TOPIC: 1962 Merc 500 piston/cylinder issues

Hello, I've been working on a 1962 Merc 500 for a couple of years now when I find the time but have run the motor very little since purchasing it. I ran it last year and it did OK but the other day had a distinct lack of power. I tested the compression and found the #3 cylinder had no compression. Upon inspection I found what you see pictured. I'm only a novice mechanic and was hoping for some advice.

The piston is obviously shot and there is some local scoring on the cylinder wall that I can distinctly feel with a fingernail. Is this something that can be corrected by honing? Does it need to go to a machine shop? Will taking it down to clean metal cause too much slop between the piston & cylinder wall? Can it be re-sleeved? If so, where can I find a sleeve? Marineengine.com doesn't list a sleeve as a separate part #.

I'd appreciate any advice offered... I'd like to fully restore this motor.

Many Moons ago I picked up an old 100hp that had a piston looking just like that. The rings were stuck/carboned-up. The hot exhaust burns right thru and melts the piston. I was able to clean up the cylinder and put a good used piston in it, and it ran fine.

Yours looks a bit worse, and you have a lot of melted aluminum on the cylinder walls. You'll have to carefully remove that; muriatic acid will dissolve aluminum while having no effect on the steel cylinder. But use it sparingly as it's quite powerful.



Once all the aluminum is gone, you can run a hone lubed with Marvel Mystery Oil or similar down the cylinder, just enough passes to clear the bore out, and then see how badly the cylinder is scored. Very Very light "finger" scores probably won't hurt, especially if they're not above the ports. But if you can really catch it with your fingernail, and you want a durable lasting repair, it's better to have that cylinder measured then bored-out to the appropriate oversize.

The most difficult issue is finding oversized pistons. I think your engine is new enough to have the larger-diameter wrist pins, and if so, a "modern" 50hp oversized piston should fit. Or maybe one of the Merc Gurus knows off the top of their head (Doc??) if O.S. pistons are available for a '62.

Even if you did have small wrist pins, you could upgrade to the newer-style rod & pistons. But would probably have to do all 4 or it may affect the engine's balance. Again, I'll defer to the experts since it's been a long time since I've gotten into those specifics. 30 years ago I would have just spouted-off all the part numbers and interchanges off the top of my head, but that was then and this is now!

Anyway, typically pistons are available in .015" and .030" oversizes for old Mercs but IDK if you can even get .030" over for a 500. Checking at marineengine.com shows the OEM .015" over pistons are NLA but you can still get the .015" rings.

Sierra Marine makes a replacement piston, P/N 18-4621. Of all places, Summit Racing lists the piston as available, shipped from mfr. So hopefully this listing isn't outdated:

www.summitracing.com/parts/rra-18-4621

At any rate, see what Doc Frankenmerc et. al. have to say and hopefully you have a shop nearby that's capable of boring a 2-stroke cylinder. And also hoping that the piston was Standard in the 1st place, if it's already worn and been bored .015" you might have better luck finding another block or complete powerhead.

Keep in mind you're gonna need a new set of Big End needle bearings and the wrist pin bearings for this cylinder, and should check the Big End bearings and crank journals on the other (3) cylinders for wear and renew as necessary. No sense throwing a lot of $$$ at the bad cyl only to find that the other rods and/or journals are bad. Check the main bearing needles and race as well.

You should also check out the upper and lower ball bearings (P/N 6206) as these are definitely wear items. Replace the upper and lower crankshaft oil seals that are pressed into the end caps. If you're gonna go deep into the rebuild, it'd be a great excuse to pick up a Harbor Freight hydraulic press! Or at least an arbor press. It's possible to do the end caps/bearings without, but so much easier if you have one.

HTH & Good Luck with the repairs..........ed

Just an update: I poked around a bit more and found that "mercury 50hp oversize pistons" was productive. Various pistons/rings in .010", .015', .020", .030" and .040" (which I was surprised to see, Merc never offered anything over .030").

So just go on eBay and paste that in. This site parses eBay links so I can't link it directly without doing TomFoolery to the link.

One of the listings showed '72 thru '85, but they really didn't make huge changes over the years except for the wrist pin size. If you post a pic of your pistons, showing a view of the wrist pin, it'll be quite apparent right away which style it is.

