I am considering the purchase of a 502 MPI. The engine was in a 31 Formula and one of the engines blew. I am considering the purchase of the non blown engine. Exhaust and mans are new.
Anyone have experience with this engine? Pros? Cons? things to have looked at prepurchase?

Thanks
Brian

Why did the other one blow? Operator error/lack of maintaince? You need to know that to make a choice on the un blown one. IOW, if it was lack of maintaince, then the twin probably had the same. How many hours on it?

At the very least do a compression test on it, or better still, a leak down test. Pull the oil filter and cut it apart and look for junk in it.

The 502 Mag is a great engine with the best hi po Chevy parts in it. It takes a bunch of abuse to break a stock one, which is why I asked why the other one broke.

Apparently the head has a plug that gave way. The Boat and engine looks well maintained. Hours are about 300. I was told that this engine has an alum head that sometime gives and some popel have had it welded to prevent this issue. Ever heard of it? I will have a comp test etc., but was looking for some feed back, THanks.

Not that the type of boat matters but a couple years ago I bought a 31 Formula with twin 502's. Blew a motor after 2 hours running. I told the mechanic to rebuild both since they were likely in similar condition.......he calls me a couple days later to tell me the non-blown motor was a new long block that had just been replaced. Turns out the marina took the boat on trade with one blown motor, fixed it, and sold it to me so I could blow the other one.

502 Mags have iron heads, not aluminum. The 525 EFI has Aluminum heads. Both motors are 502 CID, but the 525 would be painted blue, and the 502 Mag painted black

Edit: Forgot to answer the Aluminum weld question. If the head was welded, its because it cracked. Not a big deal if done correctly, BUT you would want to be sure that it was done right, Also, no there is NO reason to weld an Aluminum head to make it stronger. Someone is feeding you BS. Its starting to sound like you should run very quickly from this motor.

first let me tell you that I know nothing about anything related to boats but I did have one boat that had two off these exact motors for 110 hours and they had vapor locking problems. A fuel cooler was added and fixed the problem. They were 1996's

I had another boat that had two more of these exact motors with 350 hours both longblocks were replaced because of water reversion problems. These motors had exhaust manifolds not headers like the newer 500's and 525's. These were 1998's

Pulll the spark plugs and look at them especaiily the ones towards the back of the motor. If the plugs have been replaced than do a compression and a leak down check next, especailly if the exhaust and exhaust manifolds have been replaced.

Like I said I know nothing about anything but read my post carefully and then read the other posts and do the math.

Good luck.

OSO makes a god point......What year are these? If the 96 models didn't have the fuel cooler then vapor lock issues were likely. My 502's were 98 models with about 375 hrs when I bought it. I never had a vapor lock issue. It's my understanding that the stock exhaust manifolds are weak and will leak. (causing reversion) Before I ever took the boat out on the water I had the exhaust pulled apart, inspected, and installed all new gaskets.
Just like cars, these things need regular maintainence. If the motor was abused and regular service not performed you'll be buying one to rebuild right after you get it. With good regular service a 502 will last a long time!!!

Thanks for all your insight. I am getting more information on the boat today, year hours etc. Manifolds I new about. I think they are 98 or newer so I do not think vapor lock is an issue. COmp test and leak down are for sure.

THanks again.

My 96 has the VST, no cool fuel and I have never had a vapor lock issue.