air in cylinder to keep valve closed...true?

I buddy of mine told me he plans on taking the valve seals out of his head. Ok, but then he tells me he's going to put air in the cylinder to keep the valve closed and complete the job with the head on... this seems like it wouldn't work... anyone heard of this or tried this?

Yes. You have to bring that cylinder up to TDC and lock the crank. The air pressure can rotate the engine and allow the vale to drop into the combustion chamber.

Somebody posted that they use a segment of cloth rope pushed in through the spark plug hole which could be pulled out later. Piston comes up and squeezes it into the valve.

I have used both methods, and they both work fine.

I've never bothered doing any headwork without taking it off so i am pretty curious about... how do you re-compress the valve springs to get the keepers back in during install?
I know how to get them off fine, but back in...???
Is there really a tool for this?

The air and rope methods work fine, I've even heard of guys just dropping a balloon into the cylinder then blowing that up and tying it. When your done, just pop it and pull out.

Good Luck.

We've used the compressed air through an old modded spark plug to change valve springs and seals without removing the head. Jerry rigged a tire iron and the cap portion a valve spring compressor to depress springs and put the keepers on the stem.

I've done the air thing on my Chevy truck (350 V8) when replacing valve seals. I didn't worry about TDC, I just used lots of air - no problem. Bike still has originals with no leakage. Ed

Jack wrote:

Jerry rigged a tire iron and the cap portion a valve spring compressor to depress springs and put the keepers on the stem.

:laugh: Nice.

Something to consider....

If the seals are shot, the valves are usually coked up pretty badly. On old bronze guide heads, the guides are most always shot.....


Before going to all that trouble do this first.

Pull the spark plugs out. Turn the motor over and looking through the plug hole, have a good look at the seating face of the valves and look for heavy carbon build up.

Sometimes the intakes will be seating well enough but the exhausts will almost always be in need of refacing and seats cut.....

Larry makes a good point. If the valves don't seal, then the air trick don't work. The rope would work, but if the valves aren't sealing then you are wasting your time since the head needs to be rebuilt.

Good point Larry. Neglected to mention that this was not a street bike motor and the real mission was to replace springs not the seals.

TeK9iNe wrote:

I've never bothered doing any headwork without taking it off so i am pretty curious about... how do you re-compress the valve springs to get the keepers back in during install?)

I use a valve spring compressor that looks similar to a gear puller (tricky to use and not incredibly safe). I've seen safer ones that attach to the rocker studs/camshaft rails that have a lever that compresses the spring like a camshaft lobe would. I would prefer that style if I could find one for my bike.