Holly Crap 2 piston/ring failures in 2 weeks

[slx12001]

So about 2 weeks ago I lost compression in #1 top cylinder and found the top ring broken off destroying the piston but the cylinder looked good. Installed a new used piston/ring and all was good for about 2 weeks. Today I was doing a full compression test as the last time out top speed and rpm were down with a rough idle. I have about 35 gallons of gas through since the top piston/ring was changed. So I found #2 piston with only 75psi. I pulled the head and covers and found #2 did the exact thing that #1 did 2 weeks ago. Same exact failure same spot the top ring broke. each cylinder had 135-140+ psi depending if testing warm or cold. I have a theory why this is happening but I would like to have any input. Thanks for any replies.

 

[Jiggz]

There is almost absolutely only one reason why rings catch to the exhaust or intake ports, lack of lubrication. You need to make sure you are using 50:1 fuel oil mix and also mixing it properly. To fill the tank with gas and then pour on the oil into the tank is just not PROPER! Instead you should always pour on the oil and then fill the tank with gas. Even better is to use a gas jug (2~5 gals) and fill it with oil and then 3/4 way with gas, shake it and pour into fuel tank and then fill the tank with the proper amount of gasoline.

The other reason for lack of lubrication is too lean of a setting for the fuel air mixture screws. These are the brass screws with springs on the carbs. The minimum setting is 1 turn out from slightly seated but recommended is 1 1/8~ 1 1/4 turn out. Too much setting on this can foul the spark plugs and too lean can actually damage the cylinders.

As for repairs, I highly suggest using the Wiseco piston kit which already comes with rings and wrist pin. The only additional kit you need is the wrist bearing kit. I was able to get a kit for less than $130 for each piston including the bearing kit. When you decide to rebuild, make sure you chamfer the ports horizontal edges and also do the break in procedures.

 

[slx12001]

Thanks for the reply. Gas is carefully mixed 50-1 first oil then gas then repeat until full. I do this in 1-6 gallon increments depending on where my gas gauge is. Idle mixture is 1 turn per my manual for my 125 b model but I run 1-1/8. I need to know piston to cylinder clearance. My service manual does not state this. I have at least .007 now. I would like to see .003. I checked the bore with my cylinder bore gauge and it's with in specs. (but on the raged edge) I checked the 2 pistons that came out and there actually pretty straight and with in specs. ( but again on the raged edge) I expected to see more wear around the skirt. I also expected the cylinders to be more out of round. There is a lot of piston slap. Not that you hear but in the numbers and that you can feel if you move the piston in the bore. All 4 are about the same. There has to be a reason that #1 then #2 failed the exact same way. If anyone has ever encountered a failure like the pictures in post one I would like to know if new pistons/rings were installed and piston to cylinder clearance they had is available. Any piston to cylinder clearance would be great. I have built many 2 stroke snowmobile, dirt bike, pwc, etc engines in the past 35 years but this has me stumped. I have seen .007 or more piston to cylinder clearance on some old omc outboard engine that I have worked on but they have 3 rings. Numbers much appreciated. Please add anything that may help as well. I would consider myself a fair mechanic building engines by the book usually blueprinting them for future reference

 

[slx12001]

This thread could be continued from the thread that I started when the first ring broke

"Force 125 hp rebuild or options"

 

[Frank Acampora]

Simple: Later Force engines used a semi-keystone top ring. They tend to catch in ports and it has nothing to do with oil or anything else. They just break frequently. The semi-keystone ring uses combustion pressure to get behind the ring, pushing it out against the wall for a better seal. Unfortunately sometimes it pushes it into the port. It is also more likely to flutter than a square ring.

Damage due to lack of oil, lean running, or timing issues will almost always score the cylinder badly. In addition, the piston will usually be melted. However, it would not hurt to verify the timing and make certain that the carbs are not set too lean

If you want to keep the engine, do it right. If the cylinders are not damaged buy some used pistons from a Chrysler. These used square rings which almost never break. OR---have the block honed and use after-market pistons with better rings--like Wiseco.

When I bought my 150 I replaced two pistons with stock ones I had lying around. If and when it breaks more rings I will replace all pistons with Wiseco.

 

[slx12001]

Thanks for the reply. That makes more sense. Most later Polaris snowmobile engines use keystone rings but no where near the bevel of these. Both times I broke the top ring I could still run close to normal speed and rpm. Just down a couple. It would still plane almost the same. The first time I could hear a slight knock. The second no noise at all. In fact someone that did not know the engine and where it should be may not have even known there was a cylinder down. So the top ring breaking on the later engines is fairly common? I would still like to know factory specified piston to cylinder clearance. Wiseco always require to clear a little more due to being forged and will expand a little more.

 

[Frank Acampora]

No one publishes the piston to cylinder wall clearance.
The pistons are cam ground to account for uneven heating and expansion so in a cold (that is not operating) perfectly round cylinder you will get different measurements depending upon where you measure. IF you are either standard bore or bore to a standard oversize Wiseco and other pistons are manufactured to and will have the correct wall clearance. No need to measure. Wiseco does advertise to provide additional ring end gap in modified or racing engines. Their tolerances are so close that in all the pistons I have installed I never needed to adjust end gap--but, of course, I take my blocks to a primo machine shop. They are always right on the money

 

[slx12001]

I re measured my bore again in the light today with more patients. I'm .001~.0035 over max for cylinder bore size. I was able to do the other 3 mostly as well. They are all about the same. This explains my .014 total or .007 piston to cylinder Clearance. I have never seen a service manual state more then .005 per side or .010 total. So take the .001~.003 that I'm over with my dial bore gauge and average it to .002 and I have my .014 total clearance. (assuming factory was .005) I believe case solved. To much piston to cylinder clearance causing a ring to catch the ex port. Cylinders are worn the most in the same spot (.0035 directly at top of cylinder about 2" in) and both failures are in the same spot. (directly to the left at the top of the exhaust port)

 

[Frank Acampora]

I don't remember where I saw it but the Chrysler (and so most likely Force too) had a limit for cylinder out of round, taper, or barrel shape of .002. This surprised me because OMC had a tolerance of .005.

If you measure piston diameter at the major diameter and minor diameter you will get two different numbers. SAME if you measure at the crown, at the wrist pin, and at the skirt. All three are different. Cylinder wall clearance at the skirt is critical because this seals the crankcase and allows the piston to pump air through the engine. Too much clearance here and pumping efficiency and horsepower go down. Too little clearance and the piston will gall.

 

[slx12001]

I ended up buying new rings for the second used piston that I bought. The end gap was well over max wear. Strange thing the replacement rings were a little different. The top is the same but the bottom ring does not have the small grove on the inside top. The ring fits all the way into the ring land. The bottom ring land on the stock piston is square so I think it should be ok. The replacement rings are sierra brand but are rik branded. They replace mercury part number 39-817869A
1 Any comments or reason I may have any issue? Thanks again.

 

[slx12001]

I have about 50 gallons through the motor now since I had issues and replaced 2 pistons. I guess the bottom ring being square without the groove on the inside top is not a problem.