2 Stroke piston slap? <help plz>

I have a yamaha DT 125R (12k miles on clock) 2-stroke bike that has a noise comming from the barrel/cylinder head which i cant describe.

I have taken some sound recordings


Could someone please download it and have a listen? i am not sure if it is piston slap or something more sinister.

Symptoms are

Slight loss of power

More difficult to start up after standstill (need to use throttle)

Blowby on piston past top ring down onto skirt

rattle/knock/tap noise at random revs, not very consistant, cant really hear it during hard accel, can hear it very easily when at low revs and around idle speed, VERY obvious if i rev up let go of the trottle and slowly decel.

Sounds like it is comming from the barrel/cylinder

With 12,000 miles on a single cylinder 125cc 2 stroke I would have to say its time for a new piston. The piston is probably worn out and has more clearance than it needs. Is it more loud when the bike is cold and than as it warms up the slapping is more quiet? I'm not sure how much a piston cost for a dt125 but I doubt its much more than 100$. You need to measure the bore so you can order the proper size piston for it. Almost forgot to mention, if you have a pretty good load on the engine (kind of bogging) and it rattles, chances are that is detonation. Detonation can be caused by many things; bad gas, exsessive carbon build up on the piston, air leak and lean jetting just to name a few things. Good luck.

ditto if you havent done a new piston/rings DO IT as soon as you can thats a ton of miles for a 2T

im really surprised its lasted this long on that piston, ur lucky it aint just locked itself up and made u fork out cash for a nice new topend or more

Hi, did anyone have a listen to the noise? i actually did a top end rebuild at 11k and a small end bearing.

The noise dissapeared at first but came back not long after, could this be due to the piston-bore clearance being too great? as it has gone 12K (now 13K) miles without a rebore or hone.

The noise does not seem to go away when the bike warms up allthough it might quieten a bit.

I'll try to explain it better.

In neutral:

idle (1.6k) - no noise

1.7k rattle

1.8k rattle

1.9k rattle dies

2k no noise

2.1k no noise

2.2k no noise

2.3k rattle

It basically goes like that when i slowly increase the revs, when around town i often hold the revs at around 3-4k at a point where it doesn't rattle so i dont get strange looks from people....

When i am riding and i rev the bike upto 10k and let go of the throttle and decelerate

10k-7k no noise

7-6k you hear a slight rattle

6-4k no noise

4-3k hear a slight rattle

it follows that pattern till i reach idle

By the way these are just estimates and sometimes it makes the rattle at idle and sometimes it doesn't, i can come back from a ride and every-so often when i rev the bike in neutral the noise is quieter then usual.

Last time i opened the top end was at around 12K miles and there was blowby on the new piston and also some loose carbon buildup (this could have been my fault for not clearning everything properly?)

Flywheel has no obvious play (none that i could find), conrod has the usual slight side to side play, no up and down play however.

I could really do with some quick assistance as i have ordered an oversized piston kit and am going to have a rebore done in the next few days so i need to know if im about to waste my money.

Thanks for any help


Original link has gone dead

I re-uploaded it here


please someone check it out...

Sounds like you need to bore it over.

Since you did not atleast have the cylinder honed when you put the new piston in you probably did not get a good ring seal (indicated by your blow by). Without measuring your cylinder to piston clearance you are only guessing if you have the right clearance. I couldn't get the sound video to work, it wanted me to try and unzip a program and my unzip program expired.

I am starting to worry

The more i look on the net about main bearings and knocks and such the more uncertain i am that this will solve the problem.

Is it known for piston slap to cause noise on deceleration?

Because i have done many searches of this forum and some people say that noise on deceleration is a main bearing problem.

I took the flywheel cover off and ran the bike, it appeared to spin smoothly and while making the engine sound a bit louder it did not sound like it was comming from that area.

When running at below half revs i can hear the noise, the noise goes away with heavy acceleration and if i get it on the right spot in a gear i can ride without the noise occuring.

As an example:

constant speed 28mph - rattle

constant speed 30mph - no rattle

hard acceleration - no rattle (probably cant hear it over engine noise)

engine breaking (deceleration) - will rattle at certain revs

I know i should take it to a mechanic and have them look at it but i really am short on time lately, i cant ride my bike at the moment because of a broken clutch cable so cant get to a mechanic, there is a postal strike on and i need a head skim anyway so i ordered the top end kit aswell and hopefully it will solve the problem, if it doesn't i wont be able to afford an engine rebuild lol.

There is blowby on my piston so i guess it needs to be done anyway..

i also sometimes get a ping when comming off the throttle, it really wouldn't suprise me if the bike was jetted incorrectly as at the moment the exhaust setup is a dep silencer and a half derestricted standard front (baffle came loose, i pulled it out)

I wish we coudl help more, but we in the USA are not trusted with such machines. I woudl kill for a dt 125.

