go under the seal in the pocket if necessary.
What I can tell you is that Best's new gasket material has, for the first time, totally sealed up the back end of my 302!
First, with the rear main cap removed, I needed to make a push stick to coax the NOS asbestos rope seal that was in there out. I tried cutting down a toothbrush handle, but it broke and as I was headed back into my wood shop to find something else, I passed my son's drumset and saw 100 or so busted drumsticks in pile. EUREKA! I took a drum stick to my band saw and carved the broken head end down to match the rear main seal pocket size:
Now armed with the pusher and a rope seal removal tool (a corkscrew) I removed the original NOS rope seal.
http://www.oldGMCtrucks.com/photos/gmcforum/302/TopSealRemover.jpg
That was the easy part.
We thought we'd try to "sneak" the top half of this newer, thicker, denser, teflon/graphite rope seal up and over the main by fashioning a slip knot on a strong piece of nylon but after a great number of tries and loosening up ALL of the main caps, we could still not get it in. Getting the larger replacement seal in place on the top proved to be a much more difficult task. We tried a number of different approaches to feed it in, up, and over the crank but all just resulted in total frustration.
In the end, we removed the driveshaft and tranny and then with all four caps loosened a lot, the crank fell down a little and allowed us to finally feed the new seal in, up and over. The photo below shows the first (failed) attempt using a thick piece of plastic strapping. We ended up using double dental floss which was just strong enough to lasso the end of the seal, and while Bill's gloved hand tugged on the line, Mike fed it carefully to the slot with a small flat screwdriver while I turned the the engine over from the top by the fan belt/fan blade. This was absolutely a three man operation. NOT EASY!
http://www.oldGMCtrucks.com/photos/gmcforum/302/TopFeederSetup.jpg
http://www.oldGMCtrucks.com/photos/gmcforum/302/TopSealIn.jpg
With the top seal now in place and using the drumstick/pusher, we kept mashing the proud ends of the seal into the pocket alternating on each side and with it now mashed in there as much as possible, we tightened up the bearing caps and trimmed it up using the best spacer and exacto knife which is included in the kit.
Now the focus shifted to the lower seal half in the bearing cap.
Because I find a lot of these seals have spun in the seal pocket and spinning in the pocket is obviously counter-productive to getting it to seal, I took the additional step of drilling a small pilot hole in the seal pocket and then driving cut brad nail into the hole to secure the seal from spinning.
http://www.oldgmctrucks.com/photos/gmcforum/302/RearMainCapPin.jpg
The tools that come with the BEST kit which include a small shim spacer that leaves "just the right amount" of seal showing are very handy. The rope seal is set into the pocket and compressed and mashed in place with the drumstick and round GM seal tool which duplicates the radius and diameter of the crankshaft surface that the seal will ride on.
http://www.oldGMCtrucks.com/photos/gmcforum/302/SealInstaller.jpg
http://www.oldGMCtrucks.com/photos/gmcforum/302/BottomSealInstall.jpg
The cap is now ready to be installed:
http://www.oldGMCtrucks.com/photos/gmcforum/302/LowerBearingCapSealIn.jpg
Mike was careful to put just enough BlackRTV Gasket seal on the bearing cap to block mating surface and rope seal ends to give a little sealing of any irregularities, but not so much as to squirt out all over the place in end up in the crankcase and ultimately in the oil system. He installed, then removed the cap to inspect both surfaces carefully before installing it a final time and torquing down the bearing caps to the specified 95 ft/lbs.
The proof as they say, is in the pudding. After a half hour HIGH SPEED flogging, I returned and parked in the driveway. Normally at this time, in all of the previous four attempts, a large oil spot appeared nearly immediately.
After 1 hour of sitting, it looked like this under the truck:
http://www.oldGMCtrucks.com/photos/gmcforum/302/drycity.jpg
The kit number is 5144. Also, Best makes an EXCELLENT pan gasket and I highly recommend that too. Much better than the FelPro pan gasket.
http://www.oldgmctrucks.com/photos/gmcforum/BestGasketKit5144.jpg
If you look in the photos above, you'll see a wooden "thumb protector" for trimming the seal ends and it is shown in my post "in use". Under the knife blade (included in the kit) is the black shim which is also shown as a round end on one end and recess on the other. The recess is fitted over the seal end while lying flat on the block or bearing cap, then the wooden thumb protector is used to cut against. The trimmed seal is shown in the final shot of the bearing cap.
FINALLY! We have met the enemy, and after five seals, and much blood and crankcase oil, WE HAVE WON! Well at least for the most part. I still get a single line drip down the back of the pan and it will drop a quarter sized drop of oil on the ground, but the days of looking like a Detroit 6-71 was parked there are over!
A final note to ponder; oil is cheap. Rear main leaks will have you thinking about hanging yourself in the shower if you let them. These engines hold a boatload of oil and hot oil leaves a big puddle that looks much worse than it is. Even if you have one throwing quite a bit of oil and monitor the dipstick, you can barely see it go down even though it looks really bad. So don't hang on too tightly to having that rear main sealed up unless you have the time and energy to invest in fixing it.
