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I bought a 2nd hand 2 stroke sportbike last end april. It had been overhauled by the prev owner end 2014 with new piston, block, rings, bearings and con rod.

I sent it for its 1st full servicing under my care last december - oil & spark plug change as well as to polish the piston & block. The mech dismantled the top end, while the boss inspected my piston & block.

1)There was a dark black stripe just beneath the lower piston ring abt quarter of the piston circumference. There were light black random stains around the other areas of the piston walls. The boss told me the piston was a gone case & needs to be changed. I couldnt believe it as i hadnt really been whacking the throttle & its only abt a year old! Is there really a need for change or is he bull****ting me? The mech agrees with the boss. He said its a sign of a leak. What leak?? The gear box oil did look watery (4 mth oil). Fuel consumption still no real diff now compared to before.

2)After the piston was replaced the boss told me not to exceed 100km/h for 2 weeks. One of his mech told me not to exceed 80-90km/h for 500km. There are articles saying you must whack the throttle right from the onset (hard break in). The manual says (brand new bike) not to exceed 8000rpm for the 1st 500km & then increase the rpm by another 2k revs for another 500km - but if you follow this method, you will break the speed limit!! So what is actually the proper way of breaking in a new piston? So far Ive been riding normally. Ive kept my max revs to 6k on the highways (100km/h) with the occassional 7k revs for overtaking. Urban riding mostly kept at 4-5k revs.

3)how do i know that the piston rings are already well seated in the cylinder. Its been 2 weeks now & i have to admit that the throttle response is more lively, the bike feels more powerful compared to before the piston change. Power delivery more prompt & crisp. 1st ride after servicing, the engine did feel a little tight & sluggish.

4)Do I need to change gearbox oil after my break in period. I know for cars & 4 stroke bikes you need to change engine oil. But what abt 2 stroke engines? Metal shavings will still drop into the gearbox right??

Thanks in advance
 

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What bike is it??????????? :confused:
Brand, size, year, street, dirt, mileage. ????????????? :confused:
 

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Rev it up.

I would follow the instructions in the manual.

However, we used to put 250 cc Yamaha GP bikes engines together, take them to the track, and run them full tilt right away. As in, zero break in. It has to do with how tight the piston is when new, and the quality of the piston. The piston and the cylinder hear up at different rates. Some pistons have high spots.

With modern quality pistons, and more than 1 1/2 thousands gap, with quality oil, there should be no problems. So far you have not told us if it is air cooled or liquid cooled. So when you mention leak, I for one am puzzled if I do not know the details.

You also used the word block. There is no block on a 2 cycle motor. There is a cylinder head, a cylinder barrel, 2 parts of the crank case, and two side covers.
there is not much to run in. The gear box should be happy at any speed. The crank bearings the same. That only leaves the piston and cylinder.
Other parts might be a rotary disc valve or reed valves. But I do not know if it is a piston port engine or one of the others.

So I return to the original. Follow the manual.

I can only assume the piston failure was due to: Cheap oil, overheating, or a bad / cheap piston.

As an aside, I also find it handy to know where someone is living.

Unkle Krusty*
 

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Like Unkle Krusty said, do what the manual tells you to do.

The mech agrees with the boss. He said its a sign of a leak. What leak?? The gear box oil did look watery (4 mth oil). Fuel consumption still no real diff now compared to before.

2)After the piston was replaced the boss told me not to exceed 100km/h for 2 weeks. One of his mech told me not to exceed 80-90km/h for 500km. There are articles saying you must whack the throttle right from the onset (hard break in). The manual says (brand new bike) not to exceed 8000rpm for the 1st 500km & then increase the rpm by another 2k revs for another 500km - but if you follow this method, you will break the speed limit!! So what is actually the proper way of breaking in a new piston?

4)Do I need to change gearbox oil after my break in period. I know for cars & 4 stroke bikes you need to change engine oil. But what abt 2 stroke engines? Metal shavings will still drop into the gearbox right??

Thanks in advance
My guess is that it's an air cooled machine. The spec's say nothing of the type of cooling, that's why I guess air cooling.

A leak? Did you ask em what's leaking and from where? Do ask them.

Watery gear box oil? If water is in the oil it will look like brown/tan mud. But you're probably referring to thin oil?
I know of nothing that could leak into the gear box (thinning the oil) unless the 'wet' crankshaft seal has a MAJOR LEAK putting fuel in the gear box.
(Unlikely and the carb would have to be flooding like crazy! I don't see how it could run if that's happening).

The new piston/rings need time to wear in. The hard break in logic is a bad myth that's been going on for ever and is wrong in most all cases.
When race bikes are overhauled in between rounds you have no choice but to get back in the fight without a break in period.
But clearances (and parts) are such that it works..... usually. But it's not ideal IMHO. If possible do a few easy laps, then go for it.

Like Unkle Krusty said, do what the manual tells you to do.

The gear box has nothing to do with a top end/crankshaft rebuild. So changing the gear box oil is only needed when it's due for a change.
Of course if the wrong type of oil is in the gear box, change it.

If we have ALL the information about this bike (even stuff you don't think is important) we could be of more help.

