While out riding my yz the bike just died and would,nt go again. it now has low compression and you can push the kick start down will this be a top end rebuild the bike was been riden hard a the time
Please rate the solution as a "FixYa" if able. Thanks!
Are you having the dealer bore the cylinder?
New piston and rings > sized to match the new bore?
You are scaring me because you didn't mention rings or cylinder bore.
When you get the piston and rings, ask the parts guy or one of the mechanics to put the rings on the piston for you. It only takes a minute so they are not likely to charge you for doing it AND if a ring breaks they are responsible to replace them. If you break a ring you get to be the one to pay.
millerboi AND bookerm > call the dealer and ask the cost of a cylinder bore, piston and rings, and a gasket set.
You can do the work yourself if you have a metric socket set and 15 minutes. Doing a top end job is just about as easy as filling the gas tank. Let me emphasize one thing > > BORE THE CYLINDER!
Okay, here's the deal. Several factors at play here.You want the rings to seat properly. Fine. Lets' just use the old cylinder walls with some very minor scrapes and probably some low spots in the cylinder walls due to uneven expansion and contraction of the cylinder walls.
You want to race. Fine. Just plan on losing because the other guy has better compression than you do.
You want to save money. Fine. Just ask yourself what caused the piston to disintegrate. Couldn't be wallowing around in a distorted cylinder bore could it?
You want to attract pretty girls. Fine.
Don't be cheap. Will the new piston fit in the old cylinder? Yes it will. Would you rather have a brand new top end or save the cost of the bore job and have more piston problems in the near future?
Now make me proud by doing the right thing.
No question is a dumb question. Questions show intelligent thinking by seeking answers.
Larger hole = larger piston and rings.
I am disappointed in the dealer not steering you in the right direction at the start but perhaps he thought you wanted to lessen out of pocket cash and was trying to help. Ask the dealer the cost of boring the cylinder then tell him to change the piston/rings order.
If he is a **** about it then go to a different shop
Any chance of making the rating better?
Well guys, I may have a bit of egg on face.I checked with a service buddy at Yamaha and he says the YZ bikes have a nickle plated bore. The nickle is twice as hard as steel. Cylinder bores are not generally done so if you need a new cylinder pay $380 for a new one. PistonS are $65 to $83. Rings are $10. Aftermarket gasket kit is $60 to $70.
After all that I have decided the high cost of $15 pistons I purchased yesterday ( back in the 1970s ), were really a bargain. Cylinders were $75 and a complete engine gasket set with "O" rings was an outrageous price of $17. Guess I have been away form the parts counter far too long.
So now let's revise and explain briefly what is going on inside the engine...
Remember I first asked if there was a hole in the piston? The hole is caused by Pre-ignition, also known as 'detonation' and 'engine knock'. Carbon builds up in the head and on top of the piston. Enough buildup and it begins to get hot spots (sort of like charcoal ), and pretty soon the red hot carbon ignites the gas earlier than the spark plug does. Hence the term 'Pre-ignition".
This is hard on the engine, especially pistons. In your case the damage wasn't a hole, but rather it was deterioration of the outer rim of the piston.
Okay, here's the RE-DEAL. Several factors at play here.You want the rings to seat properly. Fine. Lets' just use the old cylinder walls with some very minor scrapes and probably some low spots in the cylinder walls due to uneven expansion and contraction of the cylinder walls. GO AHEAD SINCE THE BETTER INFORMED DEALER SAID THE CYLINDER WAS STILL USABLE.
You want to race. Fine. Just plan on losing because the other guy has better compression than you do. IN HIS DREAMS!
You want to save money. Fine. Just ask yourself what caused the piston to disintegrate. Couldn't be wallowing around in a distorted cylinder bore could it? NOPE IT COULDN'T. IT WAS PRE-IGNITION LIKE I SAID IN THE FIRST PLACE.
You want to attract pretty girls. Fine. THIS STILL HOLDS TO BE CORRECT.
Don't be cheap. Will the new piston fit in the old cylinder? Yes it will. Would you rather have a brand new top end or save the cost of the bore job and have more piston problems in the near future?
Now make me proud by doing the right thing. AND THE RIGHT THING IS TO INSTALL A STOCK PISTON AND RINGS IN THE EXISTING CYLINDER.
I hope this doesn't cause too much of a hassle on your end. I really feel bad about not knowing of the nickle coating.
By the way, to rid the bike of the detonation all you need to do is use a stock plug and use premium gas. Pull the cylinder head to remove carbon deposits on the head and on the top of the piston. Do the de-carbonizing twice a year. You can reuse the head gasket. If you STILL have detonation check for air leaks in the intake manifold. Also get a spark plug one heat range colder than stock.
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hi tombones49 thanks for your quick response i will pull the cylinder head of tommorow and have a look, i can tell you though that i do have a spark thanks again for your rapid response
i am having the exact same problem with my bike. about how much will this cost to repair?
hi ya tombones 49 striped engine down last night and found that the piston is worn and started to crumble away on edges. Have ordered new piston kit so hopefully not to hard to install. cylinder head don,t seem to be damaged apart from some of piston stuck to the very top of head, this is very minimum, was told be dealer that could proberley polish this off myself thanks again
your now scaring me as new to this piston lark, my dealer ordered me new standard piston kit and told me to fit that said that would be ok
ok i hear what your saying but this is where i get lost (sorry) if i have it rebored do i need to buy a over sized piston kit or just a normal kit, sorry if this question seems dump
ok mate thanks for all your help i,ll let you know the outcome once i have spoken to my dealer
i think i,ve gave you top rating now mate
no worries mate, did,nt have chance to make it back to the dealers today, so will just pick up new piston kit on monday and fit it, and as for egg on your face mate, your,ve tought me alot in the last few days so thanks for all your advice
can you bore out an older yamaha 125 engine.... such as a 1971 yamaha 125??????
You are welcome. Get back to me after checking the top end.
Pull off the cylinder head and check the piston. Do you have a hole in the top of the piston? Run the piston down to the bottom of it's stroke. Look at the cylinder walls. Is the cylinder wall smooth or scarred up? Do you have spark?
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