need advice on cylinders on 89 force 125

[bowhunter83]

I have a 89 force 125hp that i got off a buddy that im rebuilding......he gave it away because he bought a new motor after he broke this one..... Aparently he ran up on one too many sand bars and overheated the motor,said it was running fine and all of a sudden he heard a loud knocking,so he immediatly shut it down and got a tow back to the ramp,when he got it back to the house he did a compression check and found that 1,3, and 4 cyl had 125 compression but 2 cyl only had 85 then he just happened to find a guy with a running motor with great compression for only 500 so he jumped on that motor instead of dealing with the rebuild of his old....so here i am... i disasembled the power head and found that the motor was rebuilt not all that long ago (my buddy only had it for a year and he didnt do the rebuild) buy all 4 cyl still had the crisscross oil grooves in there.....BUT the #2 piston had some good damage on it....the other 3 had just a little bit of even wear at the very bottoms (still has grooves in piston,its just shiny down there on the bottom 3-4 grooves and the rest of the piston is dirty from oil)....so I know i need at least 1 new piston and the cyl walls are good on 1,3,and 4 and the #2 has some marks from the piston, the grooves in the cyl walls are not deep enough to have a noticeable feel to them but are visible..but the damage to the piston is VERY visible and easily felt (not sure how you can have that bad of wear on the piston and not on the cyl walls???)......and I checked the cyl's with calipers and found that ALL four cyl are bigger laft to right than they are up and down (out of round)...not exactly sure of the numbers but #1 was .006 out of round,then 2 was .009, 3 was .012, and 4 was .019!!! Is that normal!??! It makes no sense that it would have 125 compression on cyl 1,3 and 4 and 85 on #2 being like that........but other that wondering how in the heck that happens (i dont know a whole bunch about cyl/pistons), but i was wondering about boring them out......what are the specs on these cylinders? what are they SUPPOSED to be at? how much tolerance can you have out of round? how much can you bore before you need to resleeve? (i know the biggest pistons and rings are .030 so i would imagine .030 would be the max) and what does the thickness of the cyl walls need to be..as well as any other specs that i should be aware of.......sorry for such a long post,and any info is greatly appreciated,thanks

 

[john from md]

Re: need advice on cylinders on 89 force 125

Before you do anything, search and read all of Frank Acampora's posts on engine rebuilding. He has covered just about everything from boring to porting and polishing. After you have done that, come back and ask your questions.

John

 

[pnwboat]

Re: need advice on cylinders on 89 force 125

What was the actual bore diameter? I believe the maximum service limit is .002 or .003. STD. bore size for that motor should be 3.312 Maximum bore size is 3.3148. Maximum taper is .002.

 

[bowhunter83]

Re: need advice on cylinders on 89 force 125

not exactly sure of the numbers off my head id have to go get the calipers again and remeasure,but a rough measurement on dirty cyl walls, on #1 cyl was 3.313 top to bottom and left to right was somethin like 3.319 and it was like that on all four cyl.....# 1 cyl was closest to in round, then 2 was further out,3 was more out and 4 was way out of round and all the cylinders were further apart left/right than top/bottom, actually i think the top/bottom easurements on all 4 cyl were pretty close to the 3.312 std size.......what would cause the block to get that out of round? yet still have good compression and the cyl walls mot be scored as alll get out?

 

[bowhunter83]

Re: need advice on cylinders on 89 force 125

not exactly sure of the numbers off my head id have to go get the calipers again and remeasure,but a rough measurement on dirty cyl walls, on #1 cyl was 3.313 top to bottom and left to right was somethin like 3.319 and it was like that on all four cyl.....# 1 cyl was closest to in round, then 2 was further out,3 was more out and 4 was way out of round and all the cylinders were further apart left/right than top/bottom, actually i think the top/bottom easurements on all 4 cyl were pretty close to the 3.312 std size.......what would cause the block to get that out of round? yet still have good compression and the cyl walls and the pistons not be scored as alll get out?

 

[pnwboat]

Re: need advice on cylinders on 89 force 125

I think what you're seeing is probably not abnormal. It's normal to see more wear in the direction that the crankshaft spins in the cylinders on most engines whether they are 4 stroke or 2 stroke. In this case, the spinning crankshaft, centrifugal force pushes the connecting rods horizontally which in turn puts more pressure on the left or right sides of the pistons. The shorter the rod, the stronger the effect, or more push into the sides of the cylinder wall. The rods in these engines are pretty short.

The bottom portion of the crankshaft is probably subjected to more torsional or "twisting" stress as this is where the load that the engine is being subjected to is concentrated at. Right at the point where the shaft from the lower end is physically connected to the crankshaft. That's probably why you see more cylinder distortion on the lower cylinders than the top.

