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  #1  
Old January 13th, 2018, 08:18 PM
Joe Sand Joe Sand is offline
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Default Rear main seal

Hi all, just joined! I have a shop in Sacramento, and I'm working on an M715... the first I've ever seen, in fact. I'm in the process of replacing the rear main seal and haven't been able to find any published information on the subject, but my internet search has lead me to your forum!

So, after removing the lower block and seeing how the seal is attached, it was pretty clear that the upper half of the seal would not press out like the seal on a 258 (I tried anyway), but I found that just a tap would rotate out the upper half of the seal block.

My first question is, is this the correct way to get the seal out? I don't see any other way... and it's done, so the answer is somewhat moot...

Second, when I install the new seal, I'd think there should be some sort of sealant around the edge of the seal block and the engine block. The fact that it came out so easily makes me wonder if this was the source of the leak, not the seal itself. If this is correct, what would one seal this piece with, and is there any particular procedure for it's installation?

And, last, I've read that it is best to install the seal slightly rotated from the upper and lower block halves... is this true?
Any assistance is appreciated! Thanks!
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Old January 13th, 2018, 11:33 PM
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brute4c brute4c is offline
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The original military manuals, linked at the top of the forum pages, dont list a sealant...it seems to make sense but it isnt spec...you could try...

The rear main is a leak prone place...unfortunately.

As far as getting it out, yes, did that myself. Installing the new one, I had an old time engine guy who owned an engine shop since back then tell me that they put a piece of string up through the hole where the upper seal goes, then work it around so it hangs out both sides...so it goes up over the top like the upper seal should...then put a slip knot on the upper seal with the string and pull it through the groove...worked for me.
He, and others here on the site, indicated that not lining up the parting lines of the seal to the parting line of the block that holds the lower seal in place is a must to help stave off leaking.

Hope that helps...if you use sealant and it works, please post and let us know what you used and such...THANKS!!
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Old January 14th, 2018, 08:39 AM
Joe Sand Joe Sand is offline
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Thanks for the reply. It's good to know I'm on the right track, and puzzled that no sealant is known for the upper block plate... the one I took out looked like it might have had some sort of varnish-like substance on it. The key, as I see it, is to seal it without gluing it in place.

I saw the "string" thing in the archives, and I don't understand it... there's no part of the seal that you can tie a string to without damaging it. I presume I'd be installing the upper half of the seal on the upper blocking plate, then simply slide/tap the assembly around the crankshaft... just the opposite of how it came out. Since I'd be doing this with the seal slightly out of phase with the parting line, I figure that I would then place the lower seal on the lower blocking plate, but without pressing it all the way on, then temporary tap the lower plate in place on the engine, which should both press the seal all the way on and align it with the upper half, then take it off so it can be installed with the additional side and pan seals in place.

One other thing: The seal kit came with the upper and lower seal halves, the two pieces that seal the sides of the blocking plate... and a metal ring. What's the metal ring for?
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Old January 14th, 2018, 01:10 PM
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I don't remember any metal ring. What size is it?
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Old January 14th, 2018, 03:02 PM
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When I used the string method, I put a good oil coating on the seal and it pulled right through without a big fuss...driving it in could work too...I just did what the guy who had been there before did.
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Old January 15th, 2018, 10:28 AM
Joe Sand Joe Sand is offline
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I hadn't actually taken it out of the bag to look closer at it until this morning... it's a seal of some sort, one inch in diameter, and it's hollow, open on one side with a white material inside. Looks like it would be crushed when installed, like a spark plug washer. I'll have a closer look this morning and see if something occurs to me.

Having nightmares this weekend about how to put the seal in, mainly, if I should include sealant. I have a permatex product used for threads and some gaskets that is sticky and doesn't harden, but I don't want to take a chance of gluing the upper block plate in, just in case I need to start over. This is a customer's vehicle, and there's an expectation that such things are being done right the first time!

Also, (while I'm at it) the pan gasket kit came with a couple pieces of "rope" packing material, and vague instructions to "press firmly into bearing cap groove then trim flush with face of cap." I thought maybe they were a primitive version of the seal between the upper and lower block pieces... but I don't thinks so...

Last edited by Joe Sand; January 15th, 2018 at 05:10 PM.
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Old January 16th, 2018, 06:11 PM
Joe Sand Joe Sand is offline
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Well, the ring is just the gasket for the drain plug (duh), and I still have no idea what the rope packing is for.
I ended up installing the seals onto the block plates while they were off, setting them around 3/16" offset from the parting line between the two halves. I then lubed up the upper piece with some assembly lube and spun it in. The back side of the blocking plate has surfaces that I could put a long socket extension against, and spin it in with light taps. For the final taps, which had to be against the surface that mates with the lower half, I padded the extension with several layers of electrical tape. I then bolted in the lower half and called it done.

I decided against trying to seal the upper piece, partly because it would be difficult to get a meaningful amount of material up there, partly because I didn't want to take a chance of gluing it in, and because I decided against re-inventing the wheel.

The pan gasket set I received was a different style, where the rubber end pieces do not wrap around the lower blocking plates (both of them, although I don't know what you call the one in the front...) but it looks to me like it should still work fine with a dab of RTV. The pan isn't on yet, and I haven't decided if I'm going to cut off the front rubber end-piece, or use the old one, since it looks pretty healthy.

Last edited by Joe Sand; January 16th, 2018 at 06:16 PM.
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