'69 Merc 500 Piston replacement

[hohnstadt]

I need to replace a piston on a 1969 Merc 500. I figured I would be better off with a used piston then a new one not only because of price, but because when measuring mine vs. a new one, it seemed the new one was about 1/2 mm larger in diameter than any one of the 4 of mine (yes, I verified the part numbers are the same - mine must have worn a bit over the years). Seems this would put things out of balance - more weight on the new one, more compression on the cylinder with the new piston. My 3 good cylinders had 115-120 psi, so I'm guessing after I replace the busted one (sucked a reed) I don't want a whole lot more compression. Are these concerns I should be worried about? <br />Anyway, I cannot find anyone or any dealers who have a used piston(the only one I found had a crack on the head). So my next choice would be a Wiseco ($50 less than a Mercury), but even that would be pushing $100, and I've been told I could probably get an old powerhead for that if I could find one. Anyone have experience with Wiseco pistons, or know of any good places to get old pistons/powerheads?<br />Any insight to the above questions would be appreciated. If you happen to have a piston or an old powerhead of similar vintage you'd be willing to part with, let me know - sharidan@wowway.com. Thanks.

 

[Laddies]

Re: '69 Merc 500 Piston replacement

If you are only concerned about weight and not price and want to only replace one of the pistons when you say they are all worn out then get a good scale and drill the new one for balance I never heard of any like your worry before.

 

[Chinewalker]

Re: '69 Merc 500 Piston replacement

Hi SBW,<br /> Might have to buy a parts motor to get what you want. The same pistons were used in several different motors and years. The 22-cubic inch 20hp and the 44-cubic inch 50hp used the same pistons and rods from about 1966 to the early 1980s. <br /> I might have some local sources I could check - drop me a line - csr112-n@@prodigy.net.<br />- Scott

 

[hohnstadt]

Re: '69 Merc 500 Piston replacement

Thankyou both for your responses. As to worrying about the weight - I've not done this before so I'm just trying to think of what makes sense. Balance seems like a good thing when parts are moving back and forth 5000 times a minute. Good idea about weighing them - the new is .3 oz heavier than the old. I wonder if the new is one of those oversize pistons I've heard about. It seems to be the exact diameter as my bore - seems that would make it awful hard to move, but my measuring device is not the greatest. <br /><br />And cost is important - else I wouldn't be trying to get a 35-yr-old motor running. 4 new pistons would be great, but if 3 have good compression, do I really need to replace them? Will the rest of the motor last 30 more years??<br /><br />As for the search for a parts motor - that's been my tack for a few months. Your information that the 20 HP is the same piston as the 50 HP came in handy just today - I might have a lead on a used pistons from a 20 that is the same part number. Thanks, and thanks for your offer to look locally. If I strike out here I'll be in touch.

 

[Laddies]

Re: '69 Merc 500 Piston replacement

many of the fellows that race stock outboard have engines rebored and some might have thier standard size pistons around check for a club in your area, all the 44 cu.in. except some of the very earliest that use a smaller wrist pin, 44cu.in. is a D stock engine, we race them but are engines are still stock bore and we don't have any used pistons