I wouldn't call new springs a top end refresh. I had almost 7k in parts alone on mine

525's are great motors, i have friends that have put 800 hrs on them,
if compression is good i would just replace the springs
take the headers off and sell them then get a set of stainless marine ( probably cost you about the same as what you sell the cmi's for )

Our top end refresh on 500's was right around 8k for (2). Lifters and valves alone is almost 1500 a motor. Add R&R, spings, locks, keepers, gaskets...adds up fast.

I've got a 2003 Donzi 38ZX. 525efi. 500 hours. Bone stock, never opened until this winter. Leak down test showed bad head gasket between 4&6. Compression was about 120 psi. Fresh top end with valve springs, valve job, shave the heads and all new gaskets. Now the compression is about 140 psi. 140 psi is what the book rated the compression at. $3000. Did all the labor myself. Original headers pressure tested and still good! Great motors. I think a lot of guys alter and blow the engines up.

I would also suspect your zip code has a lot to do with your rebuild costs. TnT or Potter probably gets more money than a shop at LOTO.

correct statement,when the valve springs get weak the valves don,t seal well[espically the exhaust].changing just the springs won,t restore the valve seal.the heads need to be removed for a valve job and to check the flatness of the head.by this time the stock head gasket is suspect but it is getting replaced anyway,hopefully with a good quality fel pro gasket.imo the issue of weak springs could have been corrected by useing better valve springs to start with but the bean counters at mercury marine stopped that,their attitude is,JUST GET IT PAST THE WARRANTY PERIOD.generally this needs to be done at the 250 hour mark.

800 hrs without ever removing the heads?

Generally speaking, the 525 EFI camshaft, isn't very mild. I mean it is, but it isn't. Its way more camshaft than say a 502 magnum. 502 Mags have .483 valve lift. 525 EFI, has .630 valve lift. I think by 300 hours, you've done well to get that time, with and engine with a camshaft like that, and making over 1HP per cubic inch. 502 mags, 700 hours isnt an issue normally. its the mild cam that makes it live.

The 525 is kind of a hot engine, and certainly isn't going to go the distance like a mild 330 Mercury cruiser engine will time wise. The HP500, 500EFI, 575sci, are all high performance engines, and all require normal top end attention around the 250-350 hour mark. I just dont think one can "expect" them to go twice that long.

The stout cam shouldn't cause the valves to wear. I did springs, etc. at about 400 hours as maintenance. There was no need to remove the heads. I also went with stock spec springs since they've worked fine for over 400 hours and I'm not going to second guess Mercury on their choice of springs. IMO this is a very good engine. Remember that when you're on top you will have haters.

Hate to say it, but unless you removed the heads, and actually checked the guide clearances, valve seat condition, valve condition, and so on, you're statement is purely a guess, in regards to what is, or isnt' needed.

At 400 hours, I'd be willing to bet the guide clearances, seat concentricity, and valve job, is no where close to what it was when new.

After time, when valve guides wear, things get loose. The valve no longer may seat itself concentrically on the valve seat. it might "leak down test" ok, but at 5000rpm, the results may be different. The relationship between the valve and valve seat, on an endurance engine, is critical. Not even so much as power is concerned, but for valve cooling, as that is where the majority of exhaust valve cooling takes place, when it contacts the seat. Valve seat cut widths, are only as good as the concentricity of the valve to seat. If the valve is only contacting 60% of the seats cut, when closed, due to a sloppy guide or workmanship, the valve can overheat.

Valves, and valve seats, have a cycle life, just like anything. Hence why different materials are used in different applications. One might get 300 hours out of a valve job with iron or powder metal seats and stainless valves, and might get 50 hours out of a titanium valve with copper beryllium seats.

I know you get a bit sensitive about the 525 mercury engine, because you have had good luck with yours. I personally think Mercury's package are about as good as it gets. But at the end of the day, its an Edelbrock cylinder headed, crane cammed, manley valved, pac' springed 502 GM based engine. It is not above the laws of physics. The sad truth is, there are many out there who have suffered the shortcomings of this engine, and frankly, I don't think its right to keep saying in so many words, that they are doing something wrong, or simply unlucky. Running a pair of 525's in a 42 Fountain or 39 Cigarette, is quite different than in a 22 Donzi. With that being said, I know of many who run the crap out of their 525's, and they have had good luck. It is a great engine, but not perfect by any means. I think alot of guys in the past saw "525EFI" merc engines in the boat they were buying, and assumed they simply are dead nuts reliable long life engines that just won't cost them an arm and a leg to maintain. Unfortunately, for some, that just wasnt the case.