Hey guys I built a 350 with a aggressive cam, high flow heads etc etc and I have this rear main seal oil leak that absolutely CAN NOT get rid of. Replaced the 2 piece rear main seal with the poly top of the line two different times, have done all the tips and tricks with sealant in the corners and joints and I still have this damn leak. Currently I'm not running any PCV Valve and just have a breather on both valve covers. I deleted the PCV years and years ago for some reason I cant remember... since the crank case can flow freely I feel like this wouldn't contribute to the rear main leak but technically the crank case doesn't have a vacuum since it's free flow. I've heard of people putting the Wagner PCV Valve on and it curing weird leaks like this. Any chance this could be causing my issue?

 

Put in the pcv if this is a street car, doesn't hurt.
How are you sealing the rear main cap to the block?
what oil pan and gasket ?

 

I'll put it back in but I remember specially deleting for a certain reason... I'm sure it'll come right back to me once I throw it in. Black silicone between the rear cap and block especially in the corners. This thing should be bullet proof and just won't stop the drip. Very frustrating

 

Is there a bolt in this hole?

 

Yes There is. Why do you ask?

 

Because if there's not, you end up with a very difficult-to-trace leeeeek.

 

2 piece RMS? Are the seal lips facing the engine?
Did you offset the parting lines?
If YES to all, then remove and clean the rearmost oil pan bolts and apply a sealer to them.
Please report back.

 

I’ve had good results with anaerobic sealer on the 2 piece rear main cap.

 

Yes, Yes, Yes. I'm removing the whole oil pan in a couple weeks and going to add RTV the gasket in all of the corners, front seal, and in all of the corner bolt holes to see if that does the trick. Going to add a PCV valve as well to get some vacuum in the crank case. Will let you guys know how it turns out!

 

Well guys, ended up pulling the entire engine to get the F**KING rear main seal out. Oil pan couldn't drop without raising the motor and removing the trans. It sucked but its out. Absolutely zero signs of why this thing is leaking. Everything looks nice and dry when I popped the cap. I just reinstalled and did the following. Let me know if you guys have any input.

1. Cleaned with carb clean and then wiped all silicon and seal surfaces with rubbing alcohol.
2. Brand new seal
3. Front lip forward.
4. Offset seal 3/8"
5. Dap of RTV on each end of RMS.
6. Dap of RTV on rear main cap in the corners.
7. I even put a thin strip of RTV on the backside of the seal just incase it was coming around the back side which is super doubtful.
8. Re-Installed cap and torqued to 75 ft/lbs.

Did I miss anything?!?!?!?

 

Umm, maybe the leak is from elsewhere?

 

Rear china wall, or rear cam plug leak will look like a rear main seal leak too

 

Back of the engine block is total dry. I've seen it leak out of the RMS so I'm 99& sure thats it. Just saying that when I tore it apart there was nothing apparent. Anyone see any issues with my installation method? Should be bulletproof...famous last words.

 

That IS frustrating, I'm sure. Exactly what P/N seal are you using?

Did you happen to get a photo of the crank journal?

 

Fel Pro 2913 - Best two piece you can get from what I've read. I sure did, see below. The seal you see is the old one after I popped the cap.

 

Have you done a compression test and/or leakdown test?
Could be that despite your best efforts with quality parts and cautious assembly, that there's simply too much crankcase pressure to contain. That's regardless of the engine's mileage.
FTR, the PCV circuit can rectify some of that excessive pressure when it's allowing vacuum to the crankcase.

 

I have not performed either of those tests but I could once I get the engine back together. I also did buy the Wagner PCB valve so excited to see if those two items combined correct my issue

 

I've looked into that Wagner valve. Interesting item. At one time there was group purchase organized (by a member here possibly) but I missed the boat. I've spent many hours tuning with various valves looking for the operation that would have suited my last engine.

https://www.thirdgen.org/forums/tech...ions-what.html

I was dealing with a lot of blow-by from a worn shortblock. If I removed the valve from the rocker cover, it would puff like a little chimney.
There are some accounts of people setting up the PCV system to act as a sort of vacuum pump. It takes more than the OEM 3rd gen arrangement to replicate but it was an interesting approach.

 

Few know this and even fewer actually know to use sealer here:



It's a nice, straight path for an oil leak, especially if the machine shop deburred the sharp edge on the cap after align boring.