Note that the replacement pistons generally are only 2-ring; the top ring is straight-cut like yours, and the bottom ring is Keystone-type with a bevel that allows combustion pressure to push the ring against the cylinder, for better sealing. So it's generally an improved design that is far less prone to seizing in the ring groove, due to its constant and slight motion in the groove.

p.s. time for a new impeller and cooling system check, too!

Thanks Ed, you're a wealth of information! I've found a clean looking '64 powerhead on Ebay for $300/free shipping/30 day returns. Compression is supposed to be 138-148. I'll inspect it carefully, lightly hone the cylinders, replace the rings, etc.

I figured this is the quick & easy route and I can play with the old powerhead later. I think the damage existed before I bought it and only got worse in the few times I ran it. The compression was initially OK and I had replaced the impeller before ever running it. I recently gutted the lower unit so I could powder coat it, replaced the seals, gaskets, etc. When test running it, this issue reared it's ugly head when I couldn't get on plane. Otherwise it ran better than it ever had! Rebuilt the carbs, fuel pump, starter & magneto as well. Now that I disassembled everything I've decided to go ahead & do the rest of the restoration cosmetically. I'll be powder coating instead of paint & hope to have it ready by spring.

Thanks again and I'll review your advice when I get around to messing with the old powerhead. -Brian

PS... pictures you asked for

That lower unit looks Beautiful! And it's always nice to have extra parts, maybe that eBay powerhead will be easy to "refresh" and take a lot less work. I'd definitely still replace upper/lower crank seals, and upper/lower ball bearings as these are very common wear items. At least you can still get Standard rings easily enough.

Now, how 'bout a pic of the piston showing the wrist pin hole? Then I can tell you if it's a large or small wrist pin.

Is that the pin through the piston?

Yeah, that's the ticket. Lovely!

I've also found a complete '64 Merc 500 for my '64 Razorback but haven't pulled the trigger yet. The owner claims that the powerhead was replaced 40 years ago and then placed in storage having never been started. That boat currently has the deck removed and I'm vacuum bagging foam core to stiffen the hull & using carbon fiber. Still a long way to go on that one.

Is that considered large or small? Thanks again!

Like New 40 Years Ago! Well, might bear looking into.

It looks like the small wrist pin, but some of the ones I'm looking at on eBay appear to be the same size. Others have a noticeably larger diameter.

I'm gonna be dropping by Doc FrankenMerc's shop for a visit this week, I'll ask him about that and maybe we can make some measurements as well.

I really appreciate your time & experience... huge learning curve for me! My background is a lot of things besides mechanical. I do marine electronics & electrical but a lot of glass work too.

The lower unit was a first effort for me and having done a few paint jobs in my past I absolutely love the powder coating process.

You bet! Sure did come out nice. I'm a marine electrician by trade (Naval Vessel type) but outboards have been a virtually life-long "hobby" for me, ever since my late Uncle gave me a couple of Scott-Atwater 7.5hp's in a basket and said, "here, put these together". Parts are a lot more $$$ than they used to be when I was a kid, that's for sure! The same 7.5hp Elgin impeller I mail-ordered at $3.50 each back in the 70's, now cost around $75 for a repro!

And you won't be finding a MK58A on the shop bulletin board for $35 either! Cleaned the points and it was running. We threw that on a cheap old wooden ski boat and skied all summer. I also found a 700 Dockbuster in the Little Nickel circa 1982 for $25, cleaned the points, unplugged the water pump, and threw it on my Brother's 13' Sea King runabout. Man did that rig ski and we thrashed the heck out of it Summer of '82 for next to nothing! Those were the days when you could ski all weekend on $20. I can't even fill up my motorcycle for that much anymore!

I did check bearing numbers, and Merc used the same 20063 wrist pin bearing from the 1st model of 500 to the last 50hp produced. So I'd expect that the wrist pin diameters may have stayed constant and my Pea Brain is remembering the wrist pin re-size of another displacement powerhead. I'll check with the Doc in any case. Pics to follow!

I also purchased these to play with at some point... a '55 Mark 55e and a '59 Mark 55ae. The thought was to use the '55 for parts & see what I can do with the '59. Still kicking tires on a '64 known runner to match the year of the Razorback.

BTW, the '62 Merc 500 is the original motor that came with my '62 Texas Maid Astro.