View Full Version : 1985 Honda 125m pistons

06-17-2009, 08:55 PM
I am picking up a 125m sometime this week, the current owner said the the pistons and rings are worn out, keeping it from starting or running. How hard would this be to do? And how hard do you guys think the parts will be to find? The guy only wants 50 bucks for it, so I may walk into one hell of a deal, or maybe a lemon. Any input would be great.

06-18-2009, 07:14 AM
Ive had my 125m for bout 3 yrs its electric start . ... butt thats were my fun ended it has no suspention on it and my kids did nothing but complain bout it ..... so I fitted a set of forks off of an 84 or so 200 sx before the tripple tree ..... it helped but it wasnt enough ..

06-18-2009, 07:19 AM
oh ya your question ...... it wasnt any worse than any other piston to get at it and the parts that I ondered came from e-bay but if it ben sitting long dont forget to look into the carband gas tank

06-18-2009, 09:33 PM
Well, I originally asked about the tires on it, if they were decent (can't afford newbies right now) - mainly the front tire - I was willing to pay 35 bucks for the front tire, and he said that if he did that it would be hard to move it around and asked if I would give 50 for the whole bike. Everything is great besides the plastic and the pistons/rings. So I figured what the hell, why not. So I am gonna swap the tires to my 250sx, and tinker with it little by little to get it running. I did notice the lack of suspension which sucks, but for 50 I cant pass it up. Here is a picture of it minus the plastic...

06-18-2009, 09:43 PM
I did a topend job on my TRX125 and it wasn't a problem at all. Turned out, my original piston and cylinder were right at the upper limit of acceptable size (but in spec), so just a quick hone, fresh rings, and reassemble. It wasn't hard at all.

A quick check on the health of your cylinder is to put the top ring into the cylinder bore alone, square it up with the piston, and then measure the ring end gap with a feeler gauge. Assuming it was within spec when put together, you can see just how worn the ring and cylinder are by how large the gap is. On a fresh install, the end gap should be betwen 0.004" and 0.010". The service limit is 0.020". Rings are cheap, so I'd recommend fresh ones if you've gone through the trouble of tearing down the top end. The point is you don't necessarily have to bore the cylinder and buy a new piston if everything is still within spec.

You will definitely want to download the 125 service manual. The one you'll want covers the ATC 70 - 125 for 1985 and earlier (in the manuals section of the forum). It shows diagrams of the topend and gives you some of the details of how it should go back together.

06-18-2009, 09:46 PM
By the way, my TRX125 is still a blast with my 6 year old son and I riding it around my parents place... maybe it just reminds me of my childhood riding around on big red (no rear suspension...). You can still have fun, just a different kind of fun compared to the performance machines.

06-18-2009, 10:10 PM
Thanks for the info, I have already downloaded and read through the manual here before I bought it. I might be stealing the tires for my 250sx, but I plan on fixing it up for my boys (one is turning 7 next week and the other is 9) they will have a blast on it. I pick it up tomorrow and I will almost definitley be working on it (can't help myself lol) so I will keep you guys posted and I'm sure I will bombard you with questions.

06-19-2009, 10:07 PM
Ok, so I got it today, and the guy told me he had already honed the cylinder walls back to manufacturer's specs, bought a new piston head, and only thing thats left is to install the rings and piston. My question is, what are all the parts that I'm going to need to put all this back together? I know I need the rings, but are there more clips and whatnot and gaskets that go with it? I have never done this before so even after reading up a little bit I am still kinda "in the dark"

06-19-2009, 10:55 PM
You'll need a top end gasket set. I used a Vesrah kit when I did mine, and it had everything I needed.

You'll need the wrist pin circlips. I've heard that with aftermarket pistons, you aren't supposed to reuse the wrist pin circlips. I didn't know that when I did my TRX and reused the clips and it's been fine. If you don't have them though, you'll need these and they aren't in the gasket kit.

You should wash the cylinder out very well after the honing. You should literally wash it with soap and water and some sort of plastic bristle brush. You need to get all grit out of the bore. It is good to use white paper towels to dry it off and continue wipping if necessary. The towels should remain white after wiping down the bore to know it is clean enough. IMMEDIATELY put some engine oil on the freshly cleaned surfaces though.

