REPLACE the valve springs on ALL 500HP and lower Merc motors at NO MORE than 200 hrs, problems or not! You can fill the cyls with air (piston @TDC) such as doing a leakdown test, and change the springs without removing the heads. I have replaced the springs on at least 50 motors, and seen broken springs from 35 hrs to others lasting well over 400 hrs. ALSO, replace the keeper wedges that hold the spring retainers on the valve without fail !!! Those are the 2 weakest links in Merc motors, and consistently cause the most problems.... and if you lose a keeper @ speed, I assure you, you won't even have a useable core left. There are several tools for valve spring replacement that are not expensive, but depending on the room you have, may require more than 1 tool to make things easy. Don't wait, get after it!

The broken springs I have experienced you have a miss that comes and gos.The oil got dirty fast.

I got REALLY lucky,,, I bought a 502 with only 90 LBS of compression in #6 last spring,,, figured Id throw it in, run it all summer and rebuild in the off season if it held all summer,,, well,, it did,,, yanked the valve cover to start teardown,,, and there it was ,,, one of the keeper wedges came out just enough to ride the rocker arm and not let the valve close fully !!!!!!!!!

NO DAMAGE !!!!!!!! not even to the rocker arm !!!!!!!!:eek: :eek: :eek:

Scott,
The Gills can be removed without removing one of the motors. There are several ways of doing this. One way is you can take all the nuts off the studs, then slide the manifold all the way towards the other manifold. Take a pair of needle nose visegrips and from the bottom lock them on the stud between the head and the manifold and work the stud loose. Another way it can be done is loosen a nut on the stud, then take another nut and thread it on, tighten the nuts together and try turning the stud out with the inner nut. Then another way that you could remove them is with a stud remover, thats if there is enough room. There are probley other ways to remove them, you have to be imaginative. Those are ways I have removed them in the past. They can be removed with out taking a motor out. I have other ways too. Hope that I helped you out. If you have any questions feel free to e mail me. Have fun

Findme.
That is very good advise with presurizing the cylinder. That would be an excellent way off changing the springs and retainers with minimum down time.

Great info guys. But just to clarify, I do not have a boat at the moment. I was considering one with twin 500's and looking for what to expect. Thanks!!!

Also, as far as filling cylinders with air to hold the valves closed, while it sounds all good, (and have done it this way before myself)
I myself just prefer to have piston at TDC with no air and just use a spring compressor that has the fingers to compress the spring. Reason being, compressed air has a way of rotating a motor over if not positively blocked. Then you have the chance of the valve dropping down. By using no air and on TDC, the valve will just drop abit until it hits the piston. It ain't going no where. No way to drop it in the cyl.

Again, Thanks

US1,

Was contemplateing your method. Glad to hear that it works. May use combo of that & "low" air pressure.

Mark

What brand/part numbers are you guys using? There are a LOT of really good springs out there that will go several hundred hours on a cam of .650" or less lift.

Blown1500,

What springs do you recommend for a stock '99 454 magnum? Also, what seat pressure and installed height?

checkmate454mag, I don't like the springs Mercruiser uses and always file them in the round file. I like Isky the best, but Crane and Comp have some very good pieces. Always use the "premium" springs for the application and FOLLOW INSTRUCTIONS.