 

^^^ This ^^^

Esp on the driver's side. Note that there are 4 holes in the cap register surface: 1 on each side is for the bolts, the other on the pass side is for the pump drive rod to pass through, and the other on the driver's side is for the pressurized oil to come up from the pump, through the cap, and into the block. On that side therefore, there is pressurized oil trying to get out through that crack.

Use a VERY THIN schmeeer of non-hardening sealant such as thread sealer w PTFE or Permatex 200 (brown gooey gunk), NOT RTV since the parts constantly move, making sure to lay a large enough bead in the corners of the register step to seal up to the cap, where NoE drew the yellow lines. ESPECIALLY on the driver's side. The pass side can leeeeek a bit, butt the driver's side can literally SPEW if you don't get it sealed up. Obviously you don't want to use so much that it affects the main bearing clearance, or gets forced into the gap between the bearing halves or behind the shells. Those should be offset slightly anyway, much like the seal halves should be.

Note also that there should be a plug in the pressure passage on the driver's side, about an inch or so up into the block from the cap surface. That passage goes all the way from top to bottom of the block. Oil is supposed to come up into the block, be stopped by the cap, detour over to the oil filter, come back into the block above the plug, and continue upward toward everything that gets lubed. If that plug is not there, then the oil is not forced to flow through the filter; and instead of having an oil filter, you merely have a small reservoir of stagnant oil sitting off to the side, while all the oil remains dirty.

 

Hey guys, update since my last post. I got the engine back together and ran it with the PCV valve installed. Same leak from the rear main seal. It may even be worse now, getting roughly 1 drip per minute. I spoke with a mechanic who gave me a few good ideas...

1. Could the rear main seal be smashed against the crank shaft too much and not allowing pressure inside the crank case to push against the seal and instead push oil past the seal? Reference photos above.

2. Currently running a high volume/pressure oil pump. Could this be too much pressure for a pretty much stock setup and 2 piece RMS? It idles around 60 psi and gets to probably 90 when at high rev. I could definitely see this pushing oil into places where original GM engineers didn't plan for.

​​​3. Thank you sofakingdom, I'm definitely going to try this additional sealer next time I tear this apart.

4. Use a dial gauge to confirm roundness of the crank and make sure isn't some issue with the actual crank itself.

Any experience with any of these ideas?

 

1 No

2 No

3 You're welcome

4 Probably not

 

BTW when I said "Oil is supposed to come up into the block, be stopped by the cap, detour...", I should have said "plug", not "cap". It's about a ½" freeze plug type of thing.



You can also see pretty clearly in this marked-up factory drawing (not mine, I ganked it off of some other post somewhere, not my spelling mistakes) how pressurized oil is present at the rear main cap register on the driver's side and therefore why that surface needs to be sealed, as well as the oil flow path and the location & purpose of the plug I'm talking about. It often gets left out during rebuilds: it's a good idea to verify that it's installed in your block. There's a step in that vertical passage that it's supposed to be seated up against.

 

Thanks. Just to confirm, the area highlighted in green in the photo below is where you recommend a thin layer of PTFE Sealer, correct? Match on the other side as well even though there is no pressurized valley on that side? I can confirm that plug is in there because I personally installed it about 18 years ago! I'll double check next time I pop the cap and give it a smack to make sure it's still fully seated.

Side note - I spoke with a FelPro tech and showed him this photo as well. He said the RMS big lip does look flatter than it typically should when installed but doesn't think that would cause a leak. They gave me the desired journal diameter so I'll be checking that next time I pop the rear cap off. Should be between 2.430 - 4.432. This is an aftermarket forged USA crank that SHOULD be matching OEM specs.

 

Yes. Making particularly sure to get it down in the corner, where if the cap has been chamfered, will leave a little passage with a triangular cross section between the crankcase and the outside world.

Absolutely. Oil might not be pressurized over there butt the same rest of the situation can exist.

Personally the seal looks fine to me. I don't see any problem there.

Put a drop of sealer on the ends of the seal where it butts together, and leave it non-aligned to the block a bit, kinda like it is now. It may help to put a very thin schmeer of gorilla snot on the back of the seal where it goes onto the block.

Note also, in most blocks, some of the oil pan bolt holes are not blind, butt rather pass through into the crankcase, particularly the large ones at the corners. Put a drop of thread sealer on any that do so.

I would also recommend that you Scotchbrite the surface of the crank where the seal rides.