I can't think of anything special you'll need. Be careful putting the rings on. You don't want the ends of the ring to score the piston. If you are worried about doing this, put some masking tape on the piston to protect it, but don't rely on the tape.

The bottom of the cylinder is tapered enough that you won't need a ring compressor. It is pretty easy to just compress the rings with your fingers as you push the piston up into the cylinder. Go slow and use lots of engine oil on everything. Look in the manual for how to align the ring end gaps.

Let us know how it goes!

06-19-2009, 10:58 PM
Oh one more thing: make sure that the piston is oriented properly. Honda puts an "IN" mark (cast into the top of the piston) meaning put this side by the intake valve. However, Wiseco uses and arrow pointing outwards towards the edge and this is supposed to go by the exhaust side.

What piston are you using: stock OEM or aftermarket? You'll need to know if it is oversized or not when you order your rings.

06-19-2009, 11:32 PM
I have the stock piston, the cylinder is standard bore. It looks easy enough, I just didn't know what parts to replace after I took everything apart to replace the rings. I noticed you said that with aftermarket pistons you should use new wristpin circlips, but if I am running an oem piston is it ok to use the old ones? I am on a very very strict budget so I gotta save what I can. Where did you buy yours from?

06-20-2009, 08:13 PM
The clymer manual I've got says not to reuse, but I didn't follow their instructions and just reused the factory clips.

Interestingly enough, my 350x has a Wiseco piston and Wiseco will actually mail you new clips for free if you just call them and ask.

So, I guess you could do like me and just re-use your old ones. To keep them as secure as possible, be sure to only squish them down the minimum amount to actually get them in the groove. If you were to buy new ones though, I think they are only a buck or two.

06-20-2009, 08:49 PM
Yea I think I will just buy new ones if they are that inexpensive. Where did you get your top end gasket set and did the rings come with it? I am having a little trouble finding these parts.

06-20-2009, 10:21 PM
Budget! .............. I hate that word ... but I had one to and it took me neerly 2yrs to build my 350x .......... just remember not to skimp on the important stuff ........... you dont want to have to rebuild something twice because you skimped on the wrong thing ........... on my $2,000 build I spent $800 of that on my motor ........... the rest was just cosmedic .......... but that $2,000 was truely only bout $40 a month just decide what you want and set a realistic tome line you can afford .......YOU CAN OBTAIN ANYTHING WITH PATIENTS

06-20-2009, 10:55 PM
Well I'm not so much worried about the cosmetics of this one (it actually has honda 110 plastic on it) Literally the only thing it needs to be rideable is some new rings and an oil/filter. Even the electric start stuff is all there and in good working order, I'm just finding out that the parts aren't that easy to find, as most parts places I've found don't carry much if at all parts for anything less than a 200.

06-20-2009, 11:12 PM
I e-bayed the filters and rings both .........

06-20-2009, 11:16 PM
no joke just lookedmon e-bay ......... dont give up though ........ it will come around ...........

06-20-2009, 11:25 PM
ring from ATVWORKS.COM are 2nd oversize , but you can file them down to fit ......... $25 or so or the piston kits I have found for $54 ........ If its about doing it cheaply and not just the budjet can you clean the groves and rings ..... hone the thig and hope for the best

06-21-2009, 12:20 AM
How do you tell if the rings are bad? The cylinder wall has already been honed, I have all the old rings and circlips I just don't exactly know how to tell if they are really bad.

06-21-2009, 09:50 AM
If you've got a feeler gauge, you can just put the ring all by itself into the cylinder. Then use the piston to push it down an inch or so from the top. Using the piston (upside down) to push the ring down just makes sure it is square in the bore.

Then just use a feeler gauge and see how big the ring end gap is.

I've run them at the upper end of the tolerance (even beyond). Big ring gap means more blowby and less compression.

End gap is part of the equation, but also things you can't easily measure like how much tension the rings are pushing outwards against the cylinder will affect performance too.

