Jump to content

when to do top end and how

Recommended Posts

I just got a 2008 yz 125 and not sure if it needs a top end and I dont want to risk wrecking the top end, how can I know when I need to change it without testing compression? when i do the top end can i just replace the rings or should in do the whole piston?

Share this post


Link to post
Share on other sites
Just now, William1 said:

When power drops off, compression falls (for the typical rider, a drop of 15% from new is a good indicator).

If you refuse to test, then why even care?

I dont refuse to test I just dont have a compression tester. As it is my first 2 stroke im not sure how to tell what it should be like 

Share this post


Link to post
Share on other sites
2 minutes ago, mritch8 said:

I dont refuse to test I just dont have a compression tester. As it is my first 2 stroke im not sure how to tell what it should be like 

So.... your first purchase should be a compression tester. Then you read the service/owners manual.

Share this post


Link to post
Share on other sites

When it sounds like a rattle can, it's too late.

Replace the entire topend, piston, rings, and bearings.

Check the con-rod for up and down play.

Thoroughly inspect your cylinder for wear marks and the ports for any build-up and chips in the plating.

Share this post


Link to post
Share on other sites

If you don't know how old it is, consider it worn out.  125's require more frequent top end rebuilds.  Every 30-50 hours is about right.  If you just bought the bike, consider it worn out regardless of what the previous owner said.  At best you'll find a fresh but low quality aftermarket top end in there. 

  • Like 1

Share this post


Link to post
Share on other sites

Compression test for sure. But you can get a general idea of the condition by looking in the exhaust port and spark plug hole with a flashlight. Look for wear on the cylinder walls and blow by on the piston. But get a compression tester, a visual inspection isn't a substitute for a measurable test. 

Share this post


Link to post
Share on other sites
If you don't know how old it is, consider it worn out.  125's require more frequent top end rebuilds.  Every 30-50 hours is about right.  If you just bought the bike, consider it worn out regardless of what the previous owner said.  At best you'll find a fresh but low quality aftermarket top end in there. 

What's a good top end kit that's cheap I'll just get one anyway. And what tools
Will I need?
When it sounds like a rattle can, it's too late.
Replace the entire topend, piston, rings, and bearings.
Check the con-rod for up and down play.
Thoroughly inspect your cylinder for wear marks and the ports for any build-up and chips in the plating.

It's been some trouble idling even when it's not, is that the way it's tuned or the top end?

Share this post


Link to post
Share on other sites

First tool you will ever need is the manual. Then a torque wrench of some quality, bore gauge, feeler gauge, compression gauge, impact screw/hut driver, wrench and socket set or 8, 10, 12, 14, 22, 27, 32mm. Sure there are a few other bits and bobs, odds and ends, but that will get you started. Add other specialty tools as needed.

Share this post


Link to post
Share on other sites

I have to admit that I work without a torque wrench and a bore gauge.

I do own a impact driver / wrench which comes pretty handy when doing clutch or a sprocket change. You'll need some silicone for the exhaust joints. There are some good videos in YT from Rocky Mountain ATF  

  • Like 1

Share this post


Link to post
Share on other sites
1 hour ago, arnego2 said:

I have to admit that I work without a torque wrench and a bore gauge.

I do own a impact driver / wrench which comes pretty handy when doing clutch or a sprocket change. You'll need some silicone for the exhaust joints. There are some good videos in YT from Rocky Mountain ATF  

You are not alone.

Plated bores wear visibly.  The crosshatch is extremely shallow and the plating itself is also very thin.  If you have crosshatch, the plating is good.  Replace a standard size top end with a standard size top end and ride it.

I would recommend either a genuine Yamaha top end or a forged aftermarket like a Wiseco.  Pro X used to be repackaged OE but has recently begun making their own pistons which are not the same quality.  If you get one from them that is marked ART you are fine.  The ones that are marked PRO-X are low end cast items with no quality control to speak of.  I've seen one of their KX500 pistons come apart into chunks with low hours and I got one for my KDX200 that came with 10x the recommended clearance.

Share this post


Link to post
Share on other sites

Almost 40 years of riding 2-strokes (and zero engine failures) and I have never used a compression tester. Just change the top end after it has run a reasonable number of hours. If you don't know the history of your engine, now is a reasonable time.  A compression test will not tell you if the piston has a fatigue crack and is about to break. 

The shop where you buy your top end can measure the bore for you so you can order the correct piston. 

Get a set of feeler gauges to check the ring end gap and a torque wrench to install the cylinder and head properly. A beam type torque wrench is inexpensive and very accurate. 

 

  • Like 3

Share this post


Link to post
Share on other sites
2 hours ago, Woodzi said:

Almost 40 years of riding 2-strokes (and zero engine failures) and I have never used a compression tester. Just change the top end after it has run a reasonable number of hours. If you don't know the history of your engine, now is a reasonable time.  A compression test will not tell you if the piston has a fatigue crack and is about to break. 

The shop where you buy your top end can measure the bore for you so you can order the correct piston. 

Get a set of feeler gauges to check the ring end gap and a torque wrench to install the cylinder and head properly. A beam type torque wrench is inexpensive and very accurate. 

 

if I buy the kit online(dont have any shops near me) how can I tell what size to get?

Share this post


Link to post
Share on other sites

A compression test is useless for determining the condition of the top-end unless you have a test that was done with a new top-end to compare it to.

It's a used 125 two-stroke, just assume it needs one and spend the $200 to do it.

After break-in, do a compression test and write down the results. This is your baseline. Re-test every few hours, and when you start to see a measurable and consistent drop in compression, it means the rings are no longer sealing effectively, time for a new top-end.

Some people will use an arbitrary figure like a 5% loss of compression, 15%, whatever. I don't do this. When the loss is consistent and noticable it's time. If my bike tested at 220 psi with a new top-end and now consistantly tests at 210, I rebuild it. I also do not re-ring pistons, if I'm investing the time to disassemble it it's getting all new parts while I'm in there. I'm not taking chances on a metal fatigue-induced piston failure.

  • Like 1

Share this post


Link to post
Share on other sites
9 hours ago, scooter89 said:

 


Measure your piston with a dial indicator or a micrometer.

 

Don't measure the piston. It is probably worn out. Measure the cylinder bore. 

Share this post


Link to post
Share on other sites
9 hours ago, mritch8 said:


If it's just stock bore can I look at my manual?

I assume the 125's are the same as my 250. The cylinders are marked with an A, B, C or D. The cylinders are measured after they are manufactured and marked according to the actual bore size. 

There are corresponding pistons for each. 

IF the cylinder is unmolested (not re-plated, etc.) you can use the marking on the cylinder to select the correct piston. 

If you don't have a bike shop nearby that can verify the measurement, you could try a machine shop, or a shop that does automotive machine work. 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:


×