Aero 125 no compression

[RonMcG]

Hello, everyone. This is my first time posting to these forums, but I have read them for info before.

I have used the search feature, and found much information that I will (very possibly) need in the near future, but I have, what I think to be, a few simple questions regarding my Aero 125. I have vast automotive knowledge, but little experience with these simple two-cycle engines.

The bike had been giving me some starting problems the last few weeks. It would usually take several cranks to fire, anywhere from 30s to a minute or more of cranking. About 1 out of 7 starts, it would fire right up on the first few cranks, (both symptoms happened during warm and cold starts).

I was riding it last Saturday. I pulled into a parking lot, after about a 4 mile ride in stop-and-go city traffic, and she died. It died as if it ran out of gas, just sputtered to a stop.

I had a gut feeling I lost compression because when I tried to start it, the starter spun very free, as if there was no resistance.

I have a brand new spark plug. Verified it's getting a good, hot spark from the plug.

Chased fuel flow from the tank to the cylinder. There is fuel getting into the cylinder, (verified by removing the exhaust), after a few cranks I can burn off the fuel in the cylinder through the exhaust port... a little 'puff' of flame will shoot out, nothing excessive. The fuel in the cylinder appears to have oil in it, it's certainly not straight gasoline.

I used a known, working compression tester and it shows only 2-3 PSI on the cylinder.

There is no VISIBLE damage to the engine, no signs of leaks around the cylinder or head.

My questions are:

1) Do I need to have the exhaust attached to get an accurate compression reading? I know you don't in a car because of the valves and all, but again, these small engines baffle me...

2)Might there be something OTHER than a head/cylinder/ring problem that would cause a total loss of compression that I have?

I want to try anything and everything before taking the engine apart.

Thanks in advance to any and all input!

[patthesoundguy]

I have heard on one occasion and it was on a golf cart of a failed base gasket. . But I have a feeling it had had the head bolts losen up over 30 years and the gasket let go. I have also had a head gasket fail on my gas but me motor and lost compression. I would think you could have a rebuild on you hands. To find out you have to pull the head and have a look. At this point you have to. Unless you find a head bolt broken or something like that. And you will have to pull the cylinder to fix that anyway. So pull the head and have a look at the condition of the piston rings and jug. I hope its not too trashed.

[Johnniespeed]

Ron it only take a few minutes ( less than 30) to pull the jug and piston out, leave the carb in place. The muffler and cylinder heat shroud must come off, as well as both side plastics. The fan cover must be removed also. If you have no compression there is no way to fix the problem without disassembly, so the sooner you get wrenching the sooner you get scooting again.
Replacing the NH 125 piston is kind of like doing a tune up, you may as well learn how to do it fast.
There are plenty of gaskets in England, none in USA.
There is a forum for large Honda Scooters such as the Aero 125 over on MotorscooterGuide, this place is for the little guys,( 50cc and wishing they had more like us ) .

[RonMcG]

So I did get it apart, and I think it's about as bad as it can be aside from a shattered cylinder.

Both rings broke. Looks like they were broken for some time, as the top of the piston looks as if it's been beaten with BBs around the edge. The head is also beaten up at the firing chamber.

The cylinder wall has several scores on it, guessing the depth to be around .020-.025". Can't tell if it's been bored already, gonna get a caliper tomorrow and get an accurate measurement.

I don't think I can, or should, re-use this piston. Not sure if you can see in the pic, but the top ring channel is slightly malformed now, and there's several scores along the side, (this side was opposite the exhaust port when installed, fwiw). I have a clear pic of this, if you wanna see it, (can only add 3 files per post I guess).

Does it look like I can re-use this cylinder? If not, can I bolt up a cylinder from a CR 125? I read that some people would bore their cylinder to that spec and use a piston & ring set from the CR 125 because they couldn't find the Aero 125 engine rings.

What would you think be the most efficient fix here, without cutting corners? I would like to get my parts ASAP and get this thing moving again.

The bike has 6k miles on it, think this is NOT the first set of rings it's been through?

[patthesoundguy]

I'd go get the new piston and rings and get the cylinder bored to match it. Problem solved. If the cr125 piston is oversize and you can't get oversize piston and rings for that cylinder that would be the plan. Going slightly oversize will will ensure you get a good clean bore with a good hone job and you will have a better running motor than if you only hone it out and put the same size piston in there. And that orig piston is so trashed. The dents in the piston and the head is from the pieces of the rings jamming on top oofthe piston. My 2 stroke golf cart motor failed like that. The chamfers wore off the diameter of the bore got so big the rings caught in the ports and for parts of the head of the piston off.

[Johnniespeed]

That piston is trash, a paperweight or trophy. The cylinder can be bored oversize more than once. CR125 cylinder is not even close and has no chance of being used here.
Forget about the CR125 piston , this is urban legend and not true, it will fit into the cylinder, it will fit onto the rod, but will not work due to the ring stops are in the wrong locations, that means the ring end gaps will travel through a port, the ring ends will get caught on the port and self destruct immediately. Nuff said about that CR125 myth, want to know how much that education cost me ?
The aftermarket Suzuki KHC piston will fit properly but the rings are prone to breakage. Remember the 1.0 mm oversize Suzuki piston is standard bore for NH125,, 1.5 mm oversize Suzuki piston is 0.5mm oversize for NH125. The 2.0mm oversize Suzuki piston is 1.0mm oversize for NH125. and so on.
England has plenty of pistons and gaskets for this, USA has none. Remember England has no Aero 125 so don't even think about asking, but they do have the Lead 125 (NH125) and it was sold there for multiple years, not just 1984.

