exhaust port

hello, could anyone tell me the implications of the exhaust port not being fully closed at tdc....

nobby1 wrote:

hello, could anyone tell me the implications of the exhaust port not being fully closed at tdc....

All the fuel/air will leak out and it won't go.

well the bike ran.... but i get the feeling that the heat of the exhaust fumes getting into the crankcase dried out the lubrication around the big end bearing causing it to fail... teach me a lesson to take any notice of the arrow on the piston. had been running it backwards to help stop the ring breaking and as the skirt has been modified to increase the inlet port opening time its slightly shorter than the exhaust port side..... am i right ?

oops... you are correct!!
Alan

cheers greenbat, alan, its taken a while but im glad ive finally nailed that problem. hopefully now the new conrod bigend bearing wont end up the same......

I did something similar after trying to be clever fitting an RM125 54mm piston in a longstroke 125. It would not rev over 4000 and produced little power. I had the bike up on the bench and expansion chamber off, scratching my head; when I noticed the bottom of the exhaust port uncovered at TDC, durrrr ! Used a RG250 piston with a longer skirt and the engine has run well since.

I too still keep forgetting my Grandfather(BSA Pattern Maker)s advise: "check it , measure it and check it again" Its Murphy's Law that if you modify a standard Bantam part the thing next to it is affected and does not fit or work right !

The engine Im currently re-building runs uncovering the btm of the exhaust port at TDC, This was initially an accident, in the quest for more revs as we raised the barrel, the only issue it created was a little more difficult to start when cold, no other adverse effects. we were running a reed valve into the inlet port. it only became a big problem when a whole was put in the pistion inlet side to feed the rear boost port, well thats what I believed it did at the time.

I believe one of the tanks (MOD) used this arrangment in a diesel two stroke ! it never used the crankcases for scavenging though, some form of blower was used. ?

Derek