Piston replacement in 1983 90hp

[roastinears]

Bought this motor off ebay for $400. Traveled less than 190 miles round trip to pick up. Two problems though. Rectifier is literally blown and need to replace one piston. I borrowed motor lift to get the motor out of the bed of my truck and mounted on my boat (replaced a Chrysler 75hp). Started checking out the motor and found 20psi on top starboard piston. Pulled exhaust cover and both rings gone. Cylinder wall has two small scrapes in direction of piston travel, but no other visible scars. The scratches are just detectable with a fingernail. I just got my piston set (piston, wrist pin, rings, etc) and the OMC service manual (part # 393765) for this motor. My question is this: will I have to re-borrow the motor lift? Looks like just removing the intake manifold will not be enough to get to the connecting rod cap. OR will I be able to replace that piston w/o having to pull the powerhead just the intake? I will lightly hone out cylinder once I pull the old piston out. Thanks for the replies in advance!

 

[WillyBWright]

Re: Piston replacement in 1983 90hp

I can't answer that specifically, but the manifold and head has to come off one way or another. Start there. Some V's you can get at the rod bolts from the front. Others you can't.

This might aid in removing the powerhead if you have to. Let the trailer tongue way down and attach a hanging chain to the powerhead. Remove the retaining hardware and raise the tongue. The best way to support the powerhead is at the flywheel with a flywheel puller with an eyebolt in place of the center pressing bolt. The hanging bracket behind the flywheel on many is too far away from center-mass to be of much use.

 

[Walker]

Re: Piston replacement in 1983 90hp

Looks like just removing the intake manifold will not be enough to get to the connecting rod cap. OR will I be able to replace that piston w/o having to pull the powerhead just the intake? I will lightly hone out cylinder once I pull the old piston out. Thanks for the replies in advance!

Correctomundo. The case has to be split to get to the connecting rod capc. Leave the motor on the boat and drop the lower unit, then remove the powerhead. Get a couple of friends, a piece of pipe and a chain and with somebody on each end of the pipe they can lift the powerhead off once its broken loose. Takes 3 people to put it back on this way. 2 on the pipe and one lining up the powerhead to exhaust adapter.

 

[funpilot]

Re: Piston replacement in 1983 90hp

You don't have to drop the lower unit to get the powerhead off, but in this case, with a 'new to you' motor, you want to so you can replace the impeller. Be careful not to mess with (rotate) the shift rod. You will need to locate the bolts under the cowl all the way around to unbolt the power head. You can also use a come-along and a rafter to lift the powerhead or do as Walker says, use three guys.

Pick up a t-stat kit while you are at it, or test the t-stats in a pan of water with a meat probe. Should open about 145F. You will probably need a 12pt socket for the connecting rod bolts. The number one reason the top starboard piston fails is carbon, and carbon builds up from heat. Check your exhaust cover gaskets, I bet they have failed and are leaking, preventing adequate water supply to #1. Don't forget to check your ring gap. Replace all the gaskets on the motor with a gasket kit (available from iBoats). You can probably reuse the bearings, they're almost bulletproof.

good luck,
fp

 

[Walker]

Re: Piston replacement in 1983 90hp

You don't have to drop the lower unit to get the powerhead off, but in this case, with a 'new to you' motor, you want to so you can replace the impeller. Be careful not to mess with (rotate) the shift rod. You will need to locate the bolts under the cowl all the way around to unbolt the power head. You can also use a come-along and a rafter to lift the powerhead or do as Walker says, use three guys.


good luck,
fp

I always drop the Lower unit. Its real easy to damage the bottom of the crankshaft if you go the rocking the powerhead around to get it loose. And it makes resetting the powerhead much easier without having to mess with stabbing the driveshaft.

 

[roastinears]

Re: Piston replacement in 1983 90hp

forgot to ask: while i got this motor tore down, are there any particular places to check that most people would miss finding pieces of the old rings? is there anything else i should tear down while looking? thanks for the replies.