Need help on top end rebuild
 

Hi all,
Would appreciate some advice on condition of my top end. The last time I had a 2T top end off there was no talk of Nikasil, power valves, etc. :rolleyes: :o

I yanked the head to measure squish in preparation for the RB head mod and after seeing condition pulled the cyl. She's been rattling a bit too... I think I'm in need of some piston /cyl work. Both piston and cyl show some scoring and the wrist pin has noticeable wear. I can't detect any end play in the big end bearing. I think it's a Wiseco piston in there now. It looks nothing like the new-in-box Vertex TM250 piston from with the batch of parts that came with the bike. They are both marked 66.33mm.

How does one decide if a cylinder needs replating or maybe over-boring? How much wear can be fixed by replating?

Any experience with Millenium Technology as a replating service?

TIA for any advice.

Ross

Ross,

Post a pic. Clyinders can be made like new with a replate, but unless the plating is flaking, deeply scored to the aluminum, or worn very bad its probably not neccessary. Nikasil is very hard stuff. These are fixed with replating and diamond honing to size, no overboring. Even damage to the aluminum like gouges can be fixed by welding before replating so a two stroke cyl rarely needs to be replaced. As far as measurements, unless you have an accurate bore mic or take it to a shop that does, don't waste your time, the errors introduced can be greater than what you are trying to determine. Likely you can clean things up and go with a Vertex a size or two up from your cyl. size. If its making noise thats bad as the shirt fatigues and eventually cracks. Thats why you change pistons even if they appear in good shape.

I'm pretty sure a 66.33mm is an "A" size Vertex. Wiseco only used two sizes. Is your cylinder stamped "A" near the rear base?

66.33 is the "A" piston. If you end up needing replating I used Millenium last fall. They seemed very proffessional each time I called them. Turn-around was quick and the finished product was excellent.

Thanks for the info Glenn and Jostby. The Vertex is obviously an "A" piston so I assume the Wiseco(?) is too since it's the same dia. The cyl is stamped "A" too.

So how thick is that Nikasil?? From your explanation I'd say that the scoring is minimal at worst. I can still see a trace of the old crosshatching in the worst area. (can even see it in the pic) There is a barely perceptible "step" in the cyl wall at TDC.

If I can get away with just honing and an oversize piston what's the procedure for selecting the right piston? I'm guessing my buddy Fred at Silverback Performance has a bore mic. I don't think he messes with two smokes much but he could probably mic it for me.

I checked compression before I tore it apart and it was 160... 168 after squirting in a shot of WD-40 per the instructions on RBD's website.

There was only one very thin green base gasket.

The other thing I noticed is there seems to be a LOT of end play in the shaft (?) of the powervalve. The shaft (and bearings) slide back and forth between those socket head retention screws a good 1mm or more. Is that normal?

http://www.solomonrossfurniture.com/...piston/Cyl.jpg

http://www.solomonrossfurniture.com/...vertex_top.jpg

http://www.solomonrossfurniture.com/...iseco_side.jpg

http://www.solomonrossfurniture.com/...wiseco_top.jpg

And a couple more pics.

What's the idea behind that much deeper cavity above the wrist pin on the Wiseco?

http://www.solomonrossfurniture.com/...ertex_side.jpg

http://www.solomonrossfurniture.com/...iseco_port.jpg

If that were my bike, i'd get it replated. I had my kid's KTM redone by US Chrome for something like $150. I wouldn't be comfortable honing something like that.

I also used Millenium tech on a coworker's cr250r. They did a nice job, too.

for $150, i'd plate.

blitz

Ross,

The Nikasil is only a few thou thick. Is the scoring scratches in the plating, or deposited aluminum from the piston? It looks like that piston partially seized at some time. If its scored into the plating then yeah I'd replate it, especially if your going to keep it awhile. Send your "A" piston and have it finnished honed to "A" size. If those marks are aluminum stuck to the plating, they can be removed with muriatic acid, CAREFULLY. Then clean it up with a scotchbrite pad, and use a "B" or "C" Vertex piston.

Since you already have an "A" piston kit, a replate is the way to go as far as cost effectivness as long as you can wait for turnaround.

The Wiseco is forged and the Vertex is cast, so there will be some design differences.

The PV (all PVs for that matter) is a sloppy mechanism by design because it must operate in a dirty environment, however you may be missing a washer\spacer between the bearing and drum valve. This happened to someone else here once. Consult the parts manual. This is a nice PV design as its simple and uses all steel parts, easy to clean up. Also, just toss the bearings and replace as they are cheap.

Thanks blitz, Glenn

Well "kit" might be stretching it. I have a piston. No rings, wrist pin, little end bearing... but good point.

I don't feel scoring per se in the cylinder, like you say I think it's deposited alu. I'll try the muriatic acid just out of curiosity, but will probably get it replated. Millenium is only about 4 hrs away and we've got some ugly weather coming so I can wait the 7 - 10 days for Millenium to do their thing.

I will check the parts diagram and see if the PV is all there.

When you get rings, remember to get Vertex rings. Same rings for all micro size pistons (A,B,C,D).

Did I read somewhere that the Vertex piston is stock?

Then Go Fasters should have rings, wrist pin etc. no? Some other supplier for Vertex stuff?

I'm also intrigued by some of the talk I saw about port timing a while back.

http://www.gasgasrider.org/forum/sho...3296#post23296

As long as it's apart I figured I might as well check and see if it's at least close. Lots of theorizing but I didn't find any concise info on how one goes about checking it or what it "should" be. I measured the single green gasket that was on there and it's about .30mm