2005 Yamaha YZ 125

I just bought a 2005 YZ 125 with a ported cylinder & a decked head. I am a trail rider, I do not race. What are my service intervals for top-end rebuild? What brand do you recommend?

Do you how many hours are on the current piston & ring?

A good rule of thumb for trail / recreational usage 2-strokes is to monitor compression drop rather

than refurb the top end at specific intervals. 


Say a new top end gets 180psi of compression, 

checked a few times during the season by the time your 'like new' value drops 

more than 10% you should consider a refresh.  180psi x 10% = 18psi    180 - 18 = 162psi or lower.


Depending on your usage it could mean more or less once a season.


Did the previous owner specify if running race fuel was required ?

( figuring the squish band height was reduced by 'decked head' )

For trail usage, hopefully the porting isn't mid-to-top only at the cost of the decent low end the late model YZ125 has.


Edited by mlatour

Thanks for the info, mlatour. The bike was fully rebuilt and now has 12.5 hours on it. Piston & rings are fresh & motor has been broken in. It does not require race gas. I don't know anything about porting, all I know is that been done by race tech & its the expensive porting job option instead of race tech's cheaper alternative.

Edited by Caleb Kittrell

To give you an idea on hours / usage and compression drop,

I put 18 track days on my recreational MX track ridden YZ125's top end this season 

(including some sand tracks that worked the engine harder) exact hours unknown but at least 18!


At the beginning of the season my new / just broken in top end got 182psi,

by the end of the season it was down to 172psi, for a 6% drop.


Only engine mod is a reduced squish height, running 15% race fuel for a total of about 94 octane.

Edited by mlatour

Compression test is one easy way to check, although your starting point is already has some hours. The YZ125 ring tends to wear out quickly across the exhaust bridge and this drops the compression pretty fast. Add in some dust/grit and that will be even faster.

It would probably take me 15 minutes to strip the top-end, once I had washed the bike with the tank off. Measuring the piston to bore clearance, ring end cap, ring wear opposite the exhaust port, wrist pin wear would all be on the to-do list.

Have a new ring ready to put on there, unless it is a Wiseco Ti-Ni ring, which are a yellow-gold color from new, and might not be worn out yet.

Edited by Swappa

Caleb, You should also take a look at your shift stopper arm, at some point. It was a BIG problem for the 2005 and 2006 YZ125s, amongst other Yamahas and can cause terminal damage to your transmission and engine cases. If you're having trouble with shifting gears, you should definitely stop riding the bike, and check the arm.

Even if it's shifting okay, you should still check it out.

Swappa, how do I check the shift stopper arm? & how often should I do that? what can I do to prevent it from damaging the transmission?

Edited by Caleb Kittrell
36 minutes ago, Caleb Kittrell said:

Swappa, how do I check the shift stopper arm? & how often should I do that? what can I do to prevent it from damaging the transmission?

Just replace it if it has a bearing tip. Otherwise, inspect only when you replace the crank.

You can find more details if you plug in shift stopper in the search bar or check out the pinned links under the YZ FAQs.


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