scored piston - does it matter?
02GF74 - 30/7/08 at 09:52 AM
not my engine but got me thinking.
Does it matter if the piston is scored?
my understanding of the inner workings of internal combustion engine is that is that the rings, as well as sealing the cylinder, hold the piston in
place so that the piston does not contact the bores. I guess that may not be correct otherwise there would be no need to have the piton below the
bottom ring (ooh err!) to the same diameter.
So if the piston is scored, how does it affect the engine?
this is to what I refer: (BSA bantam)
and just for comparison, look how short a modern motorcycle engine piston is!
and this 
which also shows how little piston there is on the sides.
Mr Whippy - 30/7/08 at 09:59 AM
the piston does touch the walls as it changes direction, look at any used one and you will see the buffed area on the skirt. That one shown is scrap.
r1_pete - 30/7/08 at 10:04 AM
That piston looks quite servicable, the bantam isn't a performance two stroke, but it is a piston ported two stroke, so the fit between the
piston skirt and bore is important. So as long as there isn't a massive clearence it'll be ok.
02GF74 - 30/7/08 at 10:07 AM
quote:
Originally posted by Mr Whippy
the piston does touch the walls as it changes direction, look at any used one and you will see the buffed area on the skirt.
hgmmm, thought that may be the case - need to look at some pistons I removed earlier......
BenB - 30/7/08 at 10:18 AM
It doesn't look very pretty (and I'd be tempted to try and find out how it happened ? running without an air filter) but it's still the
rings doing most of the sealing. What does the bore look like- that'll be more of the problem IE if the bore looks as nasty as the edge of the
piston you're in trouble...
Mr Whippy - 30/7/08 at 10:28 AM
quote:
Originally posted by BenB
It doesn't look very pretty (and I'd be tempted to try and find out how it happened ? running without an air filter)
yip been sucking in plenty of dirt, I see this kind of damage all the time on my r/c 2 stroke model plane engines in the summer due to dust in the
air. It can damage any chrome coating on the liner and wreck it. You can see where the grit has been getting dragged against the bore and the piston,
they'll be lovely scores on that too.
MikeCapon - 30/7/08 at 11:06 AM
Don't worry about light scoring like that on a Bantam. They get light scoring like that very quickly as the air filter is a pretty rudimentary
affair which just about stops large rocks and not much else.
The liner on a Bantam is an iron jobby and it will have some similar wear but that's not a problem either. These were not exactly high
performance motors. The last Bantam was the D14/4. 14 HP and 4 gears. For a 175cc motor it's not exactly over stressed so a few scores and a
largish skirt to liner clearance is not a big deal.
Lastly, in terms of the length comparison the Bantam is an old fashioned, undersquare, piston ported two stroke. The other pistons are modern bike 4
stroke pistons so not really the same thing.
ETA Look at the rings. Unless they are very recent they are in good shape which indicates that the bore should not be too shot either.
[Edited on 30/7/08 by MikeCapon]
ReMan - 30/7/08 at 12:23 PM
For a Banty that looks mint!
oldtimer - 30/7/08 at 12:29 PM
Ah happy memories! I thought that looked pretty good for a used Bantam piston. I reused far worse and that one does not appear to have suffered any
partial seizures (yet). I would have a partial seizure every week during the summer! - I'd pull in the clutch, coast to a halt, roll a cigarette,
smoke it, and by then it had cooled down some and I'd ride on. The filter was a joke, hence the scoring.
02GF74 - 30/7/08 at 01:06 PM
parital seizure eh?
the person with this bike starts it, leaves it idling, it dies, then restarts it and repeat.
each time it becomes much harder to kick start; it got to the point when it was next to impossible to turn over - hence the taking apart of the
engine.
question is what is the likely cause?
ReMan - 30/7/08 at 02:04 PM
Overheating and siezing of the piston due to not riding off and gaining some cooling.
Also it has got some 2 stroke oil in it has'nt it?
daviep - 30/7/08 at 02:52 PM
His leg getting tired
Mr Whippy - 30/7/08 at 02:55 PM
quote:
Originally posted by 02GF74
question is what is the likely cause?
cheap oil? I use race stuff even for the still saw, never have any bother.
oldtimer - 30/7/08 at 03:22 PM
Why? - old petrol/oil mix.....incorrect petrol/oil ratio......when last decoked?....
MikeRJ - 30/7/08 at 06:41 PM
All sorts of things cause 2 strokes to seize, it's a wonder they don't do it more often tbh!
- Lean mixture. This is death to a two stroke, just as it is to a turbocharged 4 stroke engine.
- Ring gaps too tight.
- Piston/bore clearance too tight.
- Poor quality oil or inadequate oil supply.
- Incorrect ignition timing.
skodaman - 30/7/08 at 06:51 PM
I've used worse pistons. On a two-stroke they get like this very quickly. The rings look okay. As said airfilters on these in those days were
crap. Try to get a k and n to fit. Keep the timing spot on and use good two-stroke oil and it should be fairly reliable. Just be ready to whip the
clutch in just in case.