piston rings are bad... what should I do?

[ironslavery]

Okay, keeping in mind my need for saving money and impatience with wanting to get this thing done(my first ATV and I have been wanting one for some time) I have 3 options.

My trike is a 1985 yt125.

1. New rings - so far 40$ (is it possible to find cheaper generic ones?)
2. New rings and piston of the standard size - difficult to find STD piston without spending a fortune
3. New OS piston kit and get the cylinder bored, honed and chamfered. (70$ for parts plus about 50-70$ to get the block bored) 140$
add 25$ for a head gasket and new bolts. (missing 2 of em when I got it)
current situation:

Piston - several scratches, really nasty in some parts all vertical. looks like a chamfer problem.
rings - worn down to the cylinder looking in from the exhaust. they look horrible. The piston does not move side to side in the chamber though.
cylinder - no scratches, nice and smooth but can't tell if there is too much overall wear for a standard piston. couple of rust patches.

I just want to get this trike in running shape so I can mess around with it for a year or so till I move on to something else. keeping this thing in as budget of a situation as possible would be ideal and since I have to dump money into many other parts of the bike . I don't want to spring for standard size parts and only find out that I have to get it bored because there is to much wear. I want to keep the cost under 150$ and I need at minimal new hoses for everything, an air box or filter setup and a rejet, bearing set for the steering, various nuts bolts, and now I may end up needing a reed cage because I sheered a bolt head off on one of the reed mounts. Don't forget all the shipping costs. this can get pricey if don't do it right.

Would the rings looking bad even mean anything? should I take the cylinder off and see how they look that way? I can rotate from where the starter goes easily by hand and there is pressure build up but it feels like it's not what it should be. I don't have a pressure tester either and I am not willing to spring for one.(will look into borrowing one)

pics coming.

[HuffieVA]

Well you are the owner of a 27 year old two stroke (cost wise just be glad it is not a four banger), but here is my opinion... If the cylinder is good I would take the time to find a new standard piston and rings, it would make no sense to throw the old piston back in if its worn like you say, it could only lead to a short lived rebuild and possibly under the higher pressure from the higher compression it could break apart which would have the ability to destroy your crank/cylinder/cases all in one leaving you with a bolted together pile of parts... You say the cylinder is smooth with a couple of rust patches? that is definitely not "good" you could possibly get away with honing the cylinder but if your looking for long term reliability (i.e. you don't want to go through this again in another couple of years) get it bored to the next size piston, perhaps you should take the cylinder to wherever you are having it bored and have them measure it for you (to be sure what size it is now) and to suggest to you if a once over is enough or if you need to go a little more then purchase the piston to match the machinists opinion... whatever you do don't take short cuts on your engine rebuild you will not be happy with the end result (by no shortcuts I don't mean send the cylinder off to Klemm or a top builder to maximize your power, but rather that you use good quality parts so it will last you) Good Luck...

Rodd

[ironslavery]

I don't want the piston to break apart, that would be bad. I have not found a machinist yet and not to sure how to go about finding one either. I will see if I can find a machinist and just get it bored and it will save me in the long run. I have to wait for my tax return which was delayed... but when I get it I will buy all my parts in one foul swoop and just rebuild the top end all together. If I keep this project under 400$ for a full restore I could end up at least breaking even since it was free. I found a 4th over piston set for 50$ on ebay and I figure if a 4th over wont do it then it's in need of a new top end.

So 50$ plus shipping + gaskets 20$, screws 2$, airbox 30$, reed block 15$, hoses 5$, and steering bearings 40$ and this trike will be ready to ride. So 162$ in parts plus shipping and a bore job and it should be ready to ride. 220$ is what I am expecting just to get it running and useable although I might end up spending more if I break anything in the process. In the mean time I will tear it down and clean it up with my electric toothbrush . Does this sound like a reasonable amount to spend on getting this bike running, mind an extra 150$ish to get back to full stock configuration.

[Scootertrash]

First I would check the compression. Many auto parts stores will loan out tools for a small fee or for free with a small deposit you get back when you return the tool.

The only way to tell if you can use the standard sized piston is to remove the cylinder and have it measured.

The cylinder to piston clearances you are dealing with are very small. I don't have a Yammy 125 manual handy, but just for an example, the cylinder to piston clearance on a honda 200 is 0.0006 to 0.0022 of an inch. Very little room for the piston to move in the cylinder.

I'm confused by your statement that "the rings are worn down to the cylinder" If you mean the rings are worn down to the piston that is just the way it looks. Once the rings are compressed to the cylinder, they are only filling that tiny gap between the piston and the cylinder, and will certainly look like they are worn down to the piston, but that's the way it's supposed to look. The only way to tell if the rings are worn is to measure the end gap, which requires removing the head, cylinder and piston, removing the rings from the piston and placing them squarely in the cylinder and measuring the end gap.

2 bolts missing on the head? Depending on the location of the missing bolts (are they right next to each other?), you could have a leaky head gasket which could cause a loss of compression on the upstroke and an air leak on the downstroke leaning out the motor, neither of which are any good for your motor.

You will need more than a head gasket if you remove the cylinder. You will need a top end gasket set. Never reuse old gaskets/o-rings or seals. Gasket sets are cheap and if you use the old ones and the right gasket, seal or o-ring fails, you've just wasted the money you spent on the top end rebuild. It's not worth it.

I won't reuse damaged pistons in my machines or rebuilds I do for others but that's just me. I'm sure others will and do. I'm confident other members will chime in with their thoughts on reusing pistons.

