So has anybody measured their clearance. Or do you guys just fire the piston in the garbage and put a new one every year or two. My bike ran awesome just was making noise. the forged piston isn't going to break very easy. My point is that, maybe lots of them have collapsed pistons but they keep running so no one questioned it.
Has anyone every tried a cast piston? If they can last 3000 miles in a stock machine could they last 1000 in my bike. the forged one sure didn't.
Nah man... mod 400's are about as reliable and long living as they get.
I'd trust the 400 gurus way beyond anything you can get from a book.
Use their clearances and their break in procedures.
Most go real tight and use heat cycling to start with, from what I've gathered.
These are different animals from 250's and under...
I wouldn't let my polaris shop, or any of the surrounding 2 stroke builders touch mine.
It will get boxed up and sent out for service.
Probably to Rick Ritter.
I wish all you guys would join us at Gen 3 USA... especially you old polaris tech.
You're a lot of help.
So you saying go tight? or run the usual .003 (in a stock one)
So has anybody measured their clearance. Or do you guys just fire the piston in the garbage and put a new one every year or two. My bike ran awesome just was making noise. the forged piston isn't going to break very easy. My point is that, maybe lots of them have collapsed pistons but they keep running so no one questioned it.
Has anyone every tried a cast piston? If they can last 3000 miles in a stock machine could they last 1000 in my bike. the forged one sure didn't.
Stock is cast I think... it's what's still in mine for now...
I try to keep an ear open for noise...
Cast ones break when they get sloppy in the bore.
Your forged went south because you ran it that way I'd say.
__________________ '00 TRX400EX
A project in progress......brought to a screeching halt (3/09)....Time for a new 450R!
because I ran it what way? loose, ya I ran it loose cause cause it collapsed after 30 hours. The engine never seized or broke down. Which guru's are you talking about "old polaris guy"?
or the others that have said run it at .003-.0045 depending on mods
Old polaris guy has good points, but a forged piston at .002
In a motocross bike this is possible, but a motocross bike is likely to never see WFO for more than 5 seconds on a track. The plated alu liner comes into play here to.
All stock pistons are cast, doesn't matter what it is. A cast piston has many advantages, more thermally stable, better wear properties(slippery), more rigid (retain shape) But are likely to break when the clearance is out of spec. Most of the advantages are because of a high content of slicon.
So what do you guys think is actually happening inside the engine during "break-in"? Besides the rings breaking in. Some guides tell you to do a teardown early on and look for high spots on the piston, file them off and put it back together. makes sense but who would do that.
Has anyone here ever broke a relatively new cast psiton in a 400. Or are most of these cases, coming from someone putting a pipe on a half wore bike. upp the rpm 1500 or so and boom broken piston.
On the next build I might try a coated cast piston at tighter clearances. take it apart early and measure. an experiment
because I ran it what way? loose, ya I ran it loose cause cause it collapsed after 30 hours. The engine never seized or broke down. Which guru's are you talking about "old polaris guy"?
or the others that have said run it at .003-.0045 depending on mods
Old polaris guy has good points, but a forged piston at .002
No, because it was too loose to start with.
Set it tight, and break in slow/easy...
__________________ '00 TRX400EX
A project in progress......brought to a screeching halt (3/09)....Time for a new 450R!
So why does HPD and others reccomend more clearance? So they can sell more pistons.
Or is because tuning needs to be done after mods and it could get tight if it got a little lean. I don't beleive any of that. they have experience with things siezing on the dyno. I'd say
How long is break in? My cr 250 manual says 1 hour I beleive.
So then after it is broke in and it still has .002 clearance what is stoping it from seizing, if I pull a sand hill wide open for more than a minute. ????? experiment I guess.
But I'll stay close to home.
My bad... I've been on here surfing a lot late at night, due to sinus infection and double ear infection. So I haven't been real clear from one visit to this thread to the next.
I won't go against what Rick says, or OPT...
These are the people I'm saying you should listen to...
These folks have built countless 400L engines.
Now that I have a bit of medicine in me and 'think' I am thinking a little more clearly... lol...
Here are my thoughts at the moment...
1- Let the gurus do the work and earn their due.
2- If your piston is collapsed, I'm thinking it pretty much siezed brother.
Did it QUIT on you when hot at any point?
Did it ever die out sudden and bounce right back?
Or did you ever run it out of fuel quickly at high RPM/Speed, or think you did?
Well... what I really mean is that it came ever so close to seizing, but didn't actually lock up. Instead, the piston deformed some.
Nothing to do with rod angle I don't think, or the engine bending the pistion...
Instead, everything to do with almost blowing it up.
You may be a little off on your air/fuel somewhere... or off on your mix ratio.
Yet I doubt it's the mix ratio or oil type. I dunno, but I would want to say that film is all that saved you maybe. I don't know... it's hard to know what caused the piston to get too hot in this manner... Could have been the plug, or too much air/too little fuel... or too much or too little oil...
It would be very interesting to me to know the mix oil used and the ratio.
__________________ '00 TRX400EX
A project in progress......brought to a screeching halt (3/09)....Time for a new 450R!
