I was curious on what is the acceptable piston to cylinder wall clearance and proper piston ring gap specs seated inside the cylinder. I recently had my cylinder bored and it seems to be tapered in the mid section around exaust port and reed intake port and I see quite a gap in the but end of my rings. I have a namura 66.44mm piston it's classified as a 66.5 And would like to know if I get a new cylinder what size would I have it bored to fit the 66.5 piston and have a good tight clearance
what is best piston to cylinder clearance and proper ring gap specs
- Thread starter 98mod
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Least wise with a Wiseco, the new piston comes with "instructions" , minimums ring end gap and clearance according to diameter.
"Generally" speaking cast piston uses .002"clearance, forged .003".
Recently saw something about ring end gap to where it didn't affect performance until it was huge.
How are you measuring? Who measured for initial new bore?
Also "generally speaking" a 66.5 bore is just that, and piston is undersized. That is why the new piston needs to be in hand so the last hair can be honed for perfect for.
Actually "a hair" is a misnomer, because the "average" human hair is .004" .
Hopefully this tidbit illustrates why clearance is critical, because at operating temperature the clearance is only about the thickness of the film of oil.
"Generally" speaking cast piston uses .002"clearance, forged .003".
Recently saw something about ring end gap to where it didn't affect performance until it was huge.
How are you measuring? Who measured for initial new bore?
Also "generally speaking" a 66.5 bore is just that, and piston is undersized. That is why the new piston needs to be in hand so the last hair can be honed for perfect for.
Actually "a hair" is a misnomer, because the "average" human hair is .004" .
Hopefully this tidbit illustrates why clearance is critical, because at operating temperature the clearance is only about the thickness of the film of oil.
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My question is...how can you see this?
The ring end gaps can't be seen when the piston is installed. How can you "see" a taper in the bore? This makes no sense.
Didn't the machinist check the bore for taper and roundness when he was fitting the piston?
He should have been using one of these\/ \/ \/
Why would you have to have a new piston and cylinder?
What are you considering a "good tight clearance"? The reason I ask this, is because piston manufacture's all vary in clearance recommendations.
A "good tight clearance" can also cause problems depending on piston brand and type.
When I bought the blaster it had a wiseco piston but it was scarred badly and it was cocking on the cylinder so I took the jug to a machine shop ordered a namura pisto. The era so for my decision was the wiseco had alot of play at Tdc and I leterally could see the piston rings and gap where the pins are locaed within the cylinder while installed. Now i gave the jug and piston to the machine shop he was supposed to line bore my jug the next day called and said it was read he informed me he doesent know why the new piston is tapered or why jug is tapered. So i install the piston and jug run it like a normal break in period now im hearing noises i know what a piston knock sounds like. I pull jug off and inspect at tdc when puston is at deck height good fit minimal clerance i move piston downward in cylinder and i see the rings ring gap where they butt up on the piston pin and the gap is huge this is hapening while the piston comes close to the exaust port area on both sides
Yes gave the jug and piston to him when I received both Bach it was a really really tight squeeze getting jugover top of piston and in but it went but when the piston is about half way up its really loose. For example with head off and using the kick start to slowly rotate piston looking inside at instate area I see the gap in rings and about half of the flat area of the ring sliding on cylinder and then edge of piston as it come up to top dead I see rings pinch shut but a gap the size of jupiter. I'm verry dissapointed in the bore job honestly it's not close to any spec excel the top of piston on deck and the piston skirt at bottom of stroke. Honestly if the cylinder wore enough when rings do seat I would not be surprised if a ring moves over the built in pin on the piston that's supposed to hold the in position. I wish I could post pics it would help alot.
that should have been when you told him to stop immedietly and you'd be down to pick up your stuff.........
to send to someone who does know what they're doing.
He didn't make me aware untill after the job and I have few options of machine shops locally next place is 60miles and it's a minimum of 100$ to set up the machine that's why I would like to know if I would be better off buying a new jug that a straight fresh new cylinder ?
The namura piston I ordered was 66.50 and when I recieved it its lazer etched 66.44mm on dome of piston what should I do because I don't want to have a disaster and have to buy new piston and jug. My fiancee actually bought the piston for a gift and hate to toss it aside. I'm already a a@@ hole in general but trying to turn over a new leaf. That's why I'm asking yall what would you do, new jug? If so what size do I need to request to fit the namura piston that's actually 66.44mm over?? .20bore to allow .00? Clearance from piston to cyl wall? I do agree with you on that note but the job was done plus he owns a mechanic shop I hope it will hold up but it still bugs me when it's not right like it's supposed to be. As for knowing a good tolerable clearance piston to cyl wall or proper ring gap I'm still without a answer. It does deffrance between manufacturers but what is a general spec of clearances possibly ?
http://www.blasterforum.com/threads/wiseco-piston-ring-instalation-gap-pdf.62068/
Cast pistons are a lil different.
