Tomos blew up, help me figure out where I went wrong

I was riding pretty hard the other day when I heard a grinding noise. I pulled over and the engine wound down and died as I came to a stop, wasn't able to restart so I walked it home, pulled off the head and found this disaster.

My current working theory is that one of the lower stud bolts came loose and caused an air leak. Thought I'd post the pic up here though to see if it was something else so I don't make the same mistake when I rebuild, anyone have any thoughts?

Was running an A35 45mm 70cc alukit with a matching high compression head and a Malossi 4-petal reed intake with a 19mm PHBG and a Homoet exhaust. Planning to rebuild the same.

2 sub questions:

I haven't pulled the cylinder off yet to survey any damage to the sides of the piston but should I worry about any of those chunks from the piston having made their way into the crankcase or are they likely somewhere in my exhaust or on the street?

The head kind of looks like the top of the piston, all pitted and rough, but with no chunks missing. Should I replace the head with new one or am I alright to keep running it? Treats is out of new high-heads but I've got the old stock head laying around if that would work as well.

Thanks

tomosPiston.jpg

Re: Tomos blew up, help me figure out where I went wrong

Move the piston down in the bore and then take the picture. Without being able to see smearing or scoring it looks purely like mechanical failure which is either due to detonation or purely bad material. If it's detonation then I would check your timing, the burn looks good and rich for oil.

Replace the head. You won't sand the pits out and even if you did you would have to deck the head down to make up for the increased volume from the material removal.

Re: Tomos blew up, help me figure out where I went wrong

Overpriced Parts /

Seized from poor jetting/timing/tuning/and no head temp gauge.

Re: Tomos blew up, help me figure out where I went wrong

Thanks for confirming what I thought - that I'm pretty dumb and should have tuned better.

All the parts are in and I took the engine off last night to start the rebuild. I'll post some pics of the cylinder when I remove it but I'm pretty sure there wasn't any scoring on the cylinder, the exhaust port was a little ragged on the edges though.

Gonna check the timing and be less lax about tuning when I get it back together. I have a temp gauge on my hobbit that I can swap between bikes to hopefully avoid this in the future.

Re: Tomos blew up, help me figure out where I went wrong

kinda rare to chip the piston unless a ring caught. maybs your piston was upside down and caught ring

maybs bad piston size, like too small & knocking

that fracture pattern implies catching something below the piston so i dunno. maybe just seized hard enough on the bottom to tear the piston crown off. def not some debris catching a port, the fracture cone would be the other way

Re: Tomos blew up, help me figure out where I went wrong

Finally tore down the engine. Pics are attached of the piston, engine case, cylinder, and exhaust port.

The front ring was completely gone, the rear ring was jammed flush to the bottom of the piston, looks like the exhaust port was a bit chewed up, but I didn't see anything wrong with the engine case or crank.

Would a failure of the first ring cause the piston to fracture, maybe due to 4 stroking or detonation?

The bike ran for a good 4 years and, according to the odometer, over 1600 miles. I was lazy about the tuning and it 4 stroked a bit in the mid range but I'd always throttle down or up out of it, never let it go for more than a second or two, maybe it just eventually added up to kablooey?

piston.jpg
cylinder-and-exhaust.jpg
engine-cases.jpg

Re: Tomos blew up, help me figure out where I went wrong

Is that the exhaust side of the piston that is so chewed up and the port that is chipped also exhaust port? Is there any evidence of the lower ring being tack welded on either side of that port. Like a little plob of metal or a divot in the cylinder wall. I am no expert but people talk about the ring bulging into the port and getting caught , that phenomenon determines the maximum width of a port when modding. I am thinking , naively or not , that some think like that happened to the lower ring and it got caught there

Re: Tomos blew up, help me figure out where I went wrong

Yep, that's the exhaust side of the piston and the exhaust port that was chewed up. I'll take a look at the cylinder when I get to home to see if I can find any remnants of the ring.

I didn't do any porting on the cylinder or engine but maybe age and abuse just finally caught up.

Re: Tomos blew up, help me figure out where I went wrong

Chamfer port window edges. Set ring gaps. Monitor temps.

Re: Tomos blew up, help me figure out where I went wrong

#CrazyWayne™ rocks. #CrazyWayne /

Boyd - Wrote:

-------------------------------------------------------

> Chamfer port window edges. Set ring gaps. Monitor

> temps.

We have a winner!!!

Your prize.

Cocaine not included.

Re: Tomos blew up, help me figure out where I went wrong

For sure if you can fix that port and chamfer it all out and then give the cylinder a good ball hone then with a new piston, rings and clips you should be good to go unless there is metal down in the case. There is a lot of metal off the port and piston. Hopefully the experts can advise on the likelihood the case is contaminated.

Major pain in the ass to teardown a tomos.

Re: Tomos blew up, help me figure out where I went wrong

It took 6,066 posts but I finally got cryptically flamed by crazy wayne! /MA

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