1991 Force 120 Odd Piston Size???

[PaulO]

Hi All.

The machine shop informs me the piston size in my engine is 3.368". If I look at the Sierra piston guide it says the standard piston size for 120-150HP is 3.312".

It appears the right diameter is the 120L-drive, Sport Jet.

Can someone set me straight about this ASAP?

Also, do conrod cage bearings come in a set of four cause otherwise they are pretty expensive.

Thanks,

Paul from Oz

 

[Frank Acampora]

Re: 1991 Force 120 Odd Piston Size???

STANDARD bore is 3.375. The piston is sized to provide proper running clearance--don't worry about it. They should know that--It is elementary on all engines, auto and marine. The piston is also cam ground to give it a major diameter and a minor diameter to account for uneven heating as the engine runs. AND---The crown is way smaller than the skirt to account for combustion heat. SO, Where did this guy measure it? ANYWAY-- He should have "miked" the bore to determine the amount of damage and how much it needs to be overbored--NOT the piston!

Oversized pistons are available in .010, .020. and .030. It is NOT recommended to bore .040 over, even though pistons are available.

I Strongly suggest you try another machine shop since this guy apparently "don't know squat" about outboards. Since the bore is blind--that is there is a land at the bottom, it takes special skill to bore and hone to size.

 

[roscoe]

Re: 1991 Force 120 Odd Piston Size???

1990 models used 3.312 bore

1991-1998 used 3.375 bore

 

[PaulO]

Re: 1991 Force 120 Odd Piston Size???

Thanks for the quick response Chaps.

The machine shop does not do boat motors as a general rule. The closest shop would be a 4 hour drive to Melbourne.

Measurement was taken 90? to the pin bore... that was when he was showing it to me.

All I really want to know is what part number (Sierra if possible) do I need to order to make it work. I will go .010" OS for the bottom piston and use standard rings on the remaining pistions.

The engine man has not bored the cylinder as of yet but said a .010" OS would be all it needs.

Can you provide .010" OS piston part number?

Can you provide the STD rings part number?

Do cage bearings come in a set of four?

Thanks,

Paul

 

[Frank Acampora]

Re: 1991 Force 120 Odd Piston Size???

Sorry, Can't help with part numbers. I only buy Wiseco pistons.

Same thing with bearings-- I have never needed to buy a set.

 

[roscoe]

Re: 1991 Force 120 Odd Piston Size???

Your 3.369 piston will fit the factory 3.375 bore.

Here is a link to a Wiseco parts chart.

http://www.marineparts.com/mpf/mpf774.aspx

Can't find Sierra part number.

Another alternative is A mercury dealer.

 

[Faztbullet]

Re: 1991 Force 120 Odd Piston Size???

Part numbers for Sierra Force parts:
Sierra .010 piston is 18-4633
Sierra .030 piston is 18-4634
Sierra STD rings is 18-3998 (keystone rings)
Rod bearings come as a "set" for each rod, it will have all the roller and 2 cages and below is link to them..
http://cgi.ebay.com/ebaymotors/Chry...2537669QQptZBoatQ5fPartsQ5fAccessoriesQ5fGear

 

[PaulO]

Re: 1991 Force 120 Odd Piston Size???

I managed to get onto a piston (Wiseco) in Queensland (Aussie Rep.)

I found the Sierra chart confusing, look for your self Here. You will notice the bore size for 120 HP is 3.312 but mine is in line with L-drive and sport jet.

Frank you said you never worried about bearings before. Is that to say the existing bearings have been good enough for your applications?? If I can get away with using the existing bearings I will.

I found this site Here. You can get individual rollers in a pack. is this a good idea or a bad idea?

 

[Frank Acampora]

Re: 1991 Force 120 Odd Piston Size???

Normal bearings will have a slight discoloration due to "work hardening." That is, although they are hardened thoroughly during manufacture, If not abused, they will take on a bit more hardness with use--very similar to the way copper will harden with continued flexing and you must anneal it to work it again. Except, you would never anneal bearings.

SO: to make a long story short, if the bearing races and rollers are not pitted, spalled or blued due to overheating, they are re-useable.

I buy so many junk engines that I just throw out bad bearings and clean good ones. I store them on a plastic bag and a pill bottle with dessicant. I probably have a dozen races, and rollers and a 60-75 rod big end rollers and maybe a couple of hundred small end needles (which I probably will never use because Wiseco pistons need special small end kits that come with needles.). So, I have never needed to buy any.

And while most manuals say not to mix rollers in ndifferent l;ocations, I have never had a problem this way.

 

[PaulO]

Re: 1991 Force 120 Odd Piston Size???

Frank, You are restoring my confidence in this engine. Thanks.

I ordered the wiseco piston and the roller pins but it occurred to me after the fact the piston was a crossflow and while the build years relate my motor to this piston I can't recall reading anywhere about my engine being crossflow. Time will reveal all I guess.

 

[Frank Acampora]

Re: 1991 Force 120 Odd Piston Size???

NO Chrysler or Force was ever "Loop Charged." Some Force engines had a "Swirl Charged" logo on them but this was just a marketing ploy to indicate crossflow with fancy words.