My crankshaft seal leaks on my 12.5hp briggs vanguard.

the seal that goes around the crank on the bottom of the engine keeps blowing off it doesnt matter what i do it blows of any ideas? its a 12.5hp briggs vanguard

There could be a few reasons, none of which are easy to explain. the most obvious is you must have a bad seal and it must be replaced. If you have replaced it, then I would only question if 1. you got the right part, 2. seated the seal properly.
Now, if all that above is good, then the question is how are you running the engine? Alot of high RPMs cause pressure to build inside the block, not sure if its enough to blow out a seal, but possible..
Another possible thing could be an out of balance crank, causing the crank to wobble just enough to work the seal out of placement..

Did you do a pulley swap? and when doing a swap did you happen to apply heat? If so, you may have warped the block, or in most cases applying heat can ruin the crankshaft seal..


There are some guys in this forum that are far better at this subject than I am, but maybe this will give you some Ideas. Also, more information would be needed for further help im sure..

its the original seal and i had drove it like 4 times and the pop the seal came off and i dont have a pulley swap and not over reving it so should i just buy a new seal?

How old is the engine and seal? Is there a possibility that someone else did some repairs before you got the thing?

I would replace it, and observe. It it happens once again with a new seal, then assume there is a seperate problem.

Just to confirm, you are talking about the seal just above where the drive pulley is located..and when it pops loose do you lose alot of oil?

email me a picture @ junkstarbuilds@gmail.com

its possible its been replaced i just bought the engine from a guy about a month ago so i have no idea how old it is its a 12.5hp briggs vanguard ohv and i will get the picture to you as soon as i can might be tommorow tho im pretty busy today and ya its the seal above the drive pulley and ya it losses like all its oil because under that seal there is a little machined hole

On some shafts there are a spacer between the pulley and the crankseal, If that is missing, It could add to your problem. but replacing the seal might be the first step in solving this.

the drive pulley spacer looks like this:
Pulley Spacer

idk but it sure is irritating me

Ive changed the lower crank seal on a 12HP briggs - its pretty easy but be careful tapping the new one in - i used a large socket.

if you use a socket how do you get it over the crank shaft? and ia there anything i can do after i replace it to make sure it doesnt fall off

If it keeps popping off and your not over revving it or anything, check you're crankcase breathed. If its blocked pressure will build inside the block and blow the seal.

how do you check that?

greener179 wrote:

how do you check that?

you can let it run and pull the breather tube from the carb. the breather loks like this:
Briggs breather


Pull it off the carb, let it run, should see some "oil vapor" and should feel some slight preasure.

Start there, and work your way inward..if you know what I mean.

"The drain holes in the breather cavity (or breather cover on some engines) may be plugged. Access the cover and remove it. Wipe the cavity out and poke a small wire down the hole in the bottom, and check the cover for a small hole, and clean it out. The hole must be down when the cover is reinstalled. A new gasket is also required."

only problem is i cant run it because the seal comes off and loses all its oil and i think i need to buy the tube that goes into the breather any way its ripped down the middle so is there anyway to tell without it running?

greener179 wrote:

only problem is i cant run it because the seal comes off and loses all its oil and i think i need to buy the tube that goes into the breather any way its ripped down the middle so is there anyway to tell without it running?

well, if all the oil is out, might as well use some compressed air and blast all the little seep holes.. I have never done the job, but I would assume you could run a "pipe cleaner" into the holes and give them a cleaning?

the problem is, if its coming out because of built up preasure, you would have to "build" up preassure, which is only done by running it..

Did you check to see if you had that puller spacer?

i check on my motor it doesnt have it and doesnt show it on the parts diagram i did see a little thing the going into the machined hole on the diagram it says pressure release vaulve should i buy that?

greener179 wrote:

i check on my motor it doesnt have it and doesnt show it on the parts diagram i did see a little thing the going into the machined hole on the diagram it says pressure release vaulve should i buy that?

Im not a big time engine expert. I wish some of the other guys would reply for some extra "brains" on the topic. But, Could I see your diagram?

Please send your diagram so I can see what I "virtually" are working on..lol

I took the motor off , flipped it upside down ( after draining oil) and removed the oil pan - that way you know its in correctly - cost me like less the $5 for a new pan gasket and seal.

Id never changed one before, it went really easy , plus was able to check the motor/pan insides for anything out of the ordinary.

The seal is similar to one on a front brake rotor on a car - its part metal and part rubber - itll tap right in. Sounds like the bit on yours that falls down is just part of the rubber seal.


If the breather is bad you can either mod the old one or buy a new one- i dont think the rubber 'tube' is availible separate - might as well replace the whole thing anyway.