Piston to bore clearance- does it matter?

[tony.mon]

I threw an engine together last year after lots of failed welding didn't sort out a crack in the cases.
So I bought a set of cases, but the biores were a bit worn.
Ideally it needed a rebore, and oversize pistons, but I already had the standard sized JE high compression ones, so I popped on some new rings and took the chance.

It lasted round Devon and Wales forum meets, but as I'd moved the engine breathers out of the airbox and they just ran to atmosphere through a cone filter, I could see that there was a fair bit of smoke.
Now it could have been rings, it could have been valve stem seals, or something else.
It wasn't running well at the end- during the Wales run I ran it alongside another Storm and couldn't get my bike to out-accelerate his, which didn't have much tuning- just a system and jetting.

So I knew something was wrong, and when I broke the bike I left the engine sitting around, I thought I'd inspect it when I had a chance.
Well, I had a chance.

This was what I found:



But what really surprised me was the cracked piston:


Now that wouldn't have lasted very much longer....

It's worth mentioning that I have been trying out the "new" idea going around of not bothering to run engines in- just running them hard from the off.
I haven't had a problem up to now, but from the scarred skirts and damaged inter-ring lands I may reconsider.

[popkat]

Looks more like damage from a failing oil pump. with too big piston to bore you'd expect to use more oil and have some low compression.

wasn't Al when he rode it in Devon



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[lloydie]

That has seen some punishment for sure !

[Stephan]

That is why I started with checking cylinders are in tolerance. They were not, so I couldn't use std JE or Moriwaki pistons, and did fresh bore with os pistons. Expensive, but I believe I did it only once.

Seems like there was too much freeplay for piston and it was hitting the wall of cylinder.

[alanfjones1411]

Piston broke.



Well some one had to say it

[countrymick]

pissed off and definitly broke if he is rebuilding that.thats the worst piston i have seen in an engine that was still running

[bigtwinthing]

In the Very large Jenbachers i used to rebuild after a major rebuild i.e., pistons, liners, rings, gudgeon pins,shells and mains,liners turbo,intercooler and oil pump c/w oil pressure relief valves, they were run at 1000revs for 6 mins. Then at 1500 revs for 10 mins. Then we went up on load at 10 mins per KW until half load. At half load it was run for 20 mins then 3.4 load for 10 mins then Flat out for the next 2 years, apart from oil changes 2 monthly. If you didn't run them in like that, they last 1 day and fecking disintegrated.

i still to this day run my bike up until its a bit warm (sat upright) before i ride off. Oils like the bikes blood. it needs to go round first.

Bikes now have coated liners but skirts need oil and so do the ends.

[sirch345]

I bet that was not what you were expecting to see Tony

Good man for taking a look and posting up your findings,

Chris.

[KermitLeFrog]

When I used to build racing 2 strokes the piston to bore clearance was the most important thing. Then we did something very similar to BTT. Warm it up on low revs and let it cool. Really cool. Then do that a couple of times at slightly higher revs letting it cool completely between runs. Then....... rev the feck off it on the track! Never had a seizure.

[fabiostar]

much the same here with the strokers. after the second heat cycle i used to pop the barrels off and emry any high spots of the piston skirts then she was good to go.. 4Ts i would always give a good few miles cause theres a lot of moving parts to get bedded in.

[Stephan]

not sure if hard break in can cause this, maybe if engine didn´t reach operating temperature, but who knows.

Just for interest, this was my break in schedule I used, engine is now running at 3500 km without noticeable drop in performance and I hope it will last. I also did one track day after 500 km.

[KermitLeFrog]

"Don't run steady rpm". Very important. I once did just that to run an engine in. After the post run-in inspection I just threw the pistons away, they were totally shagged.

[AMCQ46]

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wasn't Al when he rode it in Devon



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Shhhhhh

[Sir-knob-head]

Holy wombles!!!!
Good to see em mate lol