[RonMcG]

Thanks a ton!

I'm getting someone to bring me a set of dial calipers tomorrow, (I know how to use, machinist background). Will they be good enough to get an accurate measurement on the cylinder and piston, or should I get a micrometer?

Ok, Johnny, I'll toss the CR125 idea. Consider it gone. Thank you VERY much for sharing your experience with me. Possibly saved me from making an expensive mistake.

I found a site before that seems to have a lot of my parts, http://www.cmsnl.com/honda-nh125-1984-a ... partslist/

They have two different oversize pistons in stock, the oversize is .25. I'm guessing that's in mm since they are in the UK. I'll dig deeper once I get a measurement.

So a few details I thought about, just to cover all bases:

Do I need to change the needle bearing, rod, or rod bearing at all?

Should I drain and/or clean the crank case?

I did not find the entire ring intact, think some of it ended up down in there?

Is there a real risk of washing any tiny metal pieces that may have fallen down there back into the cylinder?

Lastly, just to check my math, there are 25.4 mm in 1 inch, correct?

[Johnniespeed]

Ron
CMSNL is a great place to buy parts, they are in the Netherlands, at one time I thought I bought every piston they had. 0.25mm is about 0.010" 0.50mm is 0.020" 0.075 is 0.030" 1.0mm is about 0.040"
The crankcase can be disassembled to clean out, or you can rinse it out with two stroke fuel mix, ( but have to remove engine to tip upside down) on a Spree it is faster to have two guys just hold the bike upside down.
The needle bearing ( wrist pin bearing ) I have changed sometimes, and not changed other times, same results either way. The rod is one piece, the rod bearing would require pressing the crankshaft halves apart, then reassembling and truing of the crank, If you have a rod issue, just easier to get another crank.
"Bitz for bikes" in England is a great parts place also. ( we say parts they say bits )
Since two stroke engines depend on reversed pulse waves from the exhaust, there is a chance of debris inside the muffler getting sucked back into the exhaust port, at least shake and blow out the muffler ( I know it cannot be done perfectly but I try) . This is also a good time to cook the oil out of the muffler, I use my gas grill.

[RonMcG]

I really, really appreciate the information and help, yalls!!!!

I got a measurement on my cylinder and piston.

It seems this has been oversized once. The piston is 55.20-23mm and the cylinder is 55.38-40mm, (seems that it's a little elliptical now).

I'm thinking that I'd probably need to go to the .75 anyway, because the scores on the cylinder at the bottom of the stroke would hit the rings, and I think they are deeper than .05mm. I'm going to take it to an automotive machining place tomorrow and have them tell me how much they would need to take out to make it right.

If I HAD to, would I be able to put in a .25 oversize piston/ring setup with the cylinder being at that measurement, or is there too much wear?

Johnnyspeed, are these Suzuki KHC parts you mentioned available in the US? You say I would need a ring set from Honda because the Suzi rings are known to break, but the piston from Suzuki? I'm looking into this because CMS doesn't have a .75 oversize even listed, and they show the .50 oversize as 'out of stock'. I checked 'Bitz for Bikes' and they only have the standard size listed for the NH125.

[RonMcG]

Also, I found on eBay many piston sets from other years for the NH125.

Like, will one for a 1987 fit my '84? If not, what about one for an '83? Those are the two years I seem to find most, (possibly because that's the range they were made from?).

[Johnniespeed]

I strongly suggest to use a Motorcycle shop for boring and not an automotive shop. The motorcycle shop will have a boring bar setup that will accommodate a removable cylinder and they will properly know how to bore and hone it to specs. The cylinder to piston clearance is much closer than automotive guys are accustomed too. The piston to cylinder clearance on a NH125 is 0.00136 to 0.00196" that is between one and a half thousandths and two thousandths of an inch. Those are very tight tolerances and are measured after final honing.
Whatever ring set you use, it must be compatible with the piston you use, Honda top piston ring grooves are not square cut and neither are the top ring. That is not visible to the naked eye, but try the ring on upside down or in the wrong groove and you will spot the difference right away. I am pretty sure that KHC and Wiseco are both square cut ring grooves. I have not yet tried this, but I am hoping that I can use Wiseco rings on a KHC piston.
You can use Babbitts online to obtain Honda piston part numbers for the USA Aero, I would assume someone in England can obtain the Honda Factory part numbers for the 1983 and 1987 Non-USA Lead, once you have the part numbers for each you will know if they are the same.

[Johnniespeed]

A very good picture of the piston ring and piston groove is on page 63 of the Service manual here is a link.
http://motorscooterguide.net/Downloads/Downloads.html
Scroll down to Aero 125, open the PDF and go to page 63

[RonMcG]

I found a 1mm oversize piston set and ordered it.

I had to use the UK forwarding service, but it's the actual NH125 part, not the Suzuki.

Guessing it'll take two weeks to get here, but at least I got what I needed.

I will take your advice and have the cylinder bored at a motorcycle shop. Planning on taking the cylinder there with the replacement piston/ring and have them match it to the OEM clearance.

I did download that shop manual PDF, so I have that now.

Thanks for all the information and I'll let yalls know how it turns out.

[Squar]

I pick up my '84 NH125 Lead in a couple of weeks, it has just 4,000 miles from new - in 31 years ! I have to wonder what to expect....


Jim