So to do it the "cheap" way, after everything is measured and the cylinder only needs honing, and you are able to cleanup the piston and reuse it (using the prices I pay up here in MN)

Hone cylinder: 10.00
Std ring set (Service Honda) 26.65 (+ shipping)
Wrist pin circlips X 2 (service Honda) 2.64 (shipped with the above rings) I won't reuse old clips
Top end gasket set (Athena on ebay) 12.20 (free shipping)

Total 51.49 (+ any tax and shipping costs)
Add a NOS STD piston (ebay) 57.00

Total 102.98


If the cylinder needs boring:

Bore cylinder 65.00
Piston-1st o/s (ebay) 44.00
1st over ringset (Service honda) 26.65
Pin clips x 2 (Service Honda) 2.64
Top End gasket set (Athena on Ebay) 12.20

Total 150.49 (plus any tax and shipping)

ETA: Any nut's, bolts, cleaning solvents, beverages and things broken or damaged from throwing things in anger and frustration are not included in these numbers

A properly done rebuild and proper maintenance will give you a reliable machine you can sell and put the money towards your next machine and save you headaches from breaking down on the trail. Do it once and do it right the first time. YMMV

[ironslavery]

I am a mechanic (I work on planes... totally different animal) in the military so I understand reusing o rings/gaskets/seals/clips/washers/locknuts/cotter pins. Which you found a good deal on a set on ebay I see. I am on the wall about reusing the piston. It makes me uncomfortable but I am trying to keep the budget down, Thats why I am asking. I read somewhere that the rings are more important than scratches in the piston so I am unsure. I am trying to weigh the risks Vs. benefits of my options. So far it sounds like a resounding no to using a scratched up piston.

The nuts are diagonally installed. I just got this thing a couple of days ago as a gift from my brother and it came that way. I am just trying to get the darned thing to start and I noticed the rings looked way worse from the exhaust side then from the intake side. if I had extra top end gaskets on hand I would take the cylinder off and measure it at work but buying is on hold until taxes. Thanks for the info on checking the rings, I didn't know that. when I look at the rings they are smashed fairly well, with the metal looking fairly rough. Like it was beat on with a ... for a while. It's stretched out and bumpy looking. I meant to say piston... I was doing 10 things at once and wasn't paying attention.

thanks for the knowledge I have decided to get it bored out though. I found a 4th over set on ebay with everything I need minus gaskets and those darned nuts for 50$ if it is still around when taxes come I will buy it otherwise I have to look for another kit. on the good side I have other stuff to do until taxes come. I will get a plug and ether and try to start it. if that fails I will then take the top end off and get it measured and prep to rebuild the top end.

[Scootertrash]

I wasn't sure of your motor experience, so I wanted to cover all the bases for ya.

Can you post some pics of the damage?

If you go 4th oversize, you probably are at your last bore for that cylinder. If you can find a similar deal on ebay for a 1st or second oversize piston it will leave you room for any potential mishaps in the future, but that's up to you. First and foremost I would get the cylinder checked so you know how far oversize you will have to go.

I try to stay away from ether, I may be old school, but I keep an old windex bottle of gas/oil mix handy for trying to start any smaller motor (2 or 4 stroke) that is being stubborn. You've got your fuel along with some oil for a little upper cylinder lube. If it won't start on regular fuel you need to figure out why anyhow.

[RIDE-RED 250r]

Piston wear like you are describing is indicative of a cylinder that is worn out of spec and I'm sure the piston is also worn out of spec as well. Unless the cylinder is DEEPLY scored, a .010" oversize should be just fine.. If your cylinder has worn more than .010 out of spec it wouldn't run.. at least not for more than a minute or 2. Cylinder wear specs deal in ten-thousandths of an inch.. and most engines I have worked on in general, only gave you about 0.0015-0.0030 wear limits.

I would get a .010 over piston, get it bored to match and do it right. Anything short of that and your engine won't last very long.... Which could be one hour of operation, or twenty... It's a gamble and in this case the odds are not in your favor...

[ironslavery]

thats the weird thing about the cylinder, its not scored. It does however have a couple of rust patches. can I use a piston set for a 1980-81 yt125 on a 1985?

4th over is the last bore for the cylinder. I figured it would last me a good 10 years if I took proper care and by that point the trike would have well worn out it's stay with me, or will be trashed from beginners abuse or something else will have broken on it and cost too much to repair.

[RIDE-RED 250r]

Why go all the way to the 4th bore?? It's highly unlikely the cylinder is bad enough that it needs anything beyond the 1st overbore.... Did I miss something?

[4x4van]

Something is not making sense to me. How do you know that the cylinder is not scored? I'm under the impression that you have not removed the head, but only looked in from the exhaust port? If that's the case, you'll never see where the scoring has most likely occurred, which is on the exhaust side of the cylinder. You also have absolutely no idea what size piston & rings to get without removing the top end and having it inspected and measured.

If the cylinder is not damaged, then it's possible that a new piston & rings and just a hone job will suffice. If the cylinder is damaged, then a bore is required along with a piston & rings. The starting size of the cylinder and the required bore or hone is what will determine the size of the new piston & rings.

Couple of notes here: Never re-use a damaged piston. Never reuse wrist clips (EVER!). Never reuse gaskets.

[ironslavery]

I took the head off...

Why go all the way to the 4th bore?? It's highly unlikely the cylinder is bad enough that it needs anything beyond the 1st overbore.... Did I miss something?

It's just cheaper. it will save me about 30$ if I just go with the 4th over.