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I have a Craftsman walk behind mower with a Briggs model 128T05-5268-B1 engine. Recently the machine seemed to be using quite a lot of oil. It was working hard and was hot and a bit smokey. I checked the oil and it was off the stick. I topped it up and after ten minutes or so, it was smoking again. I checked the oil and it was low again and again I topped it up. This went on for a while and I decided to dig into the thing and see if I could find an obvious problem. I was unable to find anything obvious but looking around at the parts diagrams I noted the oil breather assembly and decided to have a look at it.

Once the engine was apart, there was no obvious problem with the oil breather but I tore the gasket getting the part off. A new gasket is $3 and the whole vent with a gasket is $7. I got a new vent assembly and installed it. I also replaced the spark plug, the air filter and I drained the oil and topped it up with the specified 18 ounces of straight 30 weigh oil.

I started and ran the machine. It ran OK with no more smoke than normal if I worked it hard. However, I noticed quite a bit of oil on the mower deck right under the oil vent. I checked the oil level and it actually seemed a bit high - I'm not confident that 18 ounces is the correct spec - I think 14 would be better. In any event, I checked the oil breather bolts and they took a bit of tightening and I ran the machine again. Once the machine was good and hot it started leaking again at the same place. I tore the machine down again and completely removed the oil breather. I checked the oil galleries, which were nice and clean. I did notice a bit of old gasket on the engine surface. I cleaned this off and then put Permatex FormAGasket on both sides of the gasket before reassembling and let the machine sit for two days before running it again.

My efforts made no difference. Once the engine was hot, it started dripping oil on the mower deck, right at the same place below the oil breather. There is no external orifice on the oil breather, so the oil is not coming from there. I'm quite confident the gasket is not leaking. The oil comes out around the bottom of the cylinder, but not at the head gasket. It comes out at the body of the motor. I have been assuming that the oil breather vents to the carburation system, but I don't know how. It is possible this just comes out of the engine on some vent I'm unaware of?

This only seems to happen when the engine is fully hot and working hard. An suggestions as to what is happening here?

Post Script:

Writing these things down always helps me think through them. I found a better parts diagram for this engine at https://www.m-and-d.com/pdfs/briggs/12U800-ms0989-0904.pdf and I noticed that there is an "oil breather passage", part number 697734. This appears to run under the motor and over to the carburetor side. I suspect this is leaking; possibly a bad gasket or just loose bolts. An suggestions on how to get at this component without removing the motor from mower chassis?

You are correct. The engine breather is typically connected to the carb or air filter so that engine fumes will be burned in the combustion chamber. It's important to have the engine able to "breathe" or water and other vapors will be trapped inside the engine. The breather hose may have been removed or may have become stopped up. Trace the breather system and find the 2 connection points. Clean and re-attach the hose. If not, dust and debris may be sucked inside the engine.

An older engine may simply make more "fumes" when it gets hot.

It appears that the breather is attached to the carb by this "oil breather passage", which is a casting attached to the machine by four bolts. They look difficult to get at, but maybe I can reach in there with a small wrench and check them for tightness. I may also have bad rings. My compression gage did not show a lot of compression, though the machine runs fine (and the compression gage is old and of suspect quality.) When hot, the machine may be pushing a lot of oil, which if it weren't leaking, would probably smoke heavily. I'll get in there and have a look.

18oz is the correct capacity,
actually the gasket (697734) and breather passage is on top of the engine, under the flywheel and right behind the ignition coil.
you also won't get an accurate reading of compression as the camshaft has a compression relief device on it that makes it start easier (Pull easier). Leakdown tests are better for engines with a Comp. release. but around 90PSI is usually what i get on these engines.

Scrubcadet10 said:

18oz is the correct capacity,
actually the gasket (697734) and breather passage is on top of the engine, under the flywheel and right behind the ignition coil.
you also won't get an accurate reading of compression as the camshaft has a compression relief device on it that makes it start easier (Pull easier). Leakdown tests are better for engines with a Comp. release. but around 90PSI is usually what i get on these engines.

Click to expand...

Well, that explains the crazy low compression reading in an engine that runs! Thank you.

So if the "oil breather passage" is on the top of the engine, and the oil is not coming out of the breather gasket, where else, on the bottom, valve side, of the engine could it be coming from?

