Replacing the piston on a 125 HP

What's the best advice for replacing the piston. I ordered the standard piston and pin. I don't own the piston tool set. Would it be best to take it to the boat repair shop or a local machine shop to have the piston pin removed from the old one and the new one installed?

Re: Replacing the piston on a 125 HP

Before getting to your answer: Is the cylinder in good enough condition to warrant simply replacing the piston? You said you ordered a standard piston. OR have you disassembled the engine and had it bored?

If the machine shop has a cradle to cushion the piston while installing the wrist pin and a spacer to keep from denting the keepers, then you can take it to them. If they do not AND if the local boat repair shop has the tools then pay the extra the repair shop will charge. 'Course, if you want to ride up here near Philly, I'll press it in for nothing.

Dang! I'm a really nice guy! LOL

Re: Replacing the piston on a 125 HP

Without writing a book, let me give you a little build up to the climax. When I bought the boat from my uncle, only one of two 125's were running, I did alot of diagnosis and was able to get the stbd engine running after replacing the CD modules.
In doing my diagnosis, I saw that my # 2 cylinder compression was at best 85 psi compared to 125 and 135 and 125 in the other 3. I thought I might have blown a head gasket through the water jacket since I was seeing what I thought was alot of steam in my exhaust. I replaced it, and ran it till last September, about 6 times for about 4 hours tops. Then I moved to VA, and she wouldn't start again. After a ruined fishing trip, I decided to tear them both down and rebuild them both. That's when I discovered that the top ring on #2 had broken inside the cylinder and about a 1.5 inch segment was gone. I did discover that something had bounced around in the top of the cylinder as there were impacts in the piston crown all over. There were no particles in the chamber when I replaced the head gasket the year before and the same was true when I disassembled the power head last week. And the bottom ring had been unable to expand due to having been locked into the second groove by the broken fragment, so it was likely contributing to not maintaining the pressure.

So my logic is after having measured out the bore in 8 locations in each cylinder, number 2 did have some bad spots. I'll show you pictures.

My plan was to hone out the cylinders and replace all the rings and #2 piston and rings and try to get by.:facepalm:


I'll have the boat repair facility do that for me but if I was close enough, I'd drive over to see ya. I appreciate the offer.

Re: Replacing the piston on a 125 HP

The honeing might not be enough???
If you can feel the groove in the cylinder??#2 looks pretty rough!!!
It probably needs to be bored.
Just honing aint gonna remove the groove.
The new stuff(piston,rings) has about a 50%-80% chance of doing the same thing???

Re: Replacing the piston on a 125 HP

I have to agree with jerryjerry. You have gone to the trouble of tearing it down this far, my rebore was $50 a hole, new oversized piston $100. Best of luck.

Re: Replacing the piston on a 125 HP

Yeah! Agreed! You really should have that cylinder bore miked to see if the damage is too severe to simply hone. That missing chip below the exhaust port looks severe but since it is on the bottom and no combustion pressure is against it. a new ring will ride over it with no problem--as long as the edges are not ragged and rough.

Stock bore is 3.3125 inch with only .002 tolerance for out of round, barrel shape, or taper. The shop (if it needs boring) bores and finish hones to 3.3125 plus .010, .020, or .030 Plus or minus a couple of ten thousands machining tolerance--whatever oversize it needs. Replacement pistons are sized for correct clearance and the shop does not need them to finish hone.

Also, don't bother replacing all the rings. Rings on these engines rarely wear out. The semi-keystone top ring your engine has is prone to breaking and replacing with new will not prevent it. If you are truly worried and money is not an object, replace all pistons with Wiseco. They can be assembled with no special tools and use different rings. Now: There are some who claim that Wiseco pistons are crap and will fail on you. My take is that a Forged piston will always be superior to the stock cast piston. I have used a number of Wiseco in both my own engines and in rebuilds. So far--knock on wood--I have had no problems.

At the bottom of the cylinder I see some hone marks that show incorrect honing technique. They are not at the accepted 60 degree crasshatching. Did you run a hone down to see if the damage could be reduced or was the engine rebuilt by an amateur in the past?

Re: Replacing the piston on a 125 HP

Wow, I didn't know I'd get so much activity. Anyhow, I appreciate the advice from everyone. Hmm. Maybe I ought to see how much I can get it bored out for and upsize the piston. Do you all think I would have to upsize all of them?

Also, the compression on both engines are 130, 130, 127, 130 on the port and 125, 85, 135, 125 on the stbd. The stock compression is supposed to be around 145-165, so I figured the rings could stand to be replaced.

What do you think?

Re: Replacing the piston on a 125 HP

Frank, I have no idea who worked on the engine before me. I did watch some you tube videos on honing an engine and I was surprised at how little you actually do when you hone a cylinder. I would have been at it for about ten minutes minimum, with lube of course, but I'm sure I would have destroyed it. From what I gather it's a few up and down motions about one second per stroke and that's it. I have a variable speed drill and was about to borrow a honer from Autozone and knock it out before I reassembled the engine. Was that a bad approach?

Re: Replacing the piston on a 125 HP

Honing is not a metal removal technique. It is a surface finish operation. If done by hand it does take a knack to control drill speed (low) and in and out motion. Ideally you want to have the crosshatch marks at about a 60 degree angle to each other--that is, the vee between the in and out lines should include an angle of 60 degrees. It has been empirically shown that this angle will provide the best "seating" of the rings and the best sealing and lubrication.

Was it a bad approach to try it? Probably not, if the engine does not need an overbore. If the engine needs an overbore--no foul. You tried and learned something.

Re: Replacing the piston on a 125 HP

Thanks Frank.