A 400 xplorer piped, ported, head cut (140 psi compression) 85mm wiseco piston, ran awesome for 40 hours 600 miles. then sometime in the last 10 hours piston slap got worse than nornmal. It still had all the compression, ran great. So I took it a part to check it out anyway. So the piston slap was the noise .013" clearance. had .004" when fresh. The bore looks great took two passes on the sunnen rigid hone at work to clean up. The piston has very little visible wear. But it measures .010 bigger across the wirst pin then the skirt. When new it would be the other way around. oval shaped the other way. So the piston has pretty much been bent by the motor. This is usually called a collapsed piston. Is this normal, a fact of life with a wiseco piston in a motor with to much rod angle.

How many hours before a teardown for a you mod 400 guys?

 

FIRST off the .004 original clearance was way too much !! Piston slap will occur at about .005. When I bore cylinders for take out or in shop repair the clearance after boring and honing on these 250,300,400 2-strokes I try to maintain between .0015-.002 clearance! I've had this discussion several times on the forums and if bored and honed properly this should be the clearance between piston skirt and bottom of cylinder if you want the piston to last,BUT (big but) you have to break-in properly and slowly to get maximum life from the top end. Plus if ANY excess bearing play in rod or crank bearings can cause the top end to wear out too quick by ovaling the cylinder. I've seen this a lot over the years with just top end repair only. Whether Wiseco, Sudco, OEM,etc doesnt make any difference,will wear quickly! OPT

 

Well the books I have show .0023-.0037 clearance. I have seen a really old book at the dealer that says what you are saying, .0015-.002 I realise that piston wear or collapse will excelerate when clearances increase. But I also understand that when increasing an engines power output, rpm and likelyhood of the engine to run WFO. one should also increase the clearance. Is this not as widespread of an opinion as commonly thought?

The crank bearings were also new when this engine was fresh. The rod bearing was checked for clearance with a dial indicator. The bore still measures straight and round (within a half thou)

The piston still has machine marks on it. slight wear on the intake skirt. The piston has collapsed not wore. Lots of modified scramblers, sports and others out there with wiseco pistons put together with twice the clearance you reccomend. How long do people wait before a freshening. I get a little longer out of a cr250 I guess.

So I know now what a stock motor with a cast piston can run with. What clearance do you reccomend for forged pistons running 6500-7000 rpm 15 hp more than stock. let's say a 83mm and a 87mm big bore.

Thanks for your reply, I hope to hear more from you and others.

 

What do you consider proper break-in ?

 

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.

 

The ones I've run across tended to require quite a bit of work but on a regular basis but I haven't been around tons of them. They do go like stink though. Faster than they have any right to be. I've seen old beat up popo 400s absolutely spank new sport quads much to the utter shock and horror of their owners.

 

As far as increasing piston to cylinder clearance,the ONLY ones I've ever allowed myself to do is on the pwc's because of rapid piston expansion,a lot more rapidly expanding than any atv motor.Raw cold lake water and real tight pistons dont make for a lasting engine! On the old 650,750,785 Polaris water craft the usual factory recommendation was .005 clearance with .008 max wear limit,NOT too much to play with,but with proper break-in,that is warm up slowly,no wide open throttle,allowing time to fully cool down for the first 3-4 hrs, I could get by with .004 final clearance with customers aware of how to break-in and never had a problem! Even with milled heads,cylinder porting,etc,I didn't increase clearances!! Same with atvs that are modded,I dont increase clearance! Longevity begins with slow break-in,using proper oils and filteration. Just from my experience over the years and I know some will differ from me on clearances and such,but I try to get the most life from an engine that I can. OPT

 

Well .005 in a watercraft sounds more in line with snowmobile engine. I'm not surprised by that. A snowmobile running a 85 mm piston could be as much as .006".

 

I've been racing my Polaris Sport all year long about 2-3 times a month with no problems.It has a "highly" ported cylinder and 84mm piston in it from Rick Ritter, Hot Seat head with a 12-1 compression ratio dome, 42mm TMX carb, race pipe, average size shot of nitrous oxide and a few other bolt-on parts.At the point that I'm at I have to worry about replacing belts more than anything else.

 

See... now there is some overwhelming support for what I said in my post before...




>P.S. who did the porting on the "Killer Red Tomato"?