Pump up the valves
Thread Starter
Racer
Joined: Nov 2010
Posts: 259
Likes: 70
From: MN
Pump up the valves
Ok, Im a bit spooked here. About to change the valve seals by pump up the cylinders with air method. A co-worker said I would be crazy not to bring the piston to the top of each cylinder I am working on to stop the valve in case of a mishap. Ounce of prevention thing. Another friend says no need up to do this, the valve is going no where. My compressor pumps up to 125lbs how strong will this hold up the valve? Can it take a pretty good bump if I was to accidently hit it when reinstalling the spring? Would appreciate any input. Thanks
Le Mans Master
Joined: Apr 2001
Posts: 5,941
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From: Was New Orleans but swam to Baton Rouge LA
Cruise-In IX Veteran
It does not matter if you bring the piston up or down. The air will keep that valve in place. Now if your compressor leaks, or stops working you might have an issue.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
If you fill with a lot of pressure...and the piston is not at/very near TDC or at the bottom of the stroke...the air pressure will just turn the engine over. You don't want that to happen. So, only fill with enough pressure to keep the valve in place or lock the engine with something to keep it from turning. 125 psi working on a 4" diameter piston will produce about 1500 pounds of force trying to turn the engine over. At TDC, it won't go anywhere. If it moves a few degrees of TDC, watch out.
You only need 20 psi or so to keep the valves in place.
You only need 20 psi or so to keep the valves in place.
Thread Starter
Racer
Joined: Nov 2010
Posts: 259
Likes: 70
From: MN
Couple questions... what harm would it be if the engine did turn over, that doesn't neccassarily seem like a bad thing to me. Wouldn't the piston just go to the bottom of the stroke and stay there? Also if it was to move a few degrees what would be the negative outcome of that? Thanks
Thread Starter
Racer
Joined: Nov 2010
Posts: 259
Likes: 70
From: MN
Kinda lost here. What exactly is the downside? If piston gets pushed back down the cylinder what damage/harm to the engine will be done? What physical harm will come to the engine if it gets pushed back cw/ccw? As for the car being in gear and the motor turns I can see where that would not be a good thing. Wouldn't want to be in front of it when that happens.
Le Mans Master
Joined: Dec 2007
Posts: 6,422
Likes: 591
From: McHenry Illinois
The alternative to using air to hold the valves closed is to feed a couple feet of small nylon rope into the cylinder, then roll the piston up. The rope will hold the valves closed. Just make sure to leave the end of the rope hanging out of the spark plug hole so you can remove it when done.
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What I did when I changed the valve springs, heads on car:
Take out all spark plugs, shove about 12" of bungee cord (no hooks) into spark plug hole with piston about half way to TDC, (You want longer bungee, leave some hanging out the hole) rotate engine toward TDC with wrench until resistance it felt, and your valves are not going anywhere, even if you bang them with a hammer to break spring retainer locks.
Reverse engine rotation to remove cord, move to next cylinder.
Doug
Take out all spark plugs, shove about 12" of bungee cord (no hooks) into spark plug hole with piston about half way to TDC, (You want longer bungee, leave some hanging out the hole) rotate engine toward TDC with wrench until resistance it felt, and your valves are not going anywhere, even if you bang them with a hammer to break spring retainer locks.
Reverse engine rotation to remove cord, move to next cylinder.
Doug