Top end service - Piston to replace?

[Skippa]

Ciao everybody,
I'm Luca from Italy and I ride an EC 250 my 2005.
I joint the forum as I've read lot of interesting and useful info. Thanks to administration and to you all to share. Despite several threads here on the forum about this topic, I didn't find an answer, so I try to ask you help. This is the first thread, please be patient. 
I've opened the engine to have a look to the piston/cylinder. It's the first time for me for this operation:
Piston is a Vertex "B", 66,34. Measuring it, I found 66,28 on the bottom, 66,19 on the top. I don't know how many hours it has, surely over 40, most probably around 100.
Cylinder bore seems to be 66,35. I measured several times both piston and cylinder as these numbers sound strange compared to the nominal ones, but I always found comparable values.
Do you think they are reasonable? Or measurement is not my job? 
Because now I don't know if piston should be changed or not. For the aspect I can post come pics end of the week. I can say you only that is blackened under the rings both on cold and hot sides, but finishing is still ok and cylinder appears in a good shape (still row present and no scratches).
To end, more in general:
1. On the manual I see that standard bore is 66,4 and it may be up to 66,9. Considering that I find pistons of only 3-4 dimensions, from 66,33 to 66,36 mm and suggested clearance is 0,05-0,06 mm? do I have to think that manual is wrong? Otherwise how?s it possible to have up to 0,5 mm of clearance??
2. Measure indicated on the piston, i.g. 66,34 means piston diameter? Does it mean that mine that?s 66,28 is so worn? And if my cylinder is only 66,35, which piston should I chose??

I?m a little bit confused. 

So thanks to whoever may help me and sorry for my english!!

[Skippa]

I add another info: my garage was quite cold when I measured. I suppose that nominal measures are taken in standard conditions, something nearby 20?C while when I measured, ambient was at approximately 7-8 degrees less, maybe lower too.
Thermal expansion of alluminum seems comparable with the delta of 50-60 microns I recorded.

Should it be?

[Zman]

Hello, I would measure the bore closer to 25C and contact the piston mfr for guidance. You may wish to also look at Wiseco and Woessner pistons for other options.

[JAD]

Skippa, I have also been researching this issue and I am following this thread with interest!

I have only inspected my piston via the exhaust port and it looked fine to me but I am sure I will need the same information as you in the not too distant future.

I was wondering if fitting just a new set of piston rings is a recommended interim procedure or if it is essential to fit both piston and rings at the same time?

ps - your English is excellent and considering the technical nature of the post it is perfectly understandable. Well done.

[JAD]

More to it than meets the eye... as usual! .pdf from the Wiseco site...

[Skippa]

Hei guys, thanks for your replies.
End of the week I'll repeat measures in a warmer place and I'll take some pics of the piston so that you may post your thoughts about replace...

I already bought the clutch discs being convinced mine were worn, but I later knew there's a mistake on the manual about min thickness, so no replace is needed at now! I don't wanna repeat with the piston too

An what about the max bore of 66,9 on the manual? Nobody has idea?

[Skippa]

Originally Posted by JAD

I was wondering if fitting just a new set of piston rings is a recommended interim procedure or if it is essential to fit both piston and rings at the same time?

I'm not a god of knowledge, as you already got , but I guess a ring change only is not needed on our engines unless after inspection and cleaning you find piston and cylinder ok, but worn/damaged rings (I think rarely).
Really I think that when you open the top end, it's the time for service and replace piston. So, after some hundred hours (if you're not Graham Jarvis!!).

I guess piston rings change is normally done on lower performance engines where pistons have usually a longer life.

[SS109]

If it's a cast piston, and at 100+ hours, that is a mandatory replace it situation IMO. It's not about the dimensions of the piston but the fatigue on the metal. Cast pistons like to disintegrate without warning once they reach their fatigue limit. I've known people to get way more hours on a cast piston but I won't take the risk of one coming apart and destroying the bottom end along with the cylinder. My rule is if the rings need to be replaced then I replace both when dealing with cast pistons.

Now, a forged piston on the other hand can easily be run 300 hours and only need the rings replaced 2 or 3 times during that span. Of course, putting in a new piston when the rings are worn to limit isn't a bad idea on a forged piston but not a necessity.

[JAD]

What type of piston do gas gas fit... Cast?
What is the advantage if they only last 100 hours?



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[SS109]

Originally Posted by JAD

What type of piston do gas gas fit... Cast?
What is the advantage if they only last 100 hours?

From the factory most manufacturers install cast pistons. Cast are cheaper and lighter weight than forged pistons.