I don't remember the exact number, but new rings should only cost you about $20-$30 from Honda (OEM Honda parts). I ordered my stuff from a local motorcycle shop. There was one nice side benefit from this: I ordered and paid for only a top end kit. There weren't able to get a top-end only kit, so had to get me (for the same price) a complete kit.

If you are ok with internet mail order, try Service Honda (servicehonda.com). They'll have your parts. (rings listed for $30 and piston circlips $0.58 each)

06-21-2009, 12:19 PM
sounds like WB knows whats up . Id spend the $$$ and buy the rings and top end kit

06-21-2009, 12:57 PM
Yep that is the plan, I'm not trying to skimp on anything, just trying to get the cheapest parts. I'll keep everyone posted on the progress and thanks for the input.

06-24-2009, 06:22 PM
Ok, so I will have some extra cash this weekend and I am gonna start collecting parts, but just to be sure, I'm gonna post a little list here of what I thin I need, hoping you guys will add to it if I left anything out.

-of course I will need the piston itself (got it)
- piston rings
-top end gasket set
-wristpin and clips
-lotsa oil
-a 12 pack
Missing anything?

06-24-2009, 08:55 PM
Not sure what kind of drinker you are, but you might want to get more. Better to be safe than sorry!

My exhaust on my hog alone took a 12 pack!

06-24-2009, 09:09 PM
....Well on that note, better get a case.....

Ok, so I just found all the parts I need on ebay (finally...and assuming thats all i'm gonna need) and the total comes out to be around 60 bucks shipped. This leaves me enough to go buy a new tire for my 250sx and hit the trails this weekend. Love it when a plan comes together lol

06-24-2009, 09:24 PM

06-24-2009, 09:44 PM
Hahaha that's great!

06-24-2009, 09:59 PM
Yeah, that was a kickazz show back in the day. I remember not being able to get enough of it, and always wanting to cruise around with a cigar hanging out of my mouth!

06-24-2009, 10:50 PM
dom .... do you still ride with the cigar?

250sxman what about the spark plug and oil filter you best check the air cleaner too

06-24-2009, 10:54 PM
Actually no, I usually ride with about half a tin of dip in my mouth and the front of the helmet filled with dip spit. It's actually pretty gross. HA

The cigars I used to smoke have been discontinued. I still haven't found anything to replace them.

06-25-2009, 10:05 AM
Is spitting in the helmet like pissing in the wind

06-25-2009, 10:16 AM
Very similar, with the only differences being color and taste! HA

Not, but for real though I quit dipping Monday! So, I won't be doing that anymore.

For all you kids out there tobacco products, well all drugs for that matter, are very bad!

06-25-2009, 03:47 PM
I'm addicted to my wife ........ well all women now that I think about it........ thats my drug of choice lol

06-25-2009, 03:57 PM
Did you just get married?

06-25-2009, 04:35 PM
LOL that was my thought too Dom - i love my wife and she is also my best and closest friend , but to say that i was addicted to her....i think she would run like a cat at bath time

06-25-2009, 04:38 PM
HA, I mean I was pretty gun hoe out the gate too! I think we all were.. Oh man, did I say "were?"

06-26-2009, 06:03 PM
Top end gasket, and rings/circlips ordered today, wristpin clips also. Now the long wait to get the parts and get this beotch running..........

06-26-2009, 07:40 PM
Take some pictures while you are underway.

06-26-2009, 07:51 PM
Will do. I am very impatient though, can't stand waiting for the parts......Also found a great place for some decent used rubber 5 bucks a piece, cant beat that.

06-26-2009, 07:54 PM
You talking skins (tires)?

06-26-2009, 08:03 PM
Yes sir all different kinds

06-26-2009, 08:06 PM
Dang. I wish there were a place like that around here.

06-26-2009, 08:09 PM
Well, he said there not guaranteed, but hell if I spend 20 bucks, get 4 and if only one is decent and holds air, I'm not really out any money. Call your local family type owned tire shops, most of them buy tires from junkyards, and they toss the atv tires to the side or in the back.