If you changed your pipe to a performance one, you will have to rejet. I learned thsi with a Derbi 50 I played with/

Very cool bike. Derbi 50...street legal and super moto style. I was the laughingstock of the neighborhood. You should have seen the Harley riders look at me when I pulled into the HD shop. http://www.mobcustom.com/IMG/jpg_Derbi_X_Race_SM.jpg

Thats a nice looking bike!


I just pulled my bike apart


Minimal left/right play, span it a few times checking up/down play, found none.

Small end:

how do you measure this? its a new small end but it had some side to side play, dont know if its usual, ill replace anyway.


Seems like the clearance is a bit high, with the piston inside with the rings on i could move rattle it around, inside looks fine just glazed, gonna have a rebore anyway


The piston is quite bad considering its only done about a thousand miles, there were some very slight chips on the top and quite allot of blowby, the top ring was okay, the bottom ring had failed though! it would hardly move and was continuously compressed, when i pulled it out i saw that the expander ring inside had collapsed into itself, there was also quite allot of buildup around the sides of the piston, specks of debris and such.

Could this have happened because of an almost heat sieze? when i was towards the end of my run-in i started using the bike harder and i must not have torqued the head down sufficiently and lost all my coolant shooting the temp gauge up and making me pull over and have to walk 7 miles with the bike home on a hot day in all my gear..not fun. i started it a few times when walking it but after a while it refused to start again, and it was only after 3 more stripdowns i got it to work, and since then it made the rattle.

Im suprised the bike even ran with one of the rings failing

Sounds like you need to bore it over.

At that mileage/age and never been bored, your are one luck dude. Especially since it almost seized. Take it to a reputable machinist along with piston and he will make it like new again. Be extrmemly carefull when breaking it in and you will get a long life from it again. It's not unusual for cylinders to be bored 1st, 2nd, third over before replacing the jug. Again I am stressing a reputable competent machinist, some guys don't have the experience with smaller stuff and will chatter the bit, resulting in a poor fit and ongoing rattle issues. Good luck, hope this helps.

I would have thought the guys at a yamaha dealership/workshop would know how to do a decent rebore, im gonna get it done there, does anyone know how long it would take and if i need to book it in?

These DT 125's are quite long lasting bikes as they are in a moderately low state of tune, you have to do a top end every 9k miles or so and are good for about 80mph with a dep system.

Im going for a 0.50 bore as i want to see if i can get some kind of power increase, would it make any difference?

My bike pulls 70-75mph atm and it has bad blowby and a leaking head gasket that requires more coolant every 50 miles or it overheats, once i get these fixed im hoping ill see some kind of power increase.

Way to go man you never told us the whole story of you running the bike out of coolant. If you want us to try and help you, you need to provide us with all of the information. Your rings have blow by because you put a new piston in on a glazed cylinder = the rings will not seal. You overheated the bike, seized a ring and possibly warped your head. Have the machine shop make sure your head is machined flat. Good luck.

Sorry about not giving all the information, i gave a detailed explanation of what happened on another forum and didn't get much help.

Yeah my head is definately warped or leaking, probably partially due to my attempts at scraping the old gasket off. when my bike overheated during the end of the run in i got it before it hit the red line luckily and when i stripped it down i found loads of coolant sitting in the exhaust so it was definately a head gasket failure, i was unaware of the correct run in procedure for a forged piston at that time, strangely i had done 40 below 50% rev miles and 20 below 70% and for all that time the temp gauge was fine, no coolant leak, slight rattle when breaking the powerband (i really had to push it to get into the powerband for some reason) but no noises anywhere else and as soon as i start reving the bike hard the head blows and spills coolant everywhere.

So this time im going for a new small end, +0.50 piston kit, top end gasket set, +0.50 rebore and a head and barrel skim.

Considering the state of my current piston and rings and a barrel with fairly high clearance and glazed surface would it be reasonable to assume that it would be what was causing the noise and that it will be solved once i get it sorted out, are you supposed to be able to rattle a piston about inside the barrel?

One last question if someone could answer it quickly, for a rebore and a head skim do i need to bring the replacement oversized piston with me? (im guessing its a good idea so they can get a good fit), how long does it take, do you usually need to book it in with the workshop and is it a drop it off one day collect it another type thing?

Thanks for all the help

take the piston so they can measure it and bore accordingly, no 2 pistons are really ever completely the same dimensions.

i have got a 1994 dt125r and i derestricted and ended up goin to fast for to long and went and put a hole in my piston so i have put a new piston in and small bearings in and now wen i start it up there is a tapping or rattling noise wen it is ticking over but you cannot really notice it wen u are riding tho

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