Hey Unkle Krusty, were you guys racin TZ250's? I unintentionally retired before I got to run one of those. They look like a BLAST to ride!
 

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Hey Unkle Krusty, were you guys racin TZ250's? I unintentionally retired before I got to run one of those. They look like a BLAST to ride!
The TZ250 is a 76 to 80 model. Just before that we had the TD2, and earlier the TD1. The 1 and 2 had letters after the number as the year models changed.
In the beginning I rode a Bultaco 250 single, which handled better than the Yamahas, but was down on power to the TD1. I rode a TD1C, then an early TD2 and finally a TD2B. But it may have been a C or D. First of the 6 speed gearbox, last of the large drum brake and air cooling. The next model had a disc and liquid cooling. HP was about 45, bike weight around 220. The six speed was a huge advantage on tracks with slow corners. The liquid cooled engine made no more power at first, but it did not fade. the air cooled engines would drop a little power after a while. The disc brake was also an improvement.
The TZ250 was the beginning of the hp increases, and the 250 2 strokes were up over 80 in the end. With the light weight, these were the best bikes for the track, and everyone owned one. 20 or 30 bikes was common. Put on Steve Baker and Jim Dunn, and a bunch of other hot shoes, and the racing was very fast and competitive.
Yamaha in Canada imported them. Deeley who had been the Harley importer forever, also had Yamaha and Triumph BSA.
We would buy the bikes new or slightly used, run them for a year, and sell them to Washington racers. Some were easy to upgrade with the latest stuff.
Performance of an early TD2 would be similar to a current Moto3 bike.
At the IOM the TD2 would win the 350 class because of the way it handled.
The hairpin at the IOM would be almost impossible to get around without the 6 speed.

Unkle Krusty*
 

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Discussion Starter #7
Hi Semifast & Unkle Krusty

Thanks for replying.

The bike is liquid cooled. Im using Ipone coolant. But the bike does heat up very quickly after being riden. eg 10 mins of idling while in queue or 10 mins slow riding (20-30km/h) in traffic congestion & the needle is at the 3/4 point. But it cools down very quickly once we start cruising again. needle at abt 1/4 point. I live in Singapore, so hot weather & traffic congestion are the order of the day.

Ive been using Castrol Power 1 Racing 2t oil. Its pump feed. Previous owner was using premix, same oil. My 2t consumption is now at 1:50. But im intending to try Motul 710 2t for my next top up. People tell me ester based 2t is better then fully synthetics.

The most ive riden is between 7-8k revs on the highway (110-130km/h). At junctions & lights i will accelerate off in 6-7krevs in 1st gear.

Yes, probably thin oil is the accurate word. The oil was black. I was googling around for piston with black stains on the walls & came out with this thing called blow by. So could it be i have a small case of blow by since mine is a small stripe instead? If so, then maybe what the mech means by leak, is that the fuel mixture leaks out by the side thro the worn out lower ring??

If it is indeed blow by, then im not sure how it happened as its only a 1 yr old piston & neither me or the prev owner thrashes the bike around. But phaps this 1 occassion might have caused it. I had a problem with spooge so i had 1 mech tweak the pump to reduce my 2t intake. That time i checked, the 2t consumption was reduced to abt 1:70. Much less smokey too. Then there was a night where my riding buddy found an empty braod & straight road. There i really let fly to taste the full power of the bike. So going in & coming out of that road were 2 rounds of going tro the gears at 9-10k revs. Total time spent for that activity was abt a min or so. So dunno if the 1:70 which i thought was lean could have caused the ring wear.

I'll see if I can upload a few photos of my piston. Dunno if it wld be helpful to our discussion. If you need any furter info, just lest me know.
 

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The gear box oil is completely independent of the piston, cylinder and crankshaft in a two stroke engine.
Blow by is from combustion gases 'blowing past' the rings and can not contaminate the gear box oil. (A small amount of blow by is normal).
Possibly the clutch is making the gear box oil black? Or maybe it is just way way past due for a change. Are you sure the correct oil is in the gear box?
Two cycle engine oil does NOT belong in the gear box. You can buy oil made to use in the gear box in case that's what is happening here.
Good luck and Happy Motoring. :biggrin:
 

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Discussion Starter #9
The gear box oil is completely independent of the piston, cylinder and crankshaft in a two stroke engine.
Blow by is from combustion gases 'blowing past' the rings and can not contaminate the gear box oil. (A small amount of blow by is normal).
Possibly the clutch is making the gear box oil black? Or maybe it is just way way past due for a change. Are you sure the correct oil is in the gear box?
Two cycle engine oil does NOT belong in the gear box. You can buy oil made to use in the gear box in case that's what is happening here.
Good luck and Happy Motoring. :biggrin:
Oh dont worry. I was using Castrol power 1 4t 10w40 semi synthetic previously in my gearbox. But now using Castrol power 1 racing synthetic 4t 10w40.

Theres this oil, Motul Transoil Expert 10w40 transmission oil for 2 stroke bikes. So far ive seen only 1 service shop carrying that oil. Other shops just sell a big variety of 4t engine oil or 1-2 brands of gear oil 75w80. eg Maxima.
 
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