07-01-2009, 06:07 PM
Ok.....just got the new wristpins in the mail today, and the rings and top end gasket shipped out today from North Carolina,(not too far from me) so it looks like this little project may be underway this weekend!

07-01-2009, 09:20 PM
Don't forget to document by taking some digital pictures!

07-01-2009, 10:11 PM
Oh, I won't. Pictures are a must. If I actually dont it correctly and do screw nothing up, I will post the pics and a little rundown of how I did it and any shortcuts I figured out in the Repair Guides forum. I actually am pretty excited to see how it runs compared to the 250sx, I know it's twice the motor, but very very curious.

07-03-2009, 06:04 PM
I am having a lot of trouble compressing the rings right now.....just can't seem to get them

07-03-2009, 09:01 PM
Do you have a ring tool?

07-04-2009, 12:17 AM
Yea, I had to go get one, but I still couldn't get them to fit in the cylinder wall. I am beginning to think that the guy sent me the wrong sized rings. The oil ring is fine, but the top two compression rings seem to be quite large. I gave up on it for today, although getting to them was surprisingly easy, I just couldn't seem to get the rings compressed right....

07-04-2009, 12:42 AM
I believe it was goed that mentioned to trim if needed to obtain the correct end gap on the rings

07-04-2009, 12:46 AM
How are they not fitting? Are the ends touching , meaning no gap, and still not enough or does it seem like the groove isn't quite deep enough ?

07-04-2009, 10:29 AM
It seems like the groove isn't quite deep enough. I think I am gonna file the rings down a little bit and give it another shot.

07-04-2009, 10:34 AM
What will you use to file the rings? Didn't the rings come with that piston? Is filing rings a normal requirement?

That sucks though!

07-04-2009, 10:42 AM
The rings were supposed to be for a regular bore piston (which is what I have). I heard that you could file them, but I don't think it is a requirement.

07-04-2009, 10:51 AM
How far do the rings stick out?

07-04-2009, 10:55 AM
When I just compress them with my fingers they stick out about 1/16th of an inch (I have nothing to measure that small, thats just merely an estimate) I was also reading throgh the manual, and it says that the compression rings aren't interchangeable, one is supposed to have a wedged shape to it, and the other flat. Both of them look the same, both flat.

07-04-2009, 11:04 AM
Dang man.. Sounds like they may have made a packaging mistake. I would wait till Monday and give them a call before taking anymore steps!

That is unless the modern piston setup did away with the wedge shaped ring all together. I hate having to assume stuff like that when performing projects.

07-04-2009, 11:12 AM
Well, the problem with that is, I bought them off of ebay from a seller (called motorheadtoys - apparently it is aslo a normal store) and he ordered them from another supplier (of whom I have no idea). Looking at the manual, on page 12, it shows the rings and how they are supposed to be placed, but I don't really notice any difference in the shapes of them as per the picture.

07-04-2009, 01:16 PM
Shoot man. I just looked at the manual too. It definitely says wedged shaped for the second one. My better judgment say to wait until you can contact the seller or manufacturer to confirm.

It's not worth it to install, then find out you did it wrong and have to ride it like that!

Then again, it might be right.. they might both be the same rings.

07-04-2009, 02:03 PM
Maybe, but I just test-fitted the rings into the cylinder bore, (something I didn't do yesterday) and they both fit like a glove, with very very little gap in the rings.....maybe I am not compressing them right.....I have the tool, but have never used one before, so I am not even sure if I am doing it right.

07-04-2009, 02:25 PM
Ok, well, taking a deeper look into the actual piston, in the slots where the rings go, there is a lot of build-up. I am thinking that maybe the build-up is keeping the rings from being fully compressed, but I am not sure what to clean the grooves out with. I sprayed the hell out of 'em with carb cleaner, but it still didnt get all of the crud out.

