piston installation

[johnhab]

Does anyone have any advice about installing a new piston(99 YZ400). I have been having a frustrating time trying to get the rings compressed while inserting the new piston(wiseco) back into the cylinder. The ring gaps are within spec. The oil rings don't want to go in. Is it possible to do this by hand or do you have to use a ring compressor? I have done numerous two stroke top ends with no problems.

[sirthumpalot]

IMHO a ring compressor for a 4-stroke top end job is really worth the few bucks that they cost. Once you have it together in the ring compressor you just hold it up to the cylinder and then push the piston right on in. As long as you keep it tight against the cylinder the piston should slide right in.

If the rings are having a hard time actually fitting in the piston because they're too wide or for some other reason then a ring compressor is not the answer; the rings should fit into the piston very easily. The compressor just holds them in while you push the piston into the cylinder.

[MikeOK]

I just put a piston kit in mine too, also had a terrible time the first try getting the oil rings all in there right. Someone on here suggested using a big hose clamp and it sounded like a good idea, much cheaper than a ring compressor and probably just as good. I did finally get mine in with my fingers but it was a major pain.

[Looped_Out]

I WAS going to istall a Wiseco piston in my bike too, but their top end kit had the WRONG rings! After a lot of back and forth BS with Wiseco (plus I heard that the pistons are not as reliable as stockers), I ended up putting a new stocker in. Runs great. I did use a ring compressor, but I think I could have done without. If you have the tool, use it.

[MikeOK]

I questioned that with mine too. When I first put the wiseco piston and rings in it I was getting some smoke on a cold start, just for a few seconds but enough to bother me. I took it back apart and measured the ring end gaps again (I didn't check the oil rings at first, only the compression rings). The wiseco oil ring end gaps were nearly 3 times the limit. I took a chance and ended up putting the stock oil rings in the wiseco piston and everything is fine. I have several hours on it now and no smoke, runs great. My dealer told me that wiseco always runs their oil rings loose like that but 3 times the stock limit is too much. And my stockers were still in good condition.

[WR280f]

I have a tip that might help.Try installing the piston in the cylinder before putting the piston on the con. rod,put the cylinder on your work bench upside down and install the piston in the cyl.from the bottom,you can push gently down on the piston and push the rings slowly in with something soft,I use a popsicle stick or a piece of plastic.Work slowly around the piston while pushing down,and make sure no to overlap the center end of the oil ring. Also install one of the pin clips before installing the piston.Leave the piston sticking out of the bottom of the cyl. just far enough to put the pin and second clip in.this is far easier than trying to install the cyl.over the piston while the piston is wobbling about on the rod. don't forget to put a rag around the bottom of the rod(in case you drop the clip) and to put the base gasket on before the cylinder. ?

[criss]

yeah,its a pain in the ass,i've done mine a couple of times as per the manual,and it gets easier,(but not that easy)- just make sure you maintain the proper positioning of the ring gaps relative to each other.