When installing the seal, be sure to use the little tool they give you, to prevent damaging the back surface of the seal. Install the cap half to the cap the same way as you'll have to do the block half, by rotating it around end-wise, rather than just jamming it onto the cap.

 

Do you recommend Black silicone or the PTFE Thread sealer at the butt ends of the seal halves?

 

Either thread sealer or gorilla snot. The parts move too much for silicone to work well.

 

Hey guys, small update. I called the ME Wagner PCV valve tech department and he recommended a few things before tearing the rear cap off again. With the baseline PCV valve tuning I was getting 1 drip from the RMS every 25 seconds. I then maxed out the PCV valve so it was pulling as much vacuum from the crank case as possible (I don't know what this value is but I maxed out the set screw). The leaking almost, if not completely, stopped. I got 1 drip in 15 minutes which could have even been residual. Super interesting. This is making me believe that either I have an excessive blow by issue OR the cam is so aggressive I'm getting barely any vacuum through the PCV valve and need to run it wide open to get a sufficient amount of vacuum through the crank case. Side note - I also tried completely removing the PCV valve and breather and was getting the same 1 drip every 25 seconds open to the atmosphere. Does anyone know how to measure blow by and what it should be? Is that called a leak down test? Can this be done at home without any specialized tools?

Thoughts?

 

The PCV valve has interesting flow characteristics. And different "set-points" depending on how the valve is designed. The Wagner looks to offer of range of those characteristics, more or less. If you didn't know:



There are accounts of folks that have used the PCV system as a means to develop higher levels of crankcase vacuum. Not a lot but the benefits are reportable. But any properly functioning system should impart some level of negative pressure. That's how it works. Guys that run vacuum pumps to build loads of vacuum in the crankcase have shown considerable power gains too. You have to have a well sealed engine for this.

You can observe any cylinder leakage through the leak down test you asked about. Its worth the effort as it's an excellent test for determining the relative state engine health. Once you read up on it you'll learn what it can help you with. If there's too much getting past the rings, that test will reveal that.
Leak down testers can be in expensive or not so. Harbour Freight entry level:
https://www.harborfreight.com/cylind...ter-64918.html

Along with an air compressor.

What's your idle vacuum and at what RPM? For that matter, have you posted all of your engine specs ( "Aggressive cam" isn't revealing very much.)? How about timing at idle?

 

Thanks for the reply and the link. You're right, thats definitely counter intuitive on how a standard pcv valve works...question, why wouldn't you want the pcv valve wide open no matter the RPM to always have max vacuum in the crank case?

​​I'll definitely order one of these test kits. I read the manual and understand all the issues this test can unveil. Is there a guideline to acceptable piston ring blow by? I know some leakage is normal.

I thought I gave all specs in the first thread post but just to confirm...

SBC 350
All forged lower end - after market but stock specifications.
AFR high flow heads
Holley 4650 4 barrel 750
Comp Cam ***** thumpr. I built it 10 years ago so don't know the exact model but similar to this in the link below. I know larger cams typical make your vacuum suffer. When I tuned the PCV valve I remember being right about 7-8" Hg off the back of the carb when doing the initial test. FYI this is where I have the PCV valve connected to.

https://www.compcams.com/*****-thump...RoCxvMQAvD_BwE

 

If you think of the PCV valve and the system in it's entirety, it's really a metered vacuum leak. As such you can see where you might not want it to be in play all of the time. That said, negative crankcase pressure is a desirable thing and this is why vacuum pumps are often fitted to engines that are more race orientated.
I've also come across, as mentioned earlier, PCV systems that employ a vacuum reservoir. The tank is "charged" under idle conditions via the PCV valve and engine vacuum. This imparts a significant vacuum in the crankcase which is then held, or most nearly so, for the duration of a drag strip pass. Remember that under WOT, there's little to no vacuum created so the reservoir gets emptied (in a reverse kind if way) during the several seconds it takes to make a run.

For your own experimentation, it's wouldn't be all that difficult to fit a vacuum gauge to the crankcase itself and observe firsthand how the ventilation system is influencing crankcase pressure.

As far as the cam goes, I can't find a cam card for it so as to reverse calculate what the engine idle vacuum could be, and it's is all but impossible without knowing the intake valve closing point. Not sure where the "*****' Thumper" cams have that worked out or where your particular cam was degreed to when installed.
FTR, I've run cams with 288/°294° advertised duration and installed with an intake centre line anywhere from 102° to 106° and idle vacuum ranged from 12" to 9" depending on the engine overall health and ignition advance.

