Changing Valve Stem Seals SBC
 

I need to change out my valve stem seals on a 350 at some point. I have the spark plug adapter to hook up to my air compressor. What PSI should I use to hold the valves up with out turning the motor?

why do you need to change your seals? and just run it at full pressure. probably around 100-120 psi.

Sometimes I get a little blue smoke at startup. Vacuum indicates that my valve guides are good. Motor is of unknown mileage, but good compression and oil pressure.

vacuum indicates your guides are good?

Yes, vacuum is steady. Excessive vibration that steadies as RPM increases would indicate worn guides.

I've seen plenty of cases of worn guides with steady vaccuum. I've also seen plenty of cases of shaky vacuum with excellent guides.

Hmmm, something to think about.

I always rotate the motor to TDC on the cylinder I'm working on. I wouldn't run any more than about 10-20psi or just enough to hold the valve up. This way if something happens like a sudden loss of pressure you won't drop the valve out of the guide. Running full line pressure can be enough to rotate the engine but isn't enough to allow you to break loose the locks from the retainers if they've been together awhile.

Just a note for people without compressors, or don't trust them......Take some nylon rope and shove it in the spark plug hole (leave a bit out)....turn so piston pushes it up.......change your seals.....lower piston, pull rope out.

the ol' rope a dope trick. much more reliable, and not that difficult.

i tried that with poor results. i ended up having to unbolt my headers and compress the cylinders. i also used full pressure, if i hadnt, due to the keepers being "locked" on, if i had used any less, i wouldve lost a valve. as it was, even with full pressure, some of the keepers didnt want to let go and took some effort to release.
i believe the reason it ( the rope trick) didnt work for me is because of the heads im running. the chambers arent open to the whole cylinder and when i tried to bring the piston up, the rope would get pushed up to the head before it could make contact with the valves.

Did you remove the rockers before you tried turning it over?

For the next guy wanting to use the compressor method (the way I do it)- put a large 1/2" drive socket-open end down- over the retainer and with the cylinder exactly at TDC and pressurized, whack the socket w/a large rubber mallet. This will at least separate the retainer from the locks, and if done 'right', will often eject one of the lock halves in the process. It will be caught inside the socket.

Occasionally, if not on TDC, the engine will rotate from the compressor's pressure. Keep hands and tools out of the way of the belts and fan, and for heaven's sake- turn the key to off (better yet, remove it and put the key in your pocket so a 'helper' listening to the radio or something doesn't turn the key to RUN. You wouldn't want to start the thing up by the compressor turning the engine over).

Instead of bumping the engine over w/the key (the easiest way to have the key left ON, BTW), use a breaker bar and socket/extension on the crank bolt to move from cylinder to cylinder.