Valve seals

[Mike #27545]

I'm attempting to replace the valve seals on my '79 with the heads on the motor. What air pressure should I maintain in the cylinder to keep the valves from dropping? I would assume too much pressure is going to force the piston down the cylinder?

Thanks for the help!

Mike

[motorman]

Re: Valve seals

take all the rockers off at one time and it will not make any difference if the piston goes to bottom dead center. use 100# air pressure and use a 9/16 socket and tap it with a hammer to break the keys loose so when you push down on the retainer the keys will be free.

[Mike #27545]

Re: Valve seals

Thanks for the info. I tryed to get started on this last night but I ran into trouble with the valve spring compressor. I purchased the screw down type (not the stud mount)spring compressor..all it did was pull the bottom of the coils up. I thought maybe I had to buy the stud mounted type for better results. I'll give your suggestion a try.

Thanks!

[Terry M.]

Re: Valve seals *TL*

Mike,

You may need both the screw down compressor and the lever type that attaches to the stud. The lever type is easier, but on some valves (passenger rear with AC or driver's rear with PB) there may not be clearance for it.

The symptom you described (pulling up the lower coils) is due to not breaking the keepers loose from the retainer. Use the technique moterman explained. Rap the 9/16 socket with the hammer. I just use unregulated shop pressure - around 140 lbs. Beyond the minimum necessary to hold the valves closed it doesn't matter how much pressure you apply. This is a case of a little bit is good and more is better. I would baulk at something over 200 lbs, but this high pressure is not available most places - unless you are working in a dive shop.

Good luck.

Terry

[Dale Pearman]

TERRY IS RIGHT

[Gene M.]

Re: Be safe!

I think I would prefer the pistion be at top dead center so in case a valve does not seal (with the compressed air working on it)it will not fall into the cylinder. The top of the piston would limit it's fall.

[Terry M.]

Re: GENE IS RIGHT, but..... *TL*

the piston will often go to the bottom of the stroke and it is a PITA to keep it at the top. It is also unnecessary. I can not count the number of times I have done this (probably close to 100 engines) and have yet to have a valve fall. I agree it is comforting to have the piston stay up (excuse the expression), but it is not necessary so long as the air pressure remains. I think the real risk of the valve dropping relates to losing air pressure due to a broken hose or fitting. The rings will generally seal better at the bottom of the stroke than they will at the top, unless there is some problem with the cylinder bore.

If either of the valves is not seating well, as evidenced by the sound of air escaping through the carburetor or the exhaust system (or even worse the rings are failing as evidenced by the sound of escaping air at the crankcase) one would be well advised to consider more serious repairs than the valve stem seals.

BTW: Introducing air into the cylinder in this fashion can be a test for the mechanical integrity of that cylinder. Certainly not as accurate as a cylinder leak down test, but it will give an indication of gross mechanical problems.

Terry