 

Makes total sense the crankcase wants to be in negative pressure but why would the PCV valve act inversely with vacuum pressure? You would think that when your RPMs raise you're creating more blow by pressure and would want your PCV valve to be wide open to pull as much vacuum as possible, hence my question why not run max vacuum at all times? I get the vacuum the engine produces less vacuum as RPMs raise so why not have the PCV valve act accordingly?

I'll pull the vacuum numbers off the intake on Thursday when I get back to the engine and also try see if I can detect the crank pressure with my gauge. I'll have to get a little creative. I'll respond with the results!

 

Your logic is flawed in that the actual intended purpose of the PCV valve is in no way related to blowby or leeeeeks.

Its SOLE purpose is to extract fuel vapors, combustion products (including water), etc., for emissions reasons, in a street-driven engine. Any of the rest of that is purely incidental.

Keep in mind also, that vacuum in a running engine is highest at high engine speed and low load such as when decelerating from highway speed, and lowest at WOT, mostly independent of engine speed, which is of course when there would be blowby if there was going to be. It's somewhat high at idle depending on the cam and the idle speed; in a stock motor it's pretty high at that time although not at its maximum, butt in an engine with enough cam to create "lope", it's significantly lower. It's exactly zero if the engine isn't running.

The factory produced a variety of PCV valves over the years, designed to operate after the manner of the table skinnyz posted. In the not-running mode, it's closed completely off; in the extremely high vacuum case (2nd column of the table) it's pulled fully all the way as far as it goes but leaves a small bypass orifice open, thus flows a small amount of crankcase vapor into the very high intake manifold vacuum; in the "normal operation" (ordinary street driving like moderate acceleration and cruising) it's partly open, metering a moderate amount of crankcase contents into moderate manifold vacuum; and at WOT, it's mostly closed at which point the orifices are the largest, such that it vents a small amount of crankcase vapor into very low intake manifold vacuum. The trick in some cases is to select a valve from all that are available, without any published data, that matches your engine's vacuum characteristics.

In any case it's not going to do much of anything at WOT when blowby is at its greatest, on its own; that's why people use vacuum reservoirs to maintain crankcase vacuum for short bursts of acceleration. People do that sort of thing, up to installing some kind of a vacuum pump, to reduce windage losses in the crankcase. The effect is small butt measurable. If you're in one of those racing classes where the participants are all very closely matched, then a minor tweeeeek like that, if allowed, can produce a slight advantage. It's not for covering up poorly assembled gaskets & seals.

Crutching your engine with the PCV instead of fixing the oil leaks seems kinda ... well, you do the math.

 

It's so funny, I read that chart 4 times and thought I understood it and then read your explanation, go back to the chart and realize I had it completely backwards...

This all being said, it makes the most sense then that I actually have a real rear main leak (potentially fixed by the ptfe thread sealer on the cap as you previously recommended) OR way too much blowby due to worn rings which is making too much crankcase pressure and forcing the RMS to leak and I was able to mask the issue by increasing vacuum in the crankcase so high it mitigated the blowby pressures! Super interesting.

Next step is definitely run the tests, confirm crankcase pressure and blow by and if those check out, tear that cap back off and seal appropriately. If I still get the leak after confirming blow by is acceptable and sealed the main cap appropriately, I may just need to run a higher vacuum in the crank case although it's not truly what it's meant for...

I'll post the results of the leak down and vacuum tests next week!

 

In your message above that a Felpro 2913 rear main seal is being used. I went on the Felpro website and a 2913 does not come up but a 2912 does and it’s for a gm 350. So if it’s the 2912 Felpro states it for high rpm and high vacuum engines. Also have you been lubricating the sealing lip with clean engine oil during installation? If not it will run dry at start up causing excessive heat and damage the sealing surface/tension.

 

Update -

I did the leak down test. All cylinders are leaking between 2-4psi when testing between 50-60 psi through the rings into the crankcase. From what I'm reading, that's totally normal. You guys agree? This yields between 5-10% loss. Not sure if I need to have the initial pressure higher but the instructions said don't go over 100 psi.

I also adjusted the PCV valve to flow as much as possible at idle accordingly to the tech at ME Wagner. Leaking like an absolute sivv. Roughly 1 drip every 5 seconds out of the RMS.