The fact that you still have good compression when the cylinders are out of round shows you how well the rings work. Remember, the rings on these motors are "pinned' in place. They are not free to spin around as in 4 stroke motors. I guess that as they wear, they conform to the out of round cylinders???

 

[bowhunter83]

Re: need advice on cylinders on 89 force 125

So do they need to be bored out? The number 2 piston is still intact,no cracks,holes,pitting, etc...but its scraped up pretty good and i wouldnt trust it in there again,so that is getting replaced for sure,and I looked at a kit that someone else posted on anothe thread for a rebuild of this motor for 575 inc all gaskets,bearings,pistons wrist pins and bearings thermostat,the whole 9 just about for a complete (internal) upper end rebuild and that is probably what i will go with.......my question is with these cyl being that out of round...do they NEED machined and bored over,or can they be honed or what? and as far as machining goes..... if anyone can tell me the scpecs for the machining process like how much cyl sleeve has to be left over after boring and the tollerances of the cyl to be in spec (i.e. how much out of round is acceptable in the top of they cyl,how much down at the bottom,etc) And i imagine that in a perfect world a perfect cyl would be comletely in round all the way arround top to bottom.....and i havent checked the bottom of the cyl to see if and how much they are out of round........which i wouldnt imagine that the bottom of the cyl needs to be in EXACT spec as much as the top half,being that the top half is where all the action occors (spark,ignition etc) and compression is built and stored....where as the bottom half there is very little compression that needs to be held and a whole lot less action....correct......... I know im asking a whole lot of questions,and it relly would be simpler to just order 4 new pistons and rings and bearings and all that,and just drop it off at the machinist and eat the bill later to have the crank dressed if needed and the cyl bored over..........but im trying to learn as much as i can about it rather than just dropping it off and being ignorant to what happened at the machinist and what was done to fix the situation and all that

 

[bowhunter83]

Re: need advice on cylinders on 89 force 125

I will take some pics tonight and post them to let yall see what im talking about, i wouldnt doubt that im confusing some by trying to explain whats goin on

 

[pnwboat]

Re: need advice on cylinders on 89 force 125

Maximum cylinder distortion is .0005. Maximum taper is .002. Maximum bore size for std pistons is 3.148. Sounds like you may need to go to oversize pistons to clean them up and get them back to specs. Best thing to be safe is to check with a reputable machinist and let him measure and make the final decision before you order pistons or kit w/pistons. Not sure what the minimal sleeve thickness is, but from what I've seen and been told by my machinist, generally you don't want to go more than .030 oversize. Hope this helps.

 

[javsam]

Re: need advice on cylinders on 89 force 125

Agree on taking it to a machinist to make final desicion.calipers are not very accurate for measuring bore sizes..

 

[bowhunter83]

Re: need advice on cylinders on 89 force 125

Ok,so... with them being out of round.... if that is "normal" what causes that? and if it was normal wouldnt it create marks on the piston/cyl walls from the pistons rubbing and making they cyl walls out of round? Because there arent any marks on the other 3 pistons or cyl walls indicating the pistons were rubbing the walls out of round. Or is that caused..or exagerated from something else that i should look at before reassembling (i.e. lower crank bearing or whatever)....I understand what pnwboat is saying about the centifugal force applied in excess at the bottom where the crank meets the driveshaft more so than at the top where the crank just has to spin the flywheel, and i understand that when you bore them and the cyl walls are perfectly round and install new pistons and rings that are perfectly round that they would wear the same (pistons and walls) over time,making the cyl walls and pistons out of round due to the centifugal force of the spinning crank and the rods,but wouldnt that wear be visible on the pistons,rings and cyl walls?

 

[pnwboat]

Re: need advice on cylinders on 89 force 125

In general, the cast iron cylinders wear faster than the aluminum pistons. You would think the opposite being that the aluminum is a softer metal. The pistons are not in direct contact with the cylinder walls. There is an oil film that the pistons ride on and helps protect them. The rings themselves are probably what cause most of the wear. If it weren't for the oil, the pistons would self destruct very quickly. NOTE: this is one reason why it is advisable to double the amount of oil when breaking in a rebuilt outboard 2 stroke motor. The rings are made of a steel alloy that is much harder than the cast iron cylinder walls.

You kind of have the same situation with the babbit bearings in 4 stroke motors. Crankshaft and rod bearing are basically made of an aluminum alloy. The pressurized oil system provides an oil film for the crank and rods to ride on. The surface of the crank and rods are not in direct contact with the bearing surface. Without the oil, the bearings would be severely damaged in a matter of minutes.