My Piston Ring is Spinning


New Member
May 15, 2008
Hello again. I hope that I'm using-up too much airtime, here.

Just installed a new engine, and it works great "out of the box".

I took apart the old "65/50" ThatsDax engine (now an orphaned model, per Duane), and discovered that the locating pin (top ring), has vanished. I investigated the nature of this problem in a referednce entitled "2 Stroke Tuners Handbook" (Turner), posted on this site. It states that this is a common issue with 2 stroke engines, and is happens when the ring is brutally forced back into its groove (frequently) after snagging on the exhaust port. Turner also states that eventual piston seizure may occur when the ring snags hard on the exhaust port.

I do not really care for this to happen to me, while I'm doing 25MPH down the road. Being that the "65/50" cc engine is no longer offered, ThatsDax no longer sells the piston anymore. However, I can still purchase the rings.

Would it be feasable to insert a new, more durable locating pin on the old piston? I don't want to do this unless it will be \a solid fix. I also don't want to have a machine shop do it, either, unless they would do it for a few bucks.

I would hate to lose the top half of this old engine, with very few hours (supposedly per prior owner). There is a jagged scratch right above where I think the loose ring meets TDC on the cylinder wall, on an otherwise perfect mirror-like bucket. I'll bet that this scratch was caused by dislodged locator pin, immediately before it was expelled through the exhaust port.

I have not measured the cylinder for wear, yet, as I have not invested in the guages to do such. There are no scratches at all on the piston, and the rings appear to have very little wear on them (from just "eyeballing" them). I don't want to buy too much equipment if I'm only going to be doing one or two rebuilds for myself each year. But on the other hand, I don't want to keep purchasing blocks, if they are simple to repair.

I feel that I pick-up on mechanical concepts rather quickly, and think that I would be able to do these repairs myself with a little research and investigation.

Should I just write-off the top half of this block, or should I get a few gauges or whatever else would be neccesary (to do rebuilds) without buying a full-blown set of mechanic's tools? I realize that I'll have to purchase a new set of rings from ThatsDax. I could not get the old rings off completely by hand. Will I have to buy a ring spreader for this, or are people doing it fine by hand alone? When/if I replacae the rings, how I do I ensure that I replace them properly, without damaging anything? Again, I know that I'll need to install at least one new locator pin, probably both (the second one didn't look too great, either).

Is there a good reference for doing 2-stroke rebuilds that anyone personally recommends? The Turner reference appears to present a somewhat good theoretical foundation for port tuning, but does not address hands-on rebuilding at all.

I was able to get the old engine back together by hand, but I'll have to re-gasket everything after the post replacement ring clearance is adressed.

People hear have been very helpful, and thanks for all of your input.



Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
up north now


New Member
Jun 23, 2008
Longmont, CO
A couple of notes; these engines use a nikasil type liner and can be bored/re-bored or honed. Once they are done, they are done.
I think you mean, "can't," right?

If the piston looks ok, I'd just get new rings and pins and call it a day. I wouldn't worry about measuring wear on the cylinder and all that. Once it's worn out enough to matter, new jugs are cheap enough anyway.