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  #1  
Old 2 Weeks Ago
daddyrat daddyrat is offline
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Piston size

Is there a way to determine what piston size I need without taking anything apart on my rr390 2015? It's a little smokey and while I am at it replacing rings I might aswell do the piston. Thanks
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Old 2 Weeks Ago
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Research a cheap ($20) spark plug hole inspection camera that has it's own light/torch.
There should be the piston's size visible: As it's cast into the piston's crown; methinks?
You could & should then-also inspect the state of your bore whilst you are poking at it.

I Google'd it and the below is the first thing that came up:
https://www.amazon.com/Vividia-Flexi.../dp/B004WY9F7E

Edit: I didn't realise you're after the same piston; but a new one.
In which case Roon's helpful enlightening is likely your way to go.

Piston's do wear: They wear in their ring-groves and on their skirts.
Yet as Roon implied; a new/same piston is better: If the bore is OK.

What I'm saying is: Your existing piston may-well be serviceable.
If it's not... Then you'd likely have significant bore-wear as well.

Your "smokeyness" should be contemplated as-to what it is imo.
I'd guess you'd be replacing valve-guide-seals at the very least.

Naturally: You would wiggle the valves and inspect their seats.
Inlet-valve seals can suck/burn oil during vacuum/deceleration:
Typical if uneven valve-seat-wear has levered guide/wear/slop.
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Old 2 Weeks Ago
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There is a "1" or a "2" stamped on the RHS of the barrel and is used to determine the matching piston tolerance (order a "1" or "2" piston)
This is primarily used when new bikes are being assembled on the line, but during a rebuild it would be better to measure the bore and order the corresponding tolerance piston
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Old 2 Weeks Ago
Jallopy Jallopy is offline
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I replaced my piston and rings with the updated 2016 model kit.
I also replaced valve stem seals while it was apart.
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Old 2 Weeks Ago
daddyrat daddyrat is offline
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Thank you for all the replies. I wanted to minimise down time but have now decided, after all the advise to play it save, take it apart, measure and then order. I notes reckon that most likely it's valve seals. There is no smoke when the engine is cold and lots of it after hard acceleration, like short squirter of power. As in jumping over logs.
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Old 2 Weeks Ago
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Quote:
Originally Posted by daddyrat View Post
Thank you for all the replies. I wanted to minimise down time but have now decided, after all the advise to play it save, take it apart, measure and then order. I notes reckon that most likely it's valve seals. There is no smoke when the engine is cold and lots of it after hard acceleration, like short squirter of power. As in jumping over logs.
Valve seats don't create smoke, Valve stem seals can let oil in on long deccels then give a puff of smoke when you crack the throttle.

Lots of after after had acceleration is rings or can be from breather if it's in the airbox.
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daddyrat daddyrat is offline
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checked the breather hose and blew thru it both ways and it's clear. After the weekend the strip down will begin.
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Old 2 Weeks Ago
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Quote:
Originally Posted by daddyrat View Post
checked the breather hose and blew thru it both ways and it's clear. After the weekend the strip down will begin.
You're getting too anal about bore size and skirt wear DR. Where these short-skirt pistons mostly let go is at the back of the bottom ring-land. They then drop the skirt onto the crank.

Putting new rings on worn pistons gives the new inner square edge on the ring another go at cracking the pistons' ring land.

There's a lot of work in doing a 4T top end, there's too much at stake to scrimp on a piston. EVERY top end 4T failure I've seen on Pro level ridden bikes was a dropped skirt through ring-land cracking....
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Quote:
Originally Posted by Nutty View Post
You're getting too anal about bore size and skirt wear DR. Where these short-skirt pistons mostly let go is at the back of the bottom ring-land. They then drop the skirt onto the crank.



Putting new rings on worn pistons gives the new inner square edge on the ring another go at cracking the pistons' ring land.



There's a lot of work in doing a 4T top end, there's too much at stake to scrimp on a piston. EVERY top end 4T failure I've seen on Pro level ridden bikes was a dropped skirt through ring-land cracking....


This....




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  #10  
Old 2 Weeks Ago
daddyrat daddyrat is offline
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Quote:
Originally Posted by Nutty View Post
You're getting too anal about bore size and skirt wear DR...
Don't know about getting anal.
I don't mind swapping out the lot, that is piston and rings. It just seems that 130 or so hours is not all that much particularly at my sedate pace.
Anyhow. I just hope that the bore is not to worn and it's a straight swap. I'll check valve clearances, seals and guides and that should have everything covered.
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