1991 120 HP piston replacement - advice?

[hillierp]

To the people with wisdom and knowledge...

I have a 1991 120 HP Force Outboard (H1208E91H) that needs a piston replaced (compression test 120, 120, 120, 30 from top to bottom). The number 4 piston has a hole in it, and scored cylinder.

I've hoisted it off the boat and have it down to just the powerhead.

My plan is to replace all 4 piston and rings at +20 and bore the cylinders at +20 (a couple of the scores looked deep, so I figured +20 to be safe).

Questions:
1) What gasket sets do I need for this job? I see all kinds of parts lists here: http://www.boats.net/parts/search/Merc/Force/1991/H1208E91H/CYLINDER BLOCK/parts.html

At the bottom of the list appears to be gasket "sets" but I'm not sure if they contain everything I need.

2) Also, and mandatory special tools required?
3) Anything else I should consider replacing while I am at it?
4) I'm based in Canada - any parts dealers you can recommend?
5) Is it possible to order +20 piston, rings, gasket set all from one spot?

Appreciate any input and advice you may have regarding this task. I have the shop manual and 2 mechanically inclined people (bike engine rebuild experience) helping.

Thanks!
Peter

 

[pnwboat]

A bit of advise. It's probably not a good idea to purchase a set of pistons before you measure the actual bore. Most motors that need to be rebuilt require .030 over size pistons. Not saying that yours will require that, but you need to check. It's also perfectly acceptable to replace just one piston. Also you need to check to see if your motor uses the 3.312 standard size pistons or the larger 3.375 standard pistons.

2) Also, and mandatory special tools required? Possibly. The wrist pins on the OEM style pistons are pressed in. You need at minimum a 3 ton arbor press to get them out. You can also try a hammer. A hydraulic press is the best way. If you purchase OEM style replacement pistons, you need a special spacer gauge which are rare as hen's teeth to install the wrist pins. An alternative is to use some Weisco forged pistons which have "floating" wrist pins. No press required to install the wrist pins. The wrist pins are held in place with circlips.

3) Anything else I should consider replacing while I am at it? Check all needle bearings and bearing surfaces on the crankshaft and connecting rods. If the is any signs of pitting or blue discoloration, you might want to consider replacing that item.

4) I'm based in Canada - any parts dealers you can recommend? I don't know of any dealers in Canada.

5) Is it possible to order +20 piston, rings, gasket set all from one spot? Yes there are several sites that sell individual parts or complete kits.

 

[JDusza]

Hi. My offering is not so much advice as it is a question.
Is it possible to save the cylinder without the boring?
Could you use HCl to remove the aluminum from the cylinder. Yes, you have to pay attention, but, a solution of about 15% (available at Mal-Wart) should take a couple minutes to work, giving you plenty of reaction time if something spills.
I've heard about doing this as an "old school" method but have yet to try it. Perhaps someone can comment here.
I suppose it also depends upon how deep any scoring is.
Sorry about hi-jacking your thread.
Thanks,
J

 

[HotTommy]

J,
You'd be wise to start a new thread. ... Scoring is a groove cut in the cylinder wall and is not the same as aluminum melted to the side of the cylinder.

 

[Jiggz]

Personally, I would go with just replacing the broken piston with a Wiseco piston. However, if you need to disassemble the block and the rest of the pistons for reboring, then if $$$ is not a question, you can go ahead and replace all pistons. but if $$$ is a concern, then you can just replace the one broken piston. Had the bottom cylinder not require a reboring, you can actually replace the bottom piston without dismounting and disassembling the head. And just go through the intake manifold to remove and replace the broken piston. PNW's comment is right on the mark.

 

[hillierp]

I'm not married to the idea of replacing all 4 pistons - I just thought that if I had to disassemble the whole block, I may as well do it. But if you are saying there is a way to replace just one piston, I'll look into that. I just have to double check the scoring to see if I can live with it.

Either way, I'll certainly go with the Wiseco pistons if that makes things easier.

If I replace only one piston, how does this effect getting at the wrist pin - I guess its hammer time? Via the exhaust port...

So I did provide the engine info (H1208E91H) - is there no way to find out if it uses 3.312 size pistons vs the larger 3.375?

How is the best way to judge scoring to determine if boring is needed or not.

Apologies for amateur hour - thanks everyone for the input.

Peter

 

[Jiggz]

Usually the rule of thumb is if you ran your fingernails into the cylinder and it catches, it will need re-boring. However, this is not quite definite, because if you have to replace a piston you will at least need to de-glaze the cylinder. So the way I will go with yours is to remove the piston first. This is basically how I will go by doing it.
1. Remove the head (with all obstructions attached to it) and head gasket. Save the gasket as much as possible for post diagnosis and if it's still intact it is also re-usable.
2. Dismount the lower carb, intake adapter and intake manifold. This should also include all obstructions and attached parts like choke and fuel lines. To prevent parts falling into the lower cowling place a rag or stuff the hold into the lower cowling with rags.
3. Dismount the reed v-blocks.
4. Using a flashlight look into the crankcase and push down on the piston so the con-rod cap is closes to the intake manifold. You will have to play to position the con-rod cap convenient for you.
5. Using 1/4"-12 point socket dismount the con-rod cap. CAUTION: Place a rag or something to prevent parts from falling into the lower cowling. Make sure you account for all roller bearings. If roller bearings fall into the crankcase DO NOT use magnet pick up, INSTEAD use needle nose pliers or thick grease to pick them up.
6. With the con rod cap off, push the piston up to pull it out.
7. Inspect the cylinder and take pictures and post for diagnosis.

