Piston ring end gap EC300

[Davehuge]

My replacement piston kit has arrived, wanted to check the piston ring end gap before I fit it.

I've measured it and it's 0.25mm, is that OK, should I go ahead and fit it? Or should I increase it?

What's the correct technique/procedure to increase it?

Don't think it makes any difference but my barrel is A size and I'm replacing B size piston with another B size piston, 71.94mm

Thanks, Dave.

 

[Zman]

http://www.wiseco.com/PDFs/Manuals/RingEndGap.pdf

 

[Davehuge]

Thanks for that Zman...

Out of the box my rings were at 0.25mm.

Wiseco's formula comes out at just under 0.3mm.

GasGas Workshop manual specifies minimum 0.5mm.

So based on this, I decided to split the difference and set mine at 0.4mm.

I was quite shocked when I removed my barrel and found the old ring end gap was 2.0mm.

 

[Zman]

2mm is HUGE!!! How does the piston look and how bad did it rattle?

 

[Davehuge]

Yes, 2mm is pretty big isn't it!

When I purchased the bike it was seriously over jetted, very rich, wouldn't rev clean at all. The guy I bought it from said he was selling it to buy a 4 stroke 250 because it wasn't powerful enough for him....hmmm I wonder why!

Its a 2004, I bought the bike without knowing anything about it's history, after sorting out the jetting I ran it for a short while and was quite impressed with the power. To be honest the engine didn't sound too bad.

I decided it would be a good idea to replace the piston kit, so I knew where I was.

As I said, the rings were very worn with 2mm ring end gap, even without measuring them you could spot the worn/narrow parts.

The piston looked OK, apart from a lot of dark scorches down the sides, blown past the rings.

Thankfully, the cylinder plating's in good condition and the crank main bearings and big end feel fine.

I'm also replacing the little end. I've completely stripped the power valve and given it a thorough clean. I'll be looking at the power valve mechanism/linkages as I think there's a lot of slop in the system.

Looking forward to trying it out!

 

[(F5)]

Yeah they often come too tight. Carefully filed in vice with fine file- tip just showing, soft ally jaws.
My original 300 (Vertex?) piston had huge gap. My 200 Wiseco came out barely outside brand new spec