Studying this nice parts diagram closely, I see that the crankcase is split horizontally and that there is a gasket that goes between the cylinder block and the sump. I can't see in there well enough to see exactly where the leak is, but it is in that area, just below the valves and oil breather. Is this a common failure in these motors?

If this is the source of the leak, would it be typical that the leak would get worse as the motor heats up? I don't seem to get any leaking until I work the thing hard and it's getting fully hot.

i wouldn't call it a common problem, but it's not impossible to happen.
i would also check if there is any oil ontop of the blade and crankshaft. just in case the crankshaft seal is leaking.. remove the spark plug wire if you move plan on rotating the blade.

I just watched a couple of YouTube videos showing this repair and it does not look difficult. Worst part is that I just filled the fuel tank and the machine has oil. A bit messy but not difficult. I'll pick up some good gasket goo and take the pan off. I think I've been chasing the wrong issue with the oil breather. Thanks for your help.

This is my first time with this forum and it seems to be a good one!

Let's not get the cart ahead of the horse. next thing i would do is CLEAN the engine and mower deck good enough to eat off of, then run it, or wait for it to leak, it will be easier to PIN POINT the leak.

I've got it quite clean now. I even used some carb cleaner to blow out the suspect area, but I can't really see where the oil originates from, just that it is below the oil breather, then down onto the mower deck. I can't find any other place that it could be coming from other than the pan gasket. I don't know much about splash lubrication, so I don't know how leaks in this type of motor manifest themselves. A leak at the pan gasket checks a lot of boxes though; worse when the oil is full, worse when the engine is hot, no significant exhaust smoke. I guess I could check it a little by seeing if it gets worse when the mower is tipped to the left. I can have this engine stripped and off the deck in about a half hour (I'm well practiced at the tear down - done it three times!). From there it's just seven bolts to see the gasket.

I tell you what you're luckier than me. Almost every mounting bolt on that type of engine, to the deck, i've encountered even after soaking in penetrating oil, either snaps off in the engine or rounds the head off. i usually end up drilling them out and going back with 5/16 grade 8 bolts and nuts.

Well, I don't have it off yet. But I'll take your advise and get the bolts soaking right away. I won't get a chance to work on it until next week. By then it should be loosened up, if the bolts are corroded. This machine is not that old and in pretty good condition. Chances are the bolts are not too bad.

I thought I'd confirm my theory that the oil is coming from the seal between the block and the oil pan. I started the machine (that's another issue - it does not start well now) to get it good and warm and then set it on a milk crate with the nose down to see if I could spot the leak. This didn't work well. After two hours of sitting, there was no appreciable leak. I then started the machine again and worked it hard mulching up some deep leaves. In short order I could see oil accumulating on the mower deck below the muffler. Attached is the best photo of this I was able to get. The visible bolt head is the lower bolt of the oil breather. You are looking in from the side towards the front, right under the cylinder cooling fins. My guess is that the gasket is leaking in this area, though it only seems to do so if you work the machine hard. If you just let it idle, it never starts to leak. Does that make any sense?

Has that sump base been repaired at some point? JB Weld maybe? Not sure from the picture but something looks odd there.

tom3 said:

Has that sump base been repaired at some point? JB Weld maybe? Not sure from the picture but something looks odd there.

Click to expand...

if you mean to the right of the bolt head, i think that's the edge of the valve cover/breather assembly. looks pitted

Hard to say about repairs. I got the thing when it was being tossed out by its original owner at the local transfer station maybe four years ago. The guy said it would not run and he couldn't fix it. The issue was a clogged main jet and I had it running in less than an hour. I'm pretty certain that this was a guy who was not going to take on a replacement of the sump gasket or even do a bondo job to some leak.

I'm not sure exactly when I first noticed the oil leaking. I used it hard one day (I mow the woods with it) and it started to knock badly. I shut it down and checked the oil - none on the stick and it took a bunch. Since then, I've regularly had to top it up. The valve cover/breather is brand new and I sealed it with Permatex in addition to a new gasket. I'm certain that is not the source of the leak though the oil appears right under there. Take a look a this video , about 4:20 in. I think this is my issue. the sump gasket has failed in the area under the cylinder. Now I'm wondering, should I fix it with a new gasket or use a good gasket sealer? Clearly the stock gasket, which is just paper, is not so good.

