502 Mag Refresh
 

Looking for input from others who have been down this road. I have a pair of MK IV 502 Mags carb engines that are getting close to needing refresh. Want to maybe boost them up a little to say around 450 HP. They were originally 385's. Do not want to get too crazy with wild builds or anything like that. I can stay with the stock Mercruiser Silent choice or install a some Gil/Merc's that I bought several years ago from a pair of new engine take offs. My first thoughts are to have the heads reworked and add some roller tip rockers, fresh valve job, general clean up. Have the blocks cleaned up, new bearing, rings etc, and a newer design cam. The old Quad Jets are still working pretty good, but most likely need rebuilt.
If you have been down this road, would like to hear good and bad. Thanks in advance.

Your on he right track but do a search for 502 builds and you'll find a ton of info. JMO ditch the QJ for tunability.

are you going to rebuild motor, new pistons, rings etc, or just do top end???
i would go complete roller valve train,
DUMP the QJ...

Any issues discovered during initial testing, will be addressed. If there is a need for complete rebuild it will be done. Carbs will most likely be changed to a more tunable option.

how many hours on motors??
doing a leakdown??

From his original post
"My first thoughts are to have the heads reworked and add some roller tip rockers, fresh valve job, general clean up. Have the blocks cleaned up, new bearing, rings etc, and a newer design cam"

Why a leak down?

I've seen estimates for rebuilding a 502 here ranging from a few thousand dollars all the way up to a ridiculous 20k. Do as much of the work yourself that you are able to do.

I can do it all, except pulling the motors and the machine work. Going to do a compression and leak down as soon as weather permits.

a leak down is just another tool to show you if you have one bad cylinder, etcc
so if you are to replace pistons, would give you option for bigger cam,,and go fuller roller valve train..
just my 2 cents

and 20 k for one motor rebuild is crazy, built both my 547 for 16-17,new crank , rods , pistons, complete valve train,,timing chain oil pump,,,

Probably getting to the compression/leak down test in a week or 2. Talked with guy who I trust to do a complete engine diagnostics and recommendations. Now curios of what others have used for QJ carb replacement that will work on the stock GM dual bolt pattern intake. Will likely have to use and adapter with a square bore carb. I was looking at Holley, Edlebrock and Quick Fuel. Ideas matter, but real field experiences would be appreciated.

Did you post hours yet. (Apprx if no or non working hour meters). As mentioned start with leak down when able. Even a simple compression test can red flag. Just depends on how far you want to go with your refresh but it sounds like it's about that time. What could be minimal cost now could easily turn into expensive down time when you want to enjoy your boat the most. You mentioned your not able to R&R engines. It's not as difficult as it might seem. I've used the old oak tree more than once. Not ideal considering twins... Just gotta be patient and a healthy hard wood tree with a woody. All depends on your budget but it kinda sounds like your trying to get a few more ponies with minimal cost. Understandable but not always reality once you start dissecting.

About 560 hours. Very strict about maintenance. Considering the engines still seem to be running pretty strong, I am guessing, I have some valve stem seals gone, carbs in need of some attention to begin with. Will know more after testing. Oak tree wont work for me. If needed, will just find someone who has an inside shop to rent or hire out the R/R part. It would suck to do that at the beginning of the season, but it would only be for a month if machining can be turned around in a couple weeks. Besides, got a floating living room to chill out on until the motor work is done.

You may do well to toss the current intakes for some Edelbrock air gaps and Quick Fuel carbs. I don't know what the stock manifolds are but you can search about the Air gaps, and just keeping the carbs cooler has benefits in themselves.

This time of year, its highly unlikely that you're going to find anybody that can do the machine work anytime soon.
In order to upgrade the cam, you will need to go with the Gil exhaust.

560 hours on a well maintained marine engine is getting near that time for a refresh and not that costly assuming everything is straight and or round. If bearings, cylinders, pistons etc show no unusual wear the cost should be pretty reasonable. I've seen well maintained marine engines go many more hours before rebuild. My offshore experience has typically been around 400/450 hours. It's all kind of a gamble however you might get lucky with just having the heads refurbed and some aftermarket bolt parts including exhaust if considering much of a cam swap as griff mentioned. Doing it all can get quite costly in a hurry. You might get away with another season or two on bottom end. Ya it kinda sucks to spend a fair amount of money on a basic refresh/rebuild and see no gains but is what it is. You definately need to make that call but if it were me being 100% honest considering time of year I'd go for the HP and go have some fun and see the gains. Intakes, carbs, cams and some good used exhaust.