Next step is to tear the cap off and seal per @sofakingdom recommendation. This seems like something more than the cap seal at this point because of how heavy the leak is but going to give it a shot.

 

5-10% is generally accepted as being very good.
Can you get a vacuum gauge on the dipstick and observe what the value is? This is with the adjustable valve set as you have it now and then try a range of adjustments to see what will generate the lowest pressure in the crankcase. More than few builders I've chatted with use this vacuum gauge approach while working through crankcase pan evacuation systems or vacuum pumps. You don't know what you've got unless you measure it.
Speaking of dipsticks, that's a source of leakage for any attempt at developing some negative pressure. It might be that despite working the valve as you are, it's an uphill struggle as any vacuum is just getting away.

 

That's a great idea I didn't even think of using the dip stick tube to measure crankcase vacuum. I'll try this tomorrow and give you guys an update. Super weird how this time around it was just dumping oil. Drip is coming right at the center bottom of the RMS. Can't pin point exactly where it's exiting out

 

What you want is a crankcase pressure measurement tool. There are many ways to do this. The cheapest way I have found without buying specific tool for this is use a generic 1-bar map sensor. I make a video to show how this is done

When the pressure of the crankcase drops like it should, the map sensor reports voltage decreasing, simple. You keep the voltage below the 'baseline' of 0psi atmospheric gage pressure, and walaa oil will be sucked back into the engine all the time.
Also, combustion blow-by is mostly water, and has carbon partially reacted hydrocarbons, which will circulate with oil leaving deposits leading to eventual failure. So You absolutely want some kind of vacuum-suction pumping the water and carbon out of the engine crankcase before it can dissolve into engine oil. Every gallon of gasoline produces around 1 gallon of water. Even if 1 or 2% of that goes into the crankcase you are looking at gallons of water over the course of a fuel tank just pumping into the oil system as a vapor which will dissolve into engine oil at some rate based on pressure. The pressure of the crankcase partly determines how much dissolved gas the oil will permit, more pressure = more dissolved gas, just like a bottle of soda, high pressure = lots of dissolved CO2 gas. When you remove the cap, pressure drops, CO2 has to leave the soda. So when the crankcase pressure is rising due to blow-by it is also forcing more dissolved gas products into the engine oil (water, carbon fragments, are a gas state when they leave the piston ring)

You can also measure the baseline pressure during operation without PCV. A typical small block chevy at 0psi of crankcase pressure and no PCV system shouldn't drip engine oil from the rear main. In other words, the PCV suction is helping of course, but that doesn't mean that not having PCV was the original problem en total. Something else could be causing the majority of the leak and the slight pressure is just enhancing the flow rate leaking out.

 

I'm confused. Which is it?

Have you ever measured what your idle vacuum is? A gauge off of the intake will tell you that. (Seems to me you had mentioned you'd check on that. )
Further to that, what's your idle timing?

 

Hey guys, its been awhile but FINALLY got some time to tear the pan off and redo the RMS after all of the advice I've received. I followed your instructions on using the PTFE Thread Sealer on the rear cap where it met the block face and along the chamfered corner as well as the 4 corner bolt threads and absolutely black RTV silconed the **** out of the pan gasket where it met the pan, front cover, and rear cap. I have official ran the engine for 5 minutes, 15 minutes, and 10 minutes and it is........ DRY! I'm going to run it again for 30 minutes adjusting the RPMs up and down and see if we're dry but were looking good so far. Figured I owed everyone an update since you've helped so much. Appreciate everyone's help on this!

Side note for everyone wondering - I readjusted the PCV Valve to its standard setting and completed all leak down testing which showed normal). The only thing I did not do was check the vacuum inside of the crank case. If we get no leaks on my next engine run, sounds like it really was just the oil pump pushing oil between the cap and the block. Never knew that was a leak point!

 

Come to think of it, I'm not sure the factory actually put sealer in those places... might be one of the reasons, if not the MAINMOST one, that gives the old 2-pc seals a bad rep for leeeeeking.

Took me until about the 10th Chevy motor I built, and actually WATCHED the oil coming out of the cap register crack, before I thought it all the way through, and realized that there was pressurized oil there at that unsealed spot. Then I started sealing them up like I described and NoE drew, and NEVER had a leeeeeeek there EVER again.

 

anaerobic sealer Is the best fort 2 piece rear main seal.