'83 175hp lower piston rings gone

[RobsTV]

During a head gasket and exhaust gaskets refresh, noticed top two cylinders on the right side, the inner rings were just black carbon chunks, with little to no inner rings seen. Small screwdriver tip crumbled carbon to reveal no rings left. Outer rings looked great.

Left side piston rings looked fine. All six pistons look fine. All cylinder walls look fine. No ridge even. Compression on all 6 between 75 and 80lbs. Runs great except for occasional cough at idle. Easily pulls to 5500RPM with correctly propped Wellcraft V20. Starts right up. Decarb done at least yearly. Was Florida fresh water Bass Boat motor until we bought it last year. Unknown if any previous cylinder work was done. Very clean motor.

Did I get lucky and catch this before piston or cylinder wall damage occurred, or is the visual with heads and exhaust removed, as well as compression test, not enough to accurately judge condition?

Next question is how tough will it be to pull this thing apart? Couple hours or couple days? Special tools "required". Have done several car motor and auto trans rebuilds, and regular boat motor maintenance, but never anything this deep into an outboard. Have aftermarket manuals, just didn't start reading this part yet, since issue just discovered last night, and hoping to see tips and tricks from others that are usually not listed in manuals.

Thanks in advance,
Rob

 

[tashasdaddy]

Re: '83 175hp lower piston rings gone

you need the OEM manual, aftermarkets are unclear and have errors in them. outboard books.com . it depends all on what you find as you go. take pictures as you go, mark things the way they came off and keep in pairs.

 

[Silvertip]

Re: '83 175hp lower piston rings gone

It's not a two hour job. If you have all the parts at hand, the proper tools, the power head removed, and a good place to work, the work could be done in a solid day but better figure two as you will invariably need to run for parts you forgot. Complete gasket set, carb rebuilds (essential) and thorough inspection of all bearings is essential.

 

[RobsTV]

Re: '83 175hp lower piston rings gone

Thanks.
Is this a fairly common thing to find with a running motor?

 

[emdsapmgr]

Re: '83 175hp lower piston rings gone

Not ususual to see carbon around the rings on a crossflow engine. It is unusual to see no lower rings. Usually it is the top ring that gets hot and disintegrates when the lower ring carbons up. Do either of the two pistons show any dents on the piston face? How about the head face? Unlikely the pistons lost the bottom rings and did not leave some sign of internal distress..maybe it was assembled without them, however unlikely it sounds. You probably can pull the engine apart with regular tools. A 3-bolt puller on the flywheel is required to lift the powerhead off the midsection. Once the powerhead is dismounted, you will need a thinwall socket to get the flywheel nut off. Normal overhauled compression should be around 95-100. Depending on cylinder and piston (taper) wear, you might be able to hone the cylinders, clean up your current pistons and install new rings. You also should plan on a new water pump and two new thermostats for an overhaul.

 

[RobsTV]

Re: '83 175hp lower piston rings gone

Thanks for the tips.

Carbs and thermostats were done last fall.
Water pump was in the process of being replaced during this maintenance.
No visible signs of damage to piston face or head.
We were also replacing intake gaskets this time, (double up), so already have the powerhead gasket set.
Will know more after we pull powerhead and fully disassemble.

Thanks again,
Rob

 

[RobsTV]

Re: '83 175hp lower piston rings gone

Finally got around to complete disasembly.
It was way too easy. Everything went smooth as silk.
Took only a couple hours to pull powerhead and tear it down.
Only thing I needed to get was 12 pt 5/16" thin walled socket for rod bolts.

One cylinder does have a groove in it from a ring. Too wide and deep to hone out.

I am thinking it is best to punch out all 6 and get new pistons and rings, instead of punching out the one cylinder .10 over and replacing single piston and ring. Balance?

I found on an ebay store, new OEM .30 over piston with ring, wrist pin and wrist pin clip for $30 total each, so a complete set of pistons with rings for $180 does not sound too bad.

Or, just one .10 over piston?

Do them all, right?

Thanks,
Rob

 

[Benny1963]

Re: '83 175hp lower piston rings gone

get machine shop to mi up your cyls thcen you may get by with one cyl .
check bearings well dont mix anything up.
good luck Beenyb

 

[RobsTV]

Re: '83 175hp lower piston rings gone

Thanks.
One oversize piston will not throw out balance or cause vibrations or other issues? Done all the time?

I have micrometers and telescopic gauges to check other cylinders bore and taper, just did not have time to check them yet, and thought if I had to bore out them all anyway...... At least I would check here for opinion on 1 or all first.

Thanks for the reply.

 

[emdsapmgr]

Re: '83 175hp lower piston rings gone

You can replace only one piston. I would not mix brands of pistons. If you still have OEM pistons, get an oem oversize (.030). It will weight the same as the standard pistons you already have. You should not need to rejet that carb..