How many hours before new conrod kit on 125cc?

[DomasMX]

So I have searched many different forums and still could find very little info on conrod replacement intervals. Lets say we are using good synthetic oil, washing air filter after every ride and riding trails and sometimes tracks on a 125 MX bike. We replace a ring after 15 hours and a new piston kit after 30 hours. In this case, how many hours could I expect from a new conrod kit? Or how many pistons should I replace before replacing a conrod?

I have just put a new Prox conrod kit on my Honda Cr 125 '02 crank and I wonder how long could it hold?

I know that a lot depends on riding conditions and riding style. However, I am interested in aproximate predictions and your experience :nod:

 

[_JOE_]

It's usually the bearing the goes bad. Keep an eye on all the tolerances when you take it apart and run it until it gets loose or notchy feeling. It should last over 100 hours of easy riding in good conditions. I've seen people get very long life out of bottom ends.

 

[whenfoxforks-ruled]

When ever the jug comes off, measure the thrust clearance, get a light and magnifier to inspect the cage and pins, check how much it rocks and no up and down play. Pushing the 125 pretty hard, 50~60 hours would be a miracle. But, you are seemingly getting 3 times the engine life already? So the 100+ may be ball park? b What oil ratio are you running? Vintage Bob

 

[DomasMX]

Well actually I am changing ring after 12 hours and piston kit after 24 hours. I don't race motocross yet as this is my second season on mx. I am using fully synthetic Motul 800 1:30, really good oil. My pistons looked still nice after 24 hours :whoa:

 

[2-Strokes 4-ever]

As a general rule if a bike is ridden hard (big loads, high RPM) and a lot...

125-- check lower-end each year, piston twice w/ a couple re-rings
250-- lower-end every two years, piston once per year w/ one re-ring

Clean air filters, correct jetting, quality oil at 32:1 for a 125, and 40:1 for a 250. Works for us.

 

[whenfoxforks-ruled]

Maintenance and riding habits. It sure seems everyone is different, or nobody actually measures their engine parts. "It looks good??" Laser eye has spoken? For rebuilds, a feeler gauge is priceless, literally. Every couple of pistons, you need a bore gauge to watch for ovaled bore. Measure and inspect. My oldest son put a motor together, he did not know the small end bearing was .5 mm too small. I do not know the metric system, but that equates to very sloppy engine building. He claimed after the fact, it seemed a little loose? Measuring is like painfully difficult work? Inspecting is important also, but a lot of stuff, if it looks worn its shot. A 125 ran on the rev limiter, that crank will not last 20 hours. Vintage Bob

 

[DomasMX]

Ofcourse measuring is the only way to judge on the shape of the engine part. I'm just interesting how many hours do you USUALLY get from a new conrod.

 

[Patman]

You can't say "usually" and be safe. If you want to be safe get an hour meter and a factory service manual then go by their reccomended replacement schedule. That usually will be safe.

 

[DomasMX]

I've got an hour meter and the manual as well but manual doesn't provide any schedule for the conrod, only for the piston..

 

[_JOE_]

The manual should have service limits on the crank measurements. This is like asking how long brake pads last?

 

[DomasMX]

Lets turn this question around. How many pistons do you USUALLY replace between conrod replacements?

In this case, if you are always on full throttle, then replace pistons much more often as well as conrods and so on. Would this kind of question sound better?

 

[Patman]

This is like asking how long brake pads last?

:rotfl:

 

[whenfoxforks-ruled]

And you used a pro x rod, and what bearings and thrust washers. Plus who did it. WAY TOO MANY VARIABLES. Joe gave you the best answer. I am going off racing 125's 10 years ago, and 35 years ago. They did not change much, and seemingly, if anything, the top end life got shorter, as the technologies got better/faster. Keep a log, and see how you are doing. Quit looking at it, and measure it. And if it was me, rejet the bike for 24 to 20:1. The best insurance you can get, more oil! Vintage Bob