Do I need to replace this piston? (I've read the threads, just want to be sure)

[NewToStihl]

I have an engine from a Stihl 029 Super that I plan to use for an MS290 rebuild. I'm still new at this stuff and did a bunch of searching around the forum to learn what I need to learn...

1) The cylinder is as smooth as can be, so I think I'm good there.

2) The ring end gap is .020" on the top ring & right around 0.018"-0.019" on the bottom ring. I've read that this needs to be in the .005"-.010" range, so I need new rings. It seems rings from Stihl will have the typically high cost, so it sounds like Caber is the way to go in terms quality aftermarket. Sound about right?

3) My primary concern is around the piston itself. I've attached two photos. You'll see that it still has the machining lines on the exhaust side which tells me there is no notable wear. The intake side is a different story. No scoring or damage, but certainly has worn over time. I found varying opinions on the threads I sorted through and wanted to get some feedback. This rebuilt saw will not be for professional use, just homeowner firewood cutting - I'm rebuilding it to sell to a friend who wants a "cheap 290" in his words. That said, it don't need 180+ compression and top end performance, just a good reliably running MS290. Am I crazy if I only put new rings on and keep the piston?

Thanks for the help - any feedback is appreciated!

 

[Scooterbum]

Run it.

 

[blsnelling]

Measure the piston to cylinder clearance with a feeler gauge. That'll give you a good idea.

 

[rob066]

Measure the piston to cylinder clearance with a feeler gauge. That'll give you a good idea.

I would shoot for about .006 or so max clearance

 

[NewToStihl]

Thanks guys - I'll check this shortly & report back....

 

[NewToStihl]

Okay, my feeler gauges are too wide to get a good gap measurement with the piston in the cylinder due to the curvature, so I measured the cylinder diameter versus the piston diameter.

The difference in diameter (inside cylinder vs. outside piston) is .004". This is measuring the piston near the top, specifically just under the lower ring. That said the piston seems to be a slightly larger diameter at the skirt - is that normal? If I measure the piston diameter near the bottom of the skirt the difference in diameter compared to the cylinder is then drops to just under .002".

Based on what you've told me above it sounds like it would be fine to keep the piston, correct?

 

[blsnelling]

You need to measure the piston at the very bottom of the skirt. It's tapered.

 

[T0RN4D0]

Run it.

+1

 

[kmcinms]

like blsnelling said, measure near the bottom of the skirt, it is tapered. I'd run the mess out of that one.

 

[NewToStihl]

That's great news! I was hoping I wouldn't need to sink money into a new piston. Thanks for the help guys - looks like I'll be getting some rings on order shortly and have it ready to run.