perplexing rear main seal leak

[John G.]

The mystery continues. For whatever reason the rear main seal on my 327 continues to drip oil. The engine was refreshed with the usual suspect part (main and rod bearings, rings, valves, etc.) replacements. The new gasket set rear main seal was installed very carefully, or so I thought. RTV sealant was applied to the rear main cap, as per the instructions. The seal was rotated slightly to offset the parting line of the rear main cap and 2-piece seal. The crank journal surface the rear main seal rides against was checked carefully for any grooving, burrs, etc. .. Everything seemed to be ok.

Upon first engine start up (20 minute run time at 2500 rpm) it wasn't long before oil was noted to be dripping from the rear of the oil pan.

Long story short..........The oil pan was dropped and a second replacement rear main oil seal was installed. Once again all possible areas (crank journal, rear main cap seating, etc.) were checked for any possible irregularities. Everything seemed to be ok, as in the first time around. The appropriate rear main cap areas were again sealed with RTV, the oil seal installed slightly offset from the rear main cap parting line, etc. - as in the first time around.

This second replacement rear main oil seal is the GM seal that is available over-the-counter. The idea being to install the best available rear main seal to hopefully stem the leak problem............well.............it's still there! .. The leak continues.

Until now I've never had a problem with a rear main seal leak. What gives? .. Could there be another potential area where a leak might occur? .. I checked very carefully (engine running) from below the area around the canister oil filter gasket, oil pan gasket, etc. The corners of the oil pan were sealed with RTV.

This is one stubborn leak. Has anyone had this happen? .. There was a mention here on the forum of a dye that can be added to the oil to perhaps find a 'trail' of sorts to help nail down and verify the leak source. Is there a brand name for this stuff? ..

Any ideas on where to go from here? .. Thanks for any replies.

[Clem Z.]

Re: perplexing rear main seal leak

i would not use RTV but use non hardening permatex as a seal. RTV is not a good seal where pressure is involved

[Edward J.]

Re: perplexing rear main seal leak

John, since this looks to be a nightmare, dye would be the best thing to do. but since the space between the flywheel and engine are so small its going to be tuff to make a correct dia. It would be easier to back out transmisson and bell housing, flywheel to use the black light there maybe more than meets the eye. just before you start to try the dye and run engine and make sure theres nothing coming from the top of motor.

The dye may be had at the local Napa parts store.

[Timothy B.]

Re: perplexing rear main seal leak

John,

Is there a chance the leak could be from the rubber gasket that goes over the rear main cap. Possibly from the interface where the rubber meets the pan. It may be a good idea to use some weatherstip adhesive to stick the gasket to the pan.

[Dick W.]

Re: perplexing rear main seal leak

Dye should be available at any professional parts house, i.e. NAPA, Carquest, etc. You will need a black light to seal the dye in the oil.

The flywheel acts somewhat as a fan and could spread the leaking oil away from the actual leak. Look for a vertical tell tale stream, do not worry about oil that might be spread by the flywheel. Avoid driving down the road as much as possible. The wind will also spread the oil.

[Richard T.]

Re: perplexing rear main seal leak

Sorry you are having this problem with your 327. There are several different products used for assembling an engine, all these products are different; Permatex makes them all. Like Clem said RTV is not one for your application. Assuming your crank journal and the caps and bearing mate properly you should be O.K. with a high pressure Permatex product, that dark brown stuff. Heavy based grease should hold the pan gasket in place for assembly; do not over torque the pan bolts. The dye sounds interesting if you cannot tell where the oil is coming from. Hope it turns out O.K.

[Clem Z.]

Re: perplexing rear main seal leak

use permatex in the area shown. https://www.forums.ncrs.org/attachme...24117&uid=9213

[Michael G.]

Re: perplexing rear main seal leak

John,

Assuming the engine is installed in car....let me throw out this theory. I had a similar 327 engine that went through a like kind rebuild. Upon start up using a run stand all went well for a bit then started seeing an oil drip from rear of pan. Thought I had the same issue as you.....real main seal leak.....but no. We took a closer look during the run-in and saw the leak was coming from a source higher up at rear of engine. One of the screw-in cam plugs apparently did not get enough sealant at the machine shop and was allowing a small amount of oil to drain down backside of block and then back of oil pan. Had engine been installed in car would have appeared as a rear main leak.

[Jim D.]

Re: perplexing rear main seal leak

Just a thought, was the seal lubed prior to start up? I've seen many seals fail immediately upon start up because they were installed dry. RTV is fine. I've used it many times with no leaks.

Jim

[John G.]

Re: perplexing rear main seal leak

I had to wonder after the leakage problem continued with the second go-round new replacement rear main oil seal if, in fact, the oil seal itself is actually where the problem lies. After two rear main seal installations it wouldn't seem so. .. This second seal is supposedly the 'good' one, from GM parts. The claim is that this rear main seal is used in NASCAR engines.

Cam plugs were drilled and tapped for set screws. Threads were sealed. . . Looks like the dye is next to try and nail this thing down. Thanks for all replies.