You can either go back with a new gasket, or gasket maker, I prefer Permatex Ultra Black sealer. Clean both surfaces really well, apply it to one of the surfaces. And just snug the sump bolts up. Wait 24hrs, then torque the bolts.
the torque spec for the sump is 90 Inch pounds

Sounds like a plan. I'll probably get on it next week. Have a happy Thanksgiving. I'm being grateful for good help - thank you.

BE careful doing that
The gasket sets the end float of the crankshaft
So you can end up with it flopping around like a dunny door in the wind or being as tight as a fishes bum

The 12 series. Vertical use a shim for end play

And the thickness of the sim is determined by closing up the motor, measuring the end float then placing the approppriate sized shim ( or shims ) on the crank then buttoning up the engine a second time.
I have been setting end floats on cranks that are perpendicular to the case joint since I was 12
Shim or no shim the gasket changes the end play .

I did think it was odd briggs,from what I could see only sells a .012" shim.
It would be wise to use the original gasket

Thanks for the heads-up. I'll get a genuine gasket and look for the shims and make sure they are reassembled correctly.

Should be only 1 shim on the crankshaft, just slides over the shaft and sits flush against the gear. Sometimes the camshaft will have a shim too.

I had some time this after noon, so I wend ahead and pulled the engine off the mower chassis. As warned - one of the three bolts broke. It was odd, the bolt was not seized but it would only turn a tiny bit. It was the one on the right side that you can see the tip of coming through the motor, and I could see that it was turning. I gave it lots of PB blaster and tried working it lose. I got it up to about 1/4 of a turn and then the head busted off. Fortunately, I was able to drill the bolt out pretty easily and a 3/8 bolt and nut will work fine as a substitute.

As for the gasket, it came apart when I split the cases, so it is hard to say if if was actually damaged and causing the leak. As the photos show, there is plenty of fresh oil right a the place where I see the worst evidence of a leak. I'll have a new gasket in a couple of days and I'll button it up and see how it does.

Ever heard of using UV dye to find the leaks before you tear things apart?

Nope. Please tell more....

You put dye in the oil and with a UV flashlight you use it to trace the leak. works great for cars and trucks too, i rarely use it for small engines though.

I'll remember that. It could be useful someday. As for this issue, the place where I think the leak originated from was not visible as it was right under the cylinder. I'll know the truth when I get the gasket, put the thing back together and run it. If it still leaks, I might just try the dye route!

Really a clean looking engine inside, shows good maintenance.

Well, the gasket arrived today and I reassembled the engine. After cleaning both the block and the oil pan, I greased the surfaces and pressed the gasket into position on the block. I carefully lowered the oil pan down and torqued the bolts in a criss-cross pattern. After getting the motor back on the cassis and putting all the other bits back in place I started it up. It was difficult to start and I think there is an ignition problem that I'm going to start a new post about. I ran the mower long enough to get it up to full temperature AND THE DARN THING IS STILL LEAKING! The oil leak does not seem as bad as before, but it is right in the same area with the same symptoms of oil seeping down onto the mower chassis just below the piston cylinder and somewhat to the muffler side.

There aren't any other sources for this oil that I haven't checked and put new gaskets on. I looked at the block and oil pan and there were no obvious cracks or other defects that could leak. The gasket surfaces of both parts were not severely gouged or otherwise damaged. I'm at a loss as to where the oil is coming from. I guess I'm going to have to learn to live with it and be sure to top the oil up from time to time.

Dang it, i hate to hear that.
The only thing/places it could leak oil are the sump gasket, the valve cover gasket, top and lower oil seal.
You did say you replaced the breather and breather gasket, right?

Yep. Both are brand new and I applied a coat of gasket sealer on the gasket. I'm pretty sure that's not my source, especially since the oil seems to be accumulating somewhat in front of the oil breather. It's a mystery.

Unless you try what startech said with the dye and uve light

I think I'm just going to let it leak.

Any chance it's leaking from the top seal, under the flywheel, running down the side under the tin?

When you installed the breather gasket did you by chance block the 2 little slots in the gasket? These are oil drainback passages. If they are filled with gasket sealer or such it defeats the breather from separating oil out of the crankcase vapors. Some engines will leak oil from the breather if they are blocked.