[hoops]

I've got a valve noise in a new engine and I suspect a broken spring so I want to take three pair of valve springs off with the head still on the engine. I've done this before and have the tools for doing it, but I can't remember how much air pressure I used. In doing web research the few suggested air pressures I saw went from 90 to 120 psi. I think that kind of pressure will knock the piston down to BDC even with the piston starting out at TDC. I know I didn't use that kind of pressure the two times I've done this before.

I know about the rope trick, but prefer using air.

Anyone taken springs off with less pressure than I mention above?

Thanks in advance.

[68NovaSS]

I've never used air, but why not just pressure the cylinder to where it starts to push the piston down, if it ever does.

With the piston at TDC in the cylinder, the valve won't drop all the way down, remove the spring, lift the valve and pressure it until the valve stays put.

[LATECH]

Nope. 120 psi is good. the piston will head to bdc and you should let it. I woulnt do 3 cylinders at once. you are asking for trouble. Broken valve springs should be a breeze to find withthe cover off. Sounds like your overthinking things a bit.

[LATECH]

Originally Posted by 68NovaSS

I've never used air, but why not just pressure the cylinder to where it starts to push the piston down, if it ever does.

With the piston at TDC in the cylinder, the valve won't drop all the way down, remove the spring, lift the valve and pressure it until the valve stays put.

The piston may not stay at tdc even with low air pressure.It will tend to move.and the valves may or may not stay tight with low pressure. Not worth risking. Use 120 psi or what the shop air supply is.

[1ownerT]

I have done it with 90-100psi, no problems with pushing the piston down on mine. Leave the remainder of the plugs in, could even loosen all of the rocker arms so all of the valves will be closed to aid in keeping the piston where you want it.

[cobalt327]

90-100psi is fine. "Center" the piston at TDC firing so it's at the top and it won't go anywhere. If it DOES, stop and reposition it.

Do one cylinder at a time, start to finish before going to the next hole.

You can always use soft cord down the hole w/the piston below TDC. Once the cord is threaded into the cylinder, rotate the engine towards TDC firing until the cord holds the valve closed.

[DoubleVision]

I use small rope. Bring the piston to about halfway up it`s stroke, fill the cylinder with rope, by hand move the piston up, when it stops your there. Swap the springs, reverse the procedure and move to the next cylinder.

[Jr. Mechanic]

90-100 PSI is a lot. You'll just blow most of it past the rings and your compressor will be running constantly.

It doesn't take much to to hold the valves securely in place. Through trial and error I've found that 40-50 PSI is plenty.

[BigRoy1978]

I change springs on my race engine with the heads on. Usually use around 30-45 pounds of air if i remember correctly. The piston WILL go to BDC, just happens. But the pressure will hold the valve up easily...unless you smack it with something, then it will fall into the cylinder....

[engineczar]

I've changed many sets of valve springs with the heads on. I never use any air I just make sure the piston is at TDC. With the piston at TDC the valve can only drop so far but most of the time the valve seal holds it well enough to get the job done.

[454C10]

I use shop air pressure (90 to 120psi).

piston always goes to bdc when I do it but it works just fine at btc.

sure, some air leaks by the rings but not a big deal.

you need air pressure to not only hold up the valves but to allow some extra force to be applied to break the locks free in the retainer.

[pepi]

I think the rope and piston too TDC is the way to go, air pressure and keepers and BDC sounds a bit more risky. Leaving the plugs in and loosening the rockers was a good one also.. just my 2 cents

[sbchevfreak]

You can always use soft cord down the hole w/the piston below TDC. Once the cord is threaded into the cylinder, rotate the engine towards TDC firing until the cord holds the valve closed.

! I have done it both ways but usually prefer the cord. I use starter rope, cheap and works well. If using air, use full line pressure. Unless you are absolutely perfect on TDC, eve 35-40 psi will move the piston.
As for all the air going past the rings and the compressor running constant, if that is the case you have a much more serious problem than a broken valve spring.

[Rebel1]

I like the rope trick. Works well.

[coolstuffjlw]

What if you just run a compression tester on the cylinder that you are going to be removing the springs on and turn over the motor until the compression is up? That is usually 120 or above. Does anyone besides me think that will work?