Hi All,
Try to keep this brief... bought a replacement /6 engine ('64 Valiant convertible) and after installation the neoprene rear main seal leaked pretty badly. The engine is under warranty, so they sent another seal -- but it still leaked. Bought a second one, still leaked. My mechanic suspected something else -- bad crank or bearing. The company took the engine back (had to ship it out of state), told me that it was simply the seal, installed a new rope seal, and sent it back. It's still leaking (and the pan is leaking too, and the mangled the breather cap and valve cover in the shipping). Pan is an obvious easy fix, but my question is this: I know it takes a while for a rope seal to expand and seal, but how long? I've put 200 miles on it in a week and it's down a quart (some from the pan) and still leaking. I'm hoping that it will seal up and my problem is solved, but if I give it reasonable time and it doesn't I'll have to seek some kind of remedy from the manufacturer, which likely won't be pleasant. The rebuilder (remaining nameless at this point) has a pretty good rep but they've been testy over this issue. Thanks in advance.

Sorry for the late reply to your post but here's some info I hope may assist you. Firstly,the neoprene seal. I used the Fel pro 2 piece neoprene seal in the last slant build and it's been fine,zero leaks. The rope seals,well,let's just say they are not the rope seals of days gone by. Now perhaps they used an old stock seal that I believe had some asbestos in them,if so they are reasonable,but the new asbestos free seals are a joke. They are soft and laden with graphite and almost impossible to fit into the seal recess and cleanly trim the ends. They seem to soak up the oil and then leak out a black mess of oil and graphite.I've heard of many different engines with leaks resulting from these seals. The most recent spate is the Australian Holden grey motors,only rope seals are available and I believe real gaskets in the US are working on a decent quality old skool rope seal.
As for your issues,well,the neoprene seals should work. I'd asking if the block was line bored,check the crank for damage and runout and of course bearing clearances. There's a good write up in the article section here about the paan and seal install,well worth a read. At the end of the day,it's really the engine builders issue,I really think it's only fair to send the engine back and it's his hassle,tell him to run it and send you back a leak free engine or a refund and get someone who knows what's going on to rebuild you an engine.
As a side note,many small block engines now get their blocks machined for a one piece seal,it's normal practice on Fords,especially Clevelands since Fel pro don't do the two piece seal anymore. I'm sure a similar mod could be done to the slant,but in all honesty the Fel pro two piece seems to work well for me. Good luck and I hope you have some luck with the builder.

I'd ask, but line boring is spendy and not many machine shops will do this work unless specifically needed or asked for....

I would review the article by Doc on installing the neoprene seal...which brings up a couple things to look for on those:
1) make sure the lip is facing the right way... some places have the 'apprentice' install the gaskets and assemble the engine and they might have put it in backwards....

2) if the oil pan is off, and you pulled the seal to replace it, look at the area of the crank where the seal goes... originally the crank would have some diagonal grooves like a file (but not criss-cross) in that location to help the rope seal resist leaking oil... if those grooves aren't removed or smoothed out, they will 'file' or grind down the leading lip of the neoprene seal and it will leak...


FYI....

Hey guys, thanks for the reply. 500 break in miles later, and the thing is still losing a quart every 200 miles. The company in question sounds like they're going to refund my money and I've got another engine lined up.