2 stroke guys..got a question for ya

[ram57ta]

Ok...so I blew the top end on my RMX250 last year...piston melted causing it to seize...which tore up the jug somewhat. Its gonna cost a penny or two to repair the jug and then buy new piston and ring. Thats the story.

Yesterday I found a same year RM 250 for sale on CL...the guy just finished rebuilding the topend with a new jug and new Wiseco piston. He ran it for all of about a minute and then something went wrong with the electrical system so he gave up and sold it to me for 200 bucks. Got it home and tore it down to pull the engine for a complete swap and then started looking at it. The bottom end of this thing looks pretty wore out. Looks like most of the seals leak. The kickstart lever is so worn out I cant even get it off the gear..its all chewed up. Overall I think my bottom end on the RMX is in much better shape. The jug and head is identical. Could I just pull the jug and piston off the rod and swap the whole assembly with no ill effects? It looks rather easy and I think I could even do it without pulling the piston out of the jug so I won't have to mess with compressing the ring to put it back in. Looks like some snap ring holds the wrist pin in and would easily come out. Opinions?

[Buick355]

Should be a snap ring on each end of the wrist pit. Just leave the piston in the cylinder and lift it off the bottom end and swap it to the other bike. Just check your X dimension after the swap to make sure the compression's right and there's no piston to head contact.

[SNEAKY]

Robin, just a question. could a seize engine had hurt the rod any?

[01YZ2NV]

Should be a snap ring on each end of the wrist pit. Just leave the piston in the cylinder and lift it off the bottom end and swap it to the other bike. Just check your X dimension after the swap to make sure the compression's right and there's no piston to head contact.

How are you gonna leave the piston in the cylinder and reach the clips on the wrist pin at the same time ?

[TX21]

Robin, just a question. could a seize engine had hurt the rod any?


Some do some don't. Most of the time the motor dies before any damage is done to the crank.

01YZ

The piston can be taken off the rod while still in the jug, I have done it.

[Shoot To Kill]

well, from my point of view (being someone who knows jack shit about this stuff) is that, you're basically doing the exact same thing he did. putting a new top end on the exact same bottom end. so, in my eyes, yeah, it'll work. but you gotta watch out for that electrical problem, maybe it was that new top end that caused it. :confused: but like i said, haha, i know jack shit about bikes.

[mikeb]

How are you gonna leave the piston in the cylinder and reach the clips on the wrist pin at the same time ?

My question exactly. And why would you want to do that anyway? If you are going to install used parts at least take them apart and make sure that you are not installing a part with a problem. It is not that hard to compress a couple of rings by hand and get the piston started up into the cylinder.

As far as seizing the engine, two strokes use needle bearings at each end of the rod (with thrust washers on the big end) and ball bearings for the ends of the crankshaft. Running out of oil or getting them hot enough to seize USUALLY, in my experience, does not hurt these bearings. I fixed many a yamaha whose owner let the oil tank run dry and most of the time we'd just bore the cylinder, inspect the rod and the crank, check the thrust washers for galling and excessive side to side clearance, check the crankshaft seals, and put it back together again. You want to check the rod for straight up and down slack (there should be none) and visually check the thrust washers on either side of the big end of the rod (you can usually see them) and check the small end of the rod for any galling, etc where the needle bearing runs. Also make sure that there is not metal crap in the crankcase or exhaust pipe.

One thing you do want to be careful of is to make sure that the ends of the rings line up with the locating pins in the ring lands before putting the cylinder back on. Also, when you put the circlips back into the piston to hold in the wristpin put a backward spin on them with needle nose pliers. This ensures that the clip is seated in the groove. I'd recommend getting some new circlips and a new wristpin bearing actually. Use some good sealer on the base gasket to avoid air leaks (yamabond or equivilent). Lube piston/rings/rod bearings liberally with straight 2 stroke oil.

[ram57ta]

Well...I pulled the entire top-end off the RM250 block...with the piston still in the cylinder. It was pretty easy. Cleaning off the gasket on the base was a pain but I got it done, reinstalled it on my RMX block, put everything back on..and tried it. It fired up on the second kick and ran smooth as butter. As for re-using used parts....maybe you didnt read the first post. All the parts are new. The piston looked to be fresh out of the box, it hasn't even been broken in. Since the bikes been sitting in storage for a year...the carb float leaks like a bitch so I'm gonna have to rebuild the carb. The petcock is gummed up so looks like I'm rebuilding/cleaning that too.

[mikeb]

Well...I pulled the entire top-end off the RM250 block...with the piston still in the cylinder. It was pretty easy. Cleaning off the gasket on the base was a pain but I got it done, reinstalled it on my RMX block, put everything back on..and tried it. It fired up on the second kick and ran smooth as butter. As for re-using used parts....maybe you didnt read the first post. All the parts are new. The piston looked to be fresh out of the box, it hasn't even been broken in. Since the bikes been sitting in storage for a year...the carb float leaks like a bitch so I'm gonna have to rebuild the carb. The petcock is gummed up so looks like I'm rebuilding/cleaning that too.

I read the first post, but my statement about inspecting the piston still stands. You never know if the person might have broken a ring or left out a circlip, or left in an extra circlip LOL..... Still, sounds like you got it fixed and that is all that counts.