Every piston needs to be bored to its specific size since every piston is a tad different in size. The shop doing the work should know this and should have the piston in hand before it side the maschine work as they should know that there are variables..
What does your cyl measure now currently?
Cast pistons are a lil different.
Every piston needs to be bored to its specific size since every piston is a tad different in size. The shop doing the work should know this and should have the piston in hand before it side the maschine work as they should know that there are variables..
What does your cyl measure now currently?
The piston clearance on a stock Yamaha blaster YFS200 with a stock cast piston from the factory is
right at .0015, or a little less. That's one thousandths and a half of an inch.
Actually, that's on the loose end of factory specs.They are matched up very, very close. A cast piston is supposed to be set up with close tolerances, and can be set up with close tolerances, because a cast piston stays in shape when heated. A forged piston needs more clearance for expansion because a forged piston is slammed into shape when manufactured, thus the aluminum is compressed into the shape of the piston, then machined to final dimensions. When a forged piston is heated (engine warming up) it grows larger in size by a small amount, thus needing extra room (piston to cylinder clearance) to grow or "expand". Usually a forged piston like wiseco is set up at around .0035 which is three and a half thousandths of an inch. This is very important. Wiseco recommends that clearance on the Yamaha blaster engine be set at .0025, which is actually, in my opinion too tight. That extra .001 helps a lot!
The extra clearance is necessary with a forged piston. This is why, if you don't warm it up properly, you can get a four corner seizure with a forged piston. When rapidly heated (no warm up time) a forged piston's expansion is all over the map. Areas of the piston (thinner areas) expand faster than the thicker areas causing a four corner seizure. This can also be caused by too little piston to cylinder clearance if the engine gets hot.
Hope this helps.
right at .0015, or a little less. That's one thousandths and a half of an inch.
Actually, that's on the loose end of factory specs.They are matched up very, very close. A cast piston is supposed to be set up with close tolerances, and can be set up with close tolerances, because a cast piston stays in shape when heated. A forged piston needs more clearance for expansion because a forged piston is slammed into shape when manufactured, thus the aluminum is compressed into the shape of the piston, then machined to final dimensions. When a forged piston is heated (engine warming up) it grows larger in size by a small amount, thus needing extra room (piston to cylinder clearance) to grow or "expand". Usually a forged piston like wiseco is set up at around .0035 which is three and a half thousandths of an inch. This is very important. Wiseco recommends that clearance on the Yamaha blaster engine be set at .0025, which is actually, in my opinion too tight. That extra .001 helps a lot!
The extra clearance is necessary with a forged piston. This is why, if you don't warm it up properly, you can get a four corner seizure with a forged piston. When rapidly heated (no warm up time) a forged piston's expansion is all over the map. Areas of the piston (thinner areas) expand faster than the thicker areas causing a four corner seizure. This can also be caused by too little piston to cylinder clearance if the engine gets hot.
Hope this helps.
Couple things to do.
First, start a thread in "Product Review" and out Ben Dover Repair. Or at least tell us in this thread who this is.
Demand a refund of at least cost of bore job. I don't know a lot about machining, but unless there was something wrong with the machine or set up,, they run straight.
Could try to sell piston on here to recover some funds
Generally speaking tolerances are so close that it's hard to get a piston in without oil on it.
On a side note, that's only second oversize, many oversizes left.
Who originally measured to determine proper overbore size??
First, start a thread in "Product Review" and out Ben Dover Repair. Or at least tell us in this thread who this is.
Demand a refund of at least cost of bore job. I don't know a lot about machining, but unless there was something wrong with the machine or set up,, they run straight.
Could try to sell piston on here to recover some funds
Generally speaking tolerances are so close that it's hard to get a piston in without oil on it.
On a side note, that's only second oversize, many oversizes left.
Who originally measured to determine proper overbore size??
Good explanation on the heating process. I'd like to add that the cyl also has to warm and expand.
My warm up procedure is when the cyl is uncomfortable to lay my hand on, then it's ready.
My warm up procedure is when the cyl is uncomfortable to lay my hand on, then it's ready.
Do you start it and let it idle up to temp. or do you keep revving it like in Kens video where he shows how to do heat cycles?
I blip the throttle a bit , but nothing crazy rpm. Just enough to prevent excessive amount of oil on plug.
I'd have to actually watch the video , but during break in you don't want to keep it at a constant RPM.
Even heat cycles are nothing more than warming up the engine. And no matter what I never get crazy on free reving an engine. 7,000 RPM with a load on is no where near as hard on an engine as 7 grand in neutral.
