Never done it but I have seen 80-90 psi recommended. To be safe, make sure the piston is at TDC for the cylinder you are working on so the valve can't drop if you lose pressure. I'd probably also let it sit there under pressure for a minute or so to make sure the compressor can keep up if you bleed of any pressure. A small compressor might not be able to.
put car in neutral and rotate the piston you are working on to TDC then put trans in P or gear, so air pressure will not push piston down, and then hook up air. 70+ PSI is fine and I have not had a problem holding up the valve that way. On the 327 plug access is a breeze to get the air fitting on....but on my Gen 2 LT1 car with headers no way so I do the TDC method with no air as the valve only drops about 1/2" with piston up. starting with #1 and do both valves then #6. Rotate engine 90 degrees and repeat with Cyl 5 & 8. Turn another 90 degrees for 7 & 4, turn 90 degrees 3 & 2
There is also the old school way of pushing 1/4" rope through the spark plug hole with cyl TDC
I've done both methods, air and rope. I don't like the constant hissing and losing air. I just replaced valve seals not too long ago and used the 1/4" cotton rope. I brought the piston up to 45* btdc, shoved in about 18" of rope, then rotated up until the piston was snug against the rope. All went well. How much rope you'll have to use is something you'll have to feel out. Cylinder bore, head chamber, piston design, etc all play into this.
sure you can. If you don't stop the crank from turning as a result of the air pressure that piston will go down but the air pressure will keep the valve up...as long as you don't lose pressure.
I definitely prefer the rope method if the engine is out of the car. no worries about keeping the engine from turning or having a loss of air pressure for some reason.
The reason I pulled the engine is the gentleman that built the car loved chrome and everything on the engine that was chrome leaked. The engine has very low miles on it but it sat for sometime so I thought I would replace the seals. No other problems so I don’t want to tear it down any further
Depending on how much chrome he had, valve covers timing chain oil pan etc you might be as well off buying a complete gasket set vs buying the individual gaskets. Not sure if you will get umbrella seals in the complete set or just the O rings.
The right way is to use about 90 PSI, and, turn the crank until the piston is all the way down in the bore, AT BDC, NOT AT TDC.
Why not at TDC? Well, the air pressure will push the piston down the bore, just like when it has the fuel/air mixture lit off. Think about it, where does the piston go when the cylinder pressure is raised, DOWN, and you DO NOT want that air pressure pushing the piston down the bore while doing the seal changes.
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