 

[HotTommy]

I am wrapping up the job of replacing the last of five pistons in my 1993 Force 150. It's similar to a 5 cylinder version of your engine. The hardest job was removing the wrist pin. I did it using a very large vice as a press. It would have been much easier with a real press. If you do have trouble removing the wrist pin, keep in mind that the only thing you need to protect is the connecting rod. You'll not be reusing the wrist pin bearings or the piston so its ok to use some force or heat if that makes the job easier. .... The old heads here talked me through this job about two years ago when I did my first piston replacement. The search function will reveal some advice on how to reassemble the connecting rod bearings and cap through the little holes you must work through. It can be very frustrating if you don't have a pretty good idea of how you're going to do that part of the job. .... My two favorites pieces of advice are: 1. Put that cloth down to catch small loose parts (e.g., needle bearings) that can fall down into the depths of the engine, and 2. Write down which way you installed the connecting rod (i.e., milled side up) so you can match the rod cap to it later.

 

[tommarvin]

1. I read a post on here that said if you only replace one piston or not all pistons you have to use OEM pistons so they all expand the same, and a wiseco piston won't expand the same as OEM ?

 

[HotTommy]

I'm no expert, but I replaced the #1 piston on my 1993 Force 150 with a Wiseco and saw no apparenty problem from it. I'll add that one old head here (Frank A.) said the piston/ring configuration on the stock pieces was a known weak area that sometimes failed for no apparent reason. When the #3 and #4 pistons failed a year later, I decided to replace all of them with Wiseco.

 

[jerryjerry05]

Jiggz, did you say reuse the old head gasket?
​My opinion: NEVER use an old head gasket.

There are thousands of 4cyl Force/ Chrysler running 1 oversized piston.
And running good. Ask Gregsmere.

Take the block to a machine shop and see if they can do the work.
Not all shops can do that kind of work.
Find the right shop and ask them how far out you need to go.
Going .030 on the first cut usually isn't needed.
Most times you need .020 and maybe get by with .010
Once you get to .030 you get into: Should I change the jets or is that gonna be OK?
Wisco is the way to go.
You'll need their rings and wristpin.
Their rings WON'T fit the standard OEM piston.

Most gasket kits contain all the gaskets you'll need.
The block and ALL the pieces/parts need to be taken apart and cleaned/decarbed.
The usually include the cylinder drain or recirculation gaskets and the transfer ports gaskets.
Make sure you clean and redo that system.

The motor had a problem and if you don't know the reason it died you need to make sure you check everything.

I get a can of Mercury's Powertune or OMC's Engine Tuner and soak the piston heads in it for a couple of days before cleaning.

Use bearing assembly grease when installing the needle bearings.
Make sure the alignment pins aren't smushed or damaged.
Use tcw-3 oil on the rings and any spots that need lube prior to fire up.

I buy most of the gasket and such from E-bay.
The e-bay police make sure your happy with the purchase.

 

[Jiggz]

Jerry, I sure did re-use the old gasket. In fact, even though I ordered a new head gasket, the old one was still in perfect condition and decided to re-use and it worked perfectly. The advantage of re-using an old gasket is you do not need to go back to re-torque the head after initial running which is required on new gaskets.

 

[scout-j-m]

I rebuilt my 1994 70HP inline 3 cylinder with a rebuild kit from the link below. It came with Vertex pistons, which do use a circlip for the wrist pin, but you can upgrade to WSM or Weisco as well. I'm pretty sure it came with all of the gaskets you need to do the job. It also came with emory cloth, assembly grease, crankcase sealer, and loctite. The price was actually cheaper than they had it listed at. They can be hard to reach though as they don't seem to have someone dedicated to their office, although I did always talk/email with the same guy. In a way that is a good thing because you are talking to someone who is hands on in the shop as opposed to a secretary or someone who may not know anything more than you are seeing from looking at their website. I'd recommend them even if you don't do a full rebuild as you can get a gasket set from them plus order individual pistons.

 

[flyingscott]

Jerry, I sure did re-use the old gasket. In fact, even though I ordered a new head gasket, the old one was still in perfect condition and decided to re-use and it worked perfectly. The advantage of re-using an old gasket is you do not need to go back to re-torque the head after initial running which is required on new gaskets.

What about the crush rings around the cylinders. Those gaskets are kind of a one time use no matter what kind of motor they go on. I don't know if I would take the chance on it but it's not my motor. JMO

 

[Jiggz]

What about the crush rings around the cylinders. Those gaskets are kind of a one time use no matter what kind of motor they go on. I don't know if I would take the chance on it but it's not my motor. JMO

I wouldn't call it "crush" ring but more of "compress" ring. And as long as it is not crushed and totally intact, it should be reusable. Again we are talking about a $40 part which for others is nothing but a chump change and for others like me is not. You'll be surprised that even new gaskets sometimes fail right of the bat due to improper torquing or installation. Again, reusing perfectly old gasket is not for everyone. But so far, it worked perfectly for me.