Scooterotica

Hi Gents need a little advice here and am especially looking for the cause of this. It has a Mitaka piston what has happened is the expander ring which fits under the bottom piston has failed catastrophically and taken out the barrell, head and piston the engine was still running fine and i was about to fit it back in after a year off the road and decided to whip the head off and check everything aok only to find the expander ring has broken up and come out from under the piston ring in flakes and then scratched like a hip hop DJ along the entire length of the barrell and then on to the head and piston. Can any one shed any light on it.

I always left them out tbh mate.

Not sure I can help with this one mate, but someone on here might come up with a valid reason for the Expander ring failing ( Thats the wire zig zag ring, that sits under the bottom piston ring ) Ive never hear'd of one failing before.... Can you upload a picture as you might get some more feedback...

Still no picture ..... I'm waiting.. , but you can get fed up of waiting.... I really would like to see this, as Ive fitted Quite a few Jap pistons with these expander rings and never known one of these expander rings to fail. I would like to see the damage close up and the bits that came off it.....

i agree with tony..............
have never used them..............and so have never seen one fail...................

The question dosnt appear to be who uses them.. more what caused it to fail...and as this is a kit thats on a few scoots..... An answer to the question asked, might not help me and many others on this forum... but for anyone running this type of kit if they have the expander fitted at least they would be fore warned....this must be a £100 to £200 rebuild....maybe even more....

back in the 80,s........when jap pistons came into use on lambos etc............i was always advised to NOT use expander ring..................and so never have done...........
must admit..........i never fully understood why they were there in the first place/or why it was recomended to omit these..............but as i say ,i follower their advice...........presuming that i was benefitting from their previous mistakes...........as per leaving them in.
..........many modern/high performance pistons do not have these anyway...........so .....?
what are they for............?

sean brady scooters wrote:........many modern/high performance pistons do not have these anyway...........so .....?
what are they for............?

This won`t help as far as scooters are concerned ..but..in the late `60`s and `70`s the cars we could afford had done high mileages....they used to smoke a bit because of the oil they used and also were past there best performance ....so out the engine used to come and we reconditioned them ourselves....a decoke and grinding valves was in order as was the bearing shells....instead of a rebore if the bore was still round and of a good tolerance "Hepolite Powermax" pistons were purchased and fitted....the idea was a cheaper and easier option to a rebore and really did an excellent job of returning the engine to peak performance....the piston rings were specially made for this application and as you can see from my "crappy" drawing the top ring was stepped so to avoid hitting the part of the bore right at the top that was never worn....the second ring was tapered and had to go in the way i have shown ....the third ring was made up of three parts, two thin rings with the expander ring in between....on this third ring the piston had holes around to act as drain holes.....so in operation the top piston ring when at TDC did not hit the unworn part of the bore...the second ring collected any compression the top ring let by....and the third ring acted as a oil scraper, scraping the extra oil down the supposed worn bore....the holes behind the expander ring in the piston then collected the oil and it ran down into the sump....i used these conversion pistons in a number of engines but only standard ones 1500 Ford and Austin 1100 / Austin Mini 848 and not tuned engines.....they were not recommended for tuning only for the reconditioning of an engine..............Why they are used on a scooter and cannot even hazard a guess why as they are oil scraper rings which i would of thought detrimental to a two stroke...........

..156 D.. I know the type of rings your describing, the Four stroke oil scraper ring. its slightly similar but not the same as these expander rings. these sit underneath the bottom ring as an aid to bore sealing, owing to the bottom ring recieving a lower pressure to seal the ring to the bore. It acts as a kind of spring to expand the bottom ring against the bore allowing a larger air space under the piston ring which in theory allows the lower pressure acting on the bottom ring to react better allowing a better gas seal.. but the expander ring dosnt actually come into contact with the barrel... these were used on RD's and many other Two stroke engines dating back to the early seventies.. technology that was taken from their track bikes of the day...Ive fitted alot of pistons that use these expander rings, but I didnt usually put them in as quite often I was only fitting one ring anyway, The top one. The honda 205 conversion piston uses an expander ring, which I have fitted and the pistons run for many thousands of miles with no such problems....But no picture so still cant see whats actually happened to Cryogenics piston... might never find out ...

cryogenic wrote:Hi Gents need a little advice here and am especially looking for the cause of this. It has a Mitaka piston what has happened is the expander ring which fits under the bottom piston has failed catastrophically and taken out the barrell, head and piston the engine was still running fine and i was about to fit it back in after a year off the road and decided to whip the head off and check everything aok only to find the expander ring has broken up and come out from under the piston ring in flakes and then scratched like a hip hop DJ along the entire length of the barrell and then on to the head and piston. Can any one shed any light on it.

I'm a bit confused as if it had "failed catastrophically and taken out the barrell, head and piston" how could it still run fine Are you sure you hadn't had a previous seizure that led to this "catastrophic failure"?