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Oversized pistons

Do you have the motor apart at this time ?------???----You have to put the new piston on the old rod.---If the old rod is still good.
 
Check your max spark advance and use the highest octane non-ethanol fuel you can find. Also make sure the impeller is new and you have good water pressure.

If the fitting for your water telltale isn't located at the top of the block, I'd recommend doing the mod to relocate it there. What this does is eliminate the possibility that a Hot-Spot (i.e. area where water is not and will form Steam) forms, by ensuring the cylinder block water jacket is completed flooded, all the way to the top. What you do is drill and tap for a 1/8" NPT fitting and just run the telltale hose to that fitting.

Obviously a hot spot at the top cylinder is gonna cause big problems and burnt rings/piston can be the result.

Here are the dimensions of where to drill; I imagine somewhere on this site there's a good picture of that, search for "Inline telltale mod" or something similar:

>>7/8" forward of the split line between the rear water jacket and the block, (where the gasket is).

>>3/4 inch to the left of the ridge that runs fore and aft above #1 cylinder.

If the #1 conn rod is still good, you should replace the wrist pin bearings and also the Big End caged bearings/cage. Bits of piston will have circulated into the needle & roller bearings on #1 and that isn't good for the health of the rotating assembly.

After removing the piston pin circlips, you may need to heat the piston up to around 180 deg F or so to make it easier to press out the wrist pin. There are wrist pin tools you can buy that'll help with that, the Merc pistons don't have an extremely tight press though. You could even make your own tool with some 5/16" or 3/8" All-Thread and 2 sleeves: one with O.D. larger than the wrist pin, that would bear on the piston as you're pressing-out the wrist pin with the All-Thread tool, and one slightly smaller than the wrist pin O.D., just enough to push the wrist pin thru the piston with the All-Thread tool.

Clean the rod's wrist pin surface with 380-grit wet-and-dry sandpaper (lubricate with light oil such as WD or Marvel Mystery Oil when you're sanding). For crank journals running caged roller bearings, polish the crank and rod surface with Crocus Cloth. A can of Zippo etc lighter fluid makes an excellent cleaner/degreaser for the sanded surfaces.

Hold the needle/roller bearings in place for installation with some Vaseline petroleum jelly or Merc/OMC bearing assembly lube.

I'm assuming your connecting rod is the old-style bolt-and-nut type, in that case degrease rod bolt threads and nuts, assemble to crankshaft with 1 drop of Red Loctite on the bolt threads where they're gonna contact the nuts, and torque nuts to 180 inch-lbs. Note if you have to buy a new rod, it'll have tapped holes for the rod bolts (no nuts used) and use special Stretch bolts, the tightening process is a bit different (consult your manual).

Make sure you're running the correct spark plugs (should be L-76V or equivalent). That engine is highly-stressed, probably be a good idea to mix the oil a bit rich (40:1). This also promotes piston ring sealing.

DO NOT lug this engine. It should be propped to rev at or slightly above the maximum recommended rpm range at Wide Open Throttle with a light load (just the driver in the boat). Inlines don't like to be lugged/overpropped, this is a very common cause of burnt pistons. Check #3 cylinder carefully, as the crankshaft lead on that cylinder tends to make it run with more stress/heat.

Last thought, if you don't have a good repair manual, a factory manual would be the best. Aftermarket-wise, the Seloc manual for Merc Inlines has pretty good information in it, and a lot of useful tips in the engine teardown, refurb and assembly section. Don't bother with a Chilton's, worse than Useless!

HTH & G'luck with the repairs...............ed
 
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