So, I'm working on my second 400L Sportsman trying to get it up and running. The engine decided to eat its own piston - unfortunately this seems common? It's a low-mile machine that was used on a farm, and aside from the piston (skirts), the engine's components have looked very, very good.

Anyway, I've got the jug off and the water pump cover case ("Case 3?") off as well. The primary crank case halves are still together. Now I'm staring at the counterbalance shaft... I want (need?) to split the case to remove all the bits of piston which are currently frolicking around with the crankshaft. Obviously pulling this requires a slide hammer or special tool, which I can handle, but what should I be looking for/doing when I remove it to make sure that I goes back in smoothly and in time?

The bore appears to be well within the service limits and quite true, so I don't see any reason why I can't stick with an 83mm piston. I will be redoing all my measurements before ordering a piston, however - just to be sure. The question then becomes, which piston? Namura (cast) or Wiseco (forged)? Wiseco seems the be the most well-known, but they aren't universally recommended. Seems they have a tendency to expand too much and seize? Namura is cast, so not as strong, but that doesn't necessarily mean they are weak either. Cast pistons are still quite common. Namura's seem to work well for most people from what I can see. Haven't found any threads specifically discussing the 400Ls, however. Perhaps one of you here can make an informed recommendation on that?

All the bearings and seals on this motor look good, but while I'm here, I figure I'll replace them all. Are there any cases where I should leave the OEM seal or bearing in place if it shows no sign of damage or wear? I know not all aftermarket parts are equivalent to OEMs...

What other mods/maintenance should I be doing while I've got this thing torn down?

Thanks!

 

Wiseco is the best replacement piston for these cast pistons. Higher priced but worth it.Counter balance has a mark on the gear that aligns with the crank gear mark. Rather than just guessing on a lot of things,just get a manual that goes step by step as this lower end has quite a few parts. Just let this pdf manual load up. Chapter 7 is the engine. Plenty of info on what to do.https://www.google.com/url?sa=t&rct=...JCoJXuPwcXZmBQ

 

Got that already and reviewed it several times. Thanks! Just wanted to see if there was anything else I should be paying attention to.

Is there anything special/different with those motor (breaking, clearance, etc.) I should do when installing a forged piston vs the cast one?

 

I had one, a couple years ago, here's a couple things that I found out: Previous owner had disconnected the oil injection, and ran premix. Running premix slowly clogs up the muffler, causing all kinds of troubles, including trying to set it afire. I hooked the oil injection back up, gutted the plugged muffler, suddenly it ran great. Run injection oil only in the injection. I think running premix oil in the injection, will starve it for oil. Another thing that caused a problem, bushings on the rear cradle yoke wore out and made it impossible to shift into either reverse or fwd. I made new bronze bushings, suddenly, handling was great, I could shift. It was a fun, zippy little bike once I fixed all the boo-boo's. With the forged piston, make absolutely sure that the cooling system is working 100%. That's why I mentioned, checking the muffler for being plugged. Overheating will kill it fast.

 

The early 400s manual showed to allow .003 clearance between the cylinder and piston. Later models cut down the clearance to .0012-.0022. Scramblers had a little larger clearance than say the Sportsman 400.This was for obvious reasons in that people were going to run the devil out of them and Polaris didn't want people to cold seize one. On a forged Wiseco piston I still aimed for a .002 max clearance and broke it in easy at the shop.Cranked em up,let em idle until they were warm and shut em down.When cooled down I took them out for a ride. Same thing on take out cylinders. I just told people they were set up tight,how to break them in and never had a come back on my work on these.Plus don't forget to bleed the bolt on top of the head or they will over heat from trapped air pockets.

 

Doesn't really hurt them hydrex as we had many people disconnect the oil injector and premix. Problem is the ratio that some people used was too much. The injector system was calibrated for 50/1 from the factory. 40/1 is plenty enough to premix on a stock machine. Granted if you get people that mix 32/1 or more, then you can end up with fouling plugs,clogged up exhausts plus you actually loose power by having too much oil.Probably what happened to the po of your machine. This is one oil pump that I did encourage people to disconnect because of where it was located,up front.Unless you kept an eye on it water could corrode these pumps up and ruin engines.

 

Some good advice in there. This engine had the oiling system in place still, and it seemed to be working, but I will check it out. I don't plan on removing it.

Good tips on the break-in. Sounds like it won't be too much more difficult. Perhaps I'll send it through a few extra cycles?

The shifter on this one was, apparently, an older style (no markings on handle, sticker on fender). It was rusted up pretty bad and jammed in between gears at that. I'll be replacing it with the newer style one off of a parts bike.

The exhaust pipe, actually the "silencer" I think, came unwelded somehow and is hanging loose. Kinda melted the side of the rear fenders, actually - will be swapping those out too... I plan on simply re-welding the brackets, but I'll be taking the system apart and making sure it's not blocked. There were bits of piston in the pipe, afterall.

I'm just glad that there was no other damage. Honestly, if I knew for sure I could get all the bits of piston skirt out of the crankcase, I'd could just do a top-end on this one...

Can't wait to get this one running so that I can take the pair out for some fun!

 

Hard to get all the pieces and metal sludge out after a skirt shatters,but you could fill the crank case with kerosene several times and roll the rod around and dump it out. If there is ANY up and down play on the rod,the case will have to be split and a new rod kit installed along with crank bearings.I wouldn't guarantee just a top end for any one if this happened.I saw too many of them that people just took the short cut and the new top ends were taken out when lower rod bearings spit pieces up into the cylinder.Then you have to tear it back down and do it right.Actually can cost less to do one right.

 

My plan was to simply rebuild the whole thing from the get-go. Only after I started disassembling it, and seeing how good/tight/smooth everything was, did I consider short-cutting it. I'm pretty certain I'll go ahead and split it.

Should all the bearings be replaced even if they in good condition?

 

Yes replace the crank bearings,water pump bearing and seals and be sure to check the inner and outer bearing on the counter balancer. Again if you tear it down I'd replace the rod kit.You can go for cheap oem quality which is plenty good enough.WSM Polaris 400 Connecting Rod Kit 45-305 | eBay or go to pricier kits.Polaris 1995-2000 P400L Scrambler 2x4/4x4 Hotrods Connecting Rod kit | eBay