Assuming you have to pull engines for a cam swap at the very least pull the pan of one or both of the engines and check a couple of main and rod bearings for wear. Typically the bearings furthest from the pump wear first. Check the pan and oil for any unwanted goodies. That would determine what route you choose to go.

Ive had a couple boats I drilled holes in the rear bulkhead for cam removal and install. When it's all said and done it's easier to just pull engines but does come in handy. Heck I've changed cams on the water. Literally..

put an ad in on wanted section for some good used Gil's, EMI thunders, Stainless marines etc.
Ill probably get bashed a bit for not opting rebuild first but like I said you can always determine after oil pan removal.

Can't an auto shop do the machining? They are very slow right now.

I actually have a 2 near new sets of Gil/Mercruiser exhaust that came off of a pair HP450's same as the 500's. Been sitting under my stairwell in bubble wrap for several years. No need to cut holes in the firewall, you could pull the cams, install new bearing and cams, without pulling the engines, but I would never do that w/o doing a complete block inspection etc. We'll see how the diagnostics go and figure out from there. I really appreciate the input, gives me some talking point with the guy when he comes to test everything. I know of at least 4 boats in my cove that he has rebuilt the power in over the past 4 years.

There you go OP. Snatch them up. Would be a great addition to your upgrading.

He is at LOTO, and the all shops in the area are pretty much marine and auto. Its Spring and everywhere will most likely be backed up.
Then you will have the surge of people who will have a failure on their first run or two in the Spring who flood the shops.

Ah, everything here in Michigan is on lock down and mechanics are being laid off. The shops are still mostly open though but with no work.

Machining will not be an issue if needed this season. I have other resources for that. Thanks again for commenting on my post.

Ya I can't even open my golf course until the 13th or later if it gets pushed back. Battle Creek course opened for one day before being shut down. In addition had to pay a healthy fine. Hopefully people begin to take the seriousness of this virus (bioweapon). The catholic priest here in a very small town just brought it back from his trip to Spain. Nice...

Quick update: leak down test only showed one cylinder, Stb engine #7 hole had 14% leakage through exhaust valve. All other's were at 8% or less Going to pull QJ carbs and rebuild them and run it for the season. Will pull heads in the Fall, do a refresh on the heads and decide whether or not to go further into the blocks. I also removed the 20* advance modules and installed 24* modules off of 454 mags. Set initial timing back to 10*. Full advance (34*) in at < 4000 RPM.

If you can have heads gone through great. If not at the very least take a peak at the valve with a bore scope and compare to a couple others. Concentrate along the edge of the valves. Very quick and easy. 14% not that bad considering hours. I'd probably do all at once in the fall with rebuilds and heads unless valve is noticeable. Probably valve/seat combo. Only saying this is because of the way you run and maintain the boat. Jmo.

7 and 8 will typically run hotter. In my experience it's 8 but the combo/build will dictate.

I had 2 1992 502 mags that had over 800 hard hours on them the lower ends were excellent when they were dissasembled the valves were a little beat up but the engines werein great health the only real wear were the bronze bushings in the distributoris were egg shaped and wore out .I had scrorpion racing engines turn them into 540s they made 608hp 620 ftlbs

With only having one cylinder at 14%, I wouldn't even worry about doing top ends.
A friend at LOTO had 1200+hrs on stock 502mags before he had them gone through and they still were running fine.
He did replace the Qjets with Holley carbs a couple years prior.

I did rebuild the quads, they are not leaking extra fuel into the motors now. Seem to be running smoother. Did not want to go the Holly route without different intakes. I did pull off the exhaust riser on the cylinder with the 14% leak, did not find any cracks in the manifold, replaced gasket and riser. Re-torqued other risers, which were somewhat loose, going to leave it as is for now. I would still like to see what a good combination of cams, head work, intakes/carbs would work with Gill exhaust for down the road if I get inside these motors. Just figured if I decided to refresh motors, might as well add a little extra.