Hello guys and gals, I apologize in advance for the long rambling question.

 I've built and restored vintage/ modern scooters and mopeds since I was a kid, I finally decided to take on a real motorcycle project, so I picked up an rt360 project of my own. Was a sore dick deal at 200 canadian, looked pretty complete and rust free and had "compression". 

 I finally get the bike home and get it on the bench. Hook up the compression gauge, 60psi, not good. 
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peek through the exhaust port, cracked ring with a piece missing... really not good. 
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 I pulled the head off and inspected the cylinder walls, To my surprise not scored all to hell. Pulled the jug to inspect the piston, and judging by the carbon build up, once that ring gave in it pinched the other closed and never ran again. 

I'm looking for some insight on what my next move should be. The piston has some damage from the ring caving in, but that being said with a bit of clean up the rings can move freely in their grooves again. 

I'm sure it's an dummy question but is it safe to just throw a new set of .25 os rings on it and re use the piston? Or am I stuck with the dreaded piston hunt? 

If I get a new piston would it be fine with a fresh hone and the same .25 os piston or do I need to source a .50 and bore the jug? 


Finally, has anyone had success with machining holes in a rt1 piston skirt for an rt3 motor? It's a cheaper way for me to go at this point. I'm looking at 400 Canadian for a NOS piston at the end of the day. I'm a trade student with lots of skills and limited budget so an affordable outlook is what I'm shooting at this project with.

Thanks for your time!
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 from the UK.

Forum member/moderator Enduronut is our usual first port of call for pistons and rings.

Partzilla in the US and CMSNL in Europe have been known to stock the occasional piston and rings as well.

Hmm, i hate spending money but i'd give it a hone & see how it looks, but feel it'll be a bore job in the end. RT1 piston will work even without holes but not sure how much difference it would make but no harm trying. Main thing is if the ring pegs end up missing the boost port & send ring stays above inlet port as think its lower on the piston then the one you're showing. I did run one on mine but cut an ugly big hole high up starting just under the second ring & going down, as thought stronger for the skirt. Did go ok for sure but wished i'd done two smaller holes. Was hoping i couldn't find my pics easily but here they are--so you can have a laugh. Worked great but think 'that' time i was running a 250 crank experimenting.Hmm, i hate spending money but i'd give it a hone & see how it looks, but feel it'll be a bore job in the end. RT1 piston will work even without holes but not sure how much difference it would make but no harm trying. Main thing is if the ring pegs end up missing the boost port & send ring stays above inlet port as think its lower on the piston then the one you're showing. I did run one on mine but cut an ugly big hole high up starting just under the second ring & going down, as thought stronger for the skirt. Did go ok for sure but wished i'd done two smaller holes. Was hoping i couldn't find my pics easily but here they are--so you can have a laugh. Worked great but think 'that' time i was running a 250 crank experimenting.

Cutting holes in the skirt of a cast piston is very likely to lead to cracking and catastrophic failure. 

Holes are not required.  Some good running reed engines don't have them.

But contact Enduronut and get the right piston...  determining factor is going to be piston to bore clearance....  and then best to get an experienced machinist to measure and inspect as often it might need more than one oversize to clean up.  

If it were me on a budget I'd clean it up for now and run it with just the top ring and pretend you have an MX bike with a single ring piston.    Been there, done that. Not optimal but ran okay until I could afford to get it fixed right.. 

Thank you all for the wise words! I'll definitely need to get in touch with Mr. Enduronut and get a new piston sorted out.

MarkT that's a killer idea! I've never even considered running it on just the one ring,  and I've been playing with the idea of running a mx piston to get me by. I get that it's definitely not the right way but that's the kind of outside thinking im looking for. 

hopefully it will get it running enough so I can get this ol girl down to inspection to get re-vined and back on the road where it belongs. 

Rt325 has probably forgotten more than most of us ever knew...  Note that his homemade "window" is above the skirt.  I'm guessing he knew that cutting holes down in the skirt of a cast piston often leads to the skirt cracking and breaking off resulting in major engine damage.. 

I learned that the hard way many years ago after adding reed valves to an old piston port with a friend and then putting windows in the piston skirt. 

Second attempt worked pretty good but used forged pistons after an experienced engine machinist explained why you shouldn't cut holes in a cast piston skirt.

Silly thing was the piston port was actually a little faster...  reed valves maybe made it run better at low-mid rpm.  He had a bunch of old little go karts with Kawasaki engines his Dad bought someplace cheap.  We got a couple of them running and used the rest for parts...  we never did find any new original  parts for them...  forged pistons were sourced out of a catalog by machinist from measurements.  Reed valve came from a motorcycle salvage and his Dad welded up the manifold area to take them.  Lots of filing by hand.  I think they were little snowmobile engines.  Still amazed at how we managed to get and keep those things running with almost no tools, parts, or manuals.  Just dumb luck mostly I guess.  Those were fun times.

His Dad used to say something like "someday you'll have done so much with so little for so long you'll be qualified to repair anything with nothing"

MarkT you're coming out with some great stories & i'm sure there's lots more where that came from. Yep my Big hole in the skirt--just thought anything low down on a skirt not reinforced for it is asking for trouble. Just put a felt pen down the inlet port & drew the shape on the piston & started drilling around the edge, then thought--too big by far but can't put it back now. Worked great but who knows for how long. As i said, had a 250 crank in then so wasn't getting worked as hard.

Holy cow I had a small breakthrough/facepalm!  

So like with everything I work on, it absorbs my life, and I was laying in bed 4am thinking about the build and flipping through the forums. As I scrolled past a pic of someone's rt with the jug off and piston exposed, and it all clicked.


The guy who built this put the dang piston in backwards! And that ring sheared on the exhaust port. I'm assuming judging by the piston and cylinder walls lack of damage, it probably happened on the first strokes after an os piston and bore and spat the chunk of ring into the exhaust. 
 I've mainly built 4 strokes so I didn't even clue in when I was taking the top end off. Definitely a learning moment and leads to more questions

Would this engine ever even run with with the piston backwards and the exhaust venting the crank case? 

Could it cause any damage inside the crank case? 

 

It will run.  I've bought bikes with piston in backwards that were running...  not good, but they'd run.

Probably no damage unless debris got in there. 

Someplace I have a picture of a cylinder that really got destroyed by piston in backwards.  Guy bought a new cylinder (modern chrome bore water cooled two stroke) and took it to a professional mechanic who put the piston in backwards. 

The "pro" said that he's been building engines for years and the arrow on the piston always points towards the intake. 

Guy was out over $500 just in parts and mechanic wouldn't take any responsibility.  I sent him some documentation on piston installation from several manuals so hopefully he was able to take the shop to court and get some money back at least. 

 

Probably not the cause in this case as i'd think the chances of damage to the top L shape ring would be greater than the second ring, but good thinking none the less.