502 Mag Rebuild or New 502 ???
 

Hello, First I would like to say you guys have helped out so much on this site. I have learn so much from reading the post on here. My question are this. I picked up a 1992 Baja 260 last year this time. It is powered by a 1992 Gen IV 502 Mag carb with 670 hours on it with out any overhaul done on it. The boat is in great shape. Looks like it just came off the show room floor. I picked up the boat at a Baja dealer in my area. I spent 14000.00 on it. The motor seems to me to run strong. I burned about a 1 QT of oil in 14 hours of running it this summer. I started looking into rebuilding it because of the hours it has. I had allot of guys telling me to just leave it alone because it sounds and runs great. I do about 55 mph with a B3 and a rev4 21p prop at about 4600 rpm. I don't really hold it there long because of the age and hours. I took it to a marina close to my home to have a compression and leak down test just to see how healthy the motor is just the other day. Well the number don't look good.

HOT MOTOR TEST
#8 140
#6 90
#4 110
#2 125
#7 130
#5 140
#3 110
#1 150

leak down test showed #6 60-75% leakage and #1 10% leakage. leaking from intake.

COSTS

Rebuild cost from a very good Marine engine builder cost is 6000.00 2 year warranty.

A new Gen VI 502 mag 7200.00 1 year warranty.

or just do a Valve train for 2000.00

Question is what should I do? I love the boat it looks new. Really it shocks people when they see it. Oil PSI is good. I have had some tell me not to just do valve train and some say it is okay to just do the top. I have some tell me the Gen IV motor is much better then the newer Gen VI motors. Thanks for all the help.

The leak down test should have given you an idea of how good the rings are. If the leak down test is good conventional wisdom is do the heads and keep going. Mercruiser says 450 hours before valves. Your higher hours would seem to indicate that the motor was treated gently, so a valve job sounds right to me.

I hope my advise is good because the same thing happened to me this summer. 700 hours, bad compression, ran well. Did the valves now it runs better.

I should point out that I am not a mechanic and have no idea what your leak down numbers mean

I would pull the heads, do a valve job, and check the compression/leak down again.

Was 14 hours of running an entire season for you? Are you happy with the performance you're getting from the boat? And, how much do you want to dump into the thing?

If you generally only boat for around 14 hours per season you'll likely get at least another season out of doing a valve job. You may find that you're lower end is fine, that you were losing your oil out of valve seals, and you may get better #'s after having the heads done.

If you want to upgrade your power anyhow, I wouldn't spend the money on the heads.

If money is no object - go ahead and throw in a new crate motor.

Probably just needs a top end refresh. I would guess that the head gasket is mostly burned through between #4 and #6 cylinders and on its way between #1 and #3.

Whoever did the leakdown should be able to tell where the air is leaking on each cylinder.

Your engine is probably a Gen V.

do the top end, i would if it was me, valve job and check springs hard seats, and maybe a new cam and lifters and timming chain set up

My port motor just passed 1,000 hours (1995 502 EFI), running excellent. I did the heads around 500 hours. Now having said that, my compression numbers are not so good, very similiar to yours . Probably will have it freshened up this winter or maybe not.
Starboard engine spun a rod bearing at about 800 hours.

1st off 6k to rebuild your engine ??? sounds high to me...you can get by doing the heads....but if it were me I would go thru what you have....ck with a good engine/machine shop in your area...then find somebody to R&R...probably going to find the cam on the way out as well...

Thanks for all the help. I have had guys tell me that if I just to the top I could blow out the bottom half. Any thoughts on that?
the 6000.00 rebuild was also to R&R it. Do you think it still to high?

502 all come with forged bottom end,
with stock heads and cam I doubt youll have a problem "blowing" the bottom end especially if you still have good oil pressure. You can always have your engine oil analyzed to see if there is any metal in it.

6k seems a bit high. I am in the exact same spot and my compression tests looked very similar to yours. During tear down the Head Gaskets were found to be the problem. I am just doing a top end refresh and sending injectors off etc. I considered doing full rebuilds and doing the cams and stuff but in the grand scheme of things It would be double the cost and the additional power would have created a host of problems down the road with the current drive set up etc. My plan is to get another 3-400 hours out of them with fresh top ends and then next go around do full re-builds and go with a different drive set up.