07-04-2009, 03:03 PM
Alot of that is probably carbon buildup - spraying and soaking doesn't do much good . There is a special tool for cleaning out grooves or if your'e patient and careful you can use a thin file and "chisel" it out -don't "file" it use it more as a chisel or a scraper , you don't want to distort the shape of the groove. This is what i was looking for when i asked if it just didn't seem deep enough - gunk buildup

Don't file the rings unless the gap is too tight - to check this , put just the ring in the bore and with feeler gauge , measure gap
If you do have to file ( it's kinda rare ) don't get too carried away , a couple of strokes can mean the difference between perfect and garbage

07-04-2009, 03:09 PM
Alot of that is probably carbon buildup - spraying and soaking doesn't do much good . There is a special tool for cleaning out grooves or if your'e patient and careful you can use a thin file and "chisel" it out -don't "file" it use it more as a chisel or a scraper , you don't want to distort the shape of the groove. This is what i was looking for when i asked if it just didn't seem deep enough - gunk buildup

Couldn't have said something earlier, could ya'? ;) Yea, I am gonna clean the grooves out (very carefully) and I beleive that will solve my problem cuz the shit is thick.

07-04-2009, 03:56 PM
Wait.. I thought you were putting in a new piston and rings? Now you are just putting new rings on the old piston?

I'm lost now.

07-04-2009, 04:16 PM
Yea, old piston, new rings. At first I was gonna get a new piston, had one ordered, but cancelled it due to the fact that the "old" piston was still useable according to the Honda parts dealer here. Sorry to lose you there man.

07-04-2009, 04:47 PM
Tha was the problem guys. Got all the build-up cleaned out, fit the rings on, and pushed it through the bore, no problems at all......excpet it started raining about 30 seconds afterward and no garage means no more work till it's done......uugghhh

07-04-2009, 06:50 PM
Awesome! You need to make yourself a little shelter area!

07-04-2009, 08:19 PM
Yep. got it all back together. Rings went in surprisingly easy. Now on to the next problem - it's not starting. I pull (no battery for the electric start at the moment) and the headlight land nuetral light come on, but it's not wanting to start at all. Is there anything special I am supposed to do after taking the top end apart that would make it not start?

07-05-2009, 08:49 AM
Nothing special that I know of. You have the ignition, wire, and plug all connected back up? You have a spark for sure?

07-05-2009, 11:44 AM
Whats the best way to check if I am getting spark?

07-05-2009, 12:06 PM
stick your finger in it and crank the motor ......... LOL ........ no realy though pull the wire off the plug and place it over the end of a screw driver then hold the insolated end of the driver with the exposed metal portion of it close to the engine bout 1/4 in or less do not touch, have some one turn over the motor and watch the spark jump from the driver to the engine

07-05-2009, 12:14 PM
Yep, getting spark.

07-05-2009, 12:41 PM
stick your finger in it and crank the motor .....

That really is the best way!

Well, let's see.

Are you sure the carburetor works? Your timing is correct on the engine too?

Do you have some starting fluid?

I would hit up some starting fuel and see if you get anything to catch.

07-05-2009, 12:49 PM
The timing I'm not sure about....I wasn't exactly sure how to set the timing to TDC

07-05-2009, 12:55 PM
Was there marks on the sprockets?

You didn't hear any crunching noises when you pulled over the engine right?

07-05-2009, 01:02 PM
The sprocket on the cam chain? And no, I didn't hear any type of crunching noises.

07-05-2009, 01:04 PM
And didn't see the "T" or "F" marks either

07-05-2009, 01:24 PM
Ok, I see the "O" mark on the sprocket, but am not sure what it aligns to.

07-05-2009, 02:23 PM
SX, have you read the manual? there are so many questions that can be answered for you in it

07-05-2009, 03:03 PM
Yea, I have, but I don't quite underdsand it. I looked for the marks to align everything, but couldn't find anything but the "o" marks on the sprocket and rotor.

07-05-2009, 09:04 PM
You have to align the timing marks or you will not be going anywhere. What I have always done and still do (well with cars, esp twin cam units) is get some white out and mark my sprockets and the engine really good so there isn't confusion later. There should be some obvious marks on there and you need to figure the correct locations before you get that chain on there.

Keep us posted.

07-05-2009, 09:44 PM
Well that must be the problem then......damn first timers.....time to take it all apart again.:(

07-06-2009, 10:26 AM
S'all good! A mistake that can be fixed with a short amount of time and no money is an easy lesson learned.