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Post Info TOPIC: Piston ring positions


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Piston ring positions
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Hi there, has anyone noticed during a rebuild that there top two piston rings are positioned closer together than expected?

 

I noticed this in both the 325 and 350 I split recently but 100% know the 350 was spot on with spacings some 200 miles ago? The 325 had had more miles on and were closely aligned.

 

To be clear I checked that end gaps on the 350 too, they were fine.

 

Has anyone else noticed this or have an explanation? 

 

Thanks



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Daniel Lee



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This is Wiseco's interpretation of where the gaps are on their pistons.

The Wiseco diagram is unhelpful in that it shows the front of the engine to the left of the diagram (I assume for a car engine?) whereas for our engines I assume the front is at the top of the diagram?

Wiseco piston ring gaps.jpg

 

Yamaha's diagram differs! This is copied from their service manual:

Yamaha piston ring gaps.jpg



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That's weird that on the first image the piston centreline is in line with the front of the engine!

Both of the top two piston rings were were pointing to the rear but about 45 degrees from each other when the barrel was removed, I'm not surprised the 325 and 350 were burning a bit of oil!

I followed the aftermarket guidance for the 325 and 350 rebuild but having the 350 apart after 200 miles and seeing where the gaps moved to has me a bit concerned



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Daniel Lee



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dlee22 wrote:

Both of the top two piston rings were were pointing to the rear but about 45 degrees from each other when the barrel was removed, I'm not surprised the 325 and 350 were burning a bit of oil!


Isn't that exactly where the Yamaha diagram suggests the gaps should be Dan? 

Was there evidence of blowby marks on the piston to show where the problem lay?



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The Wiseco diagram will be a generic one based on car engines, hence the front being in line with the gudgeon pin, as you suggest Brian. The pin axis is more important than where the "front" is.
No explanation as to why they migrate though, sorry!

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Not 45 degrees apart though, it's strange.

I spoke to a friend who regularly performance engines (Cars sadly) but he said he quite often finds pistons like that... Weird

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