Need help on top end rebuild

[rpduc]

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.

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

[GMP]

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?

[jostby]

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.

[rpduc]

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.

Likely you can clean things up and go with a Vertex a size or two up from your cyl. size.

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?

[rpduc]

And a couple more pics.

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

[blitz11]

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

[gasgasman]

If you can't snag your finger nail on the grooves, you can use a ball type hone to clean up the cylinder.
I use one on my cylinder to knock the glaze off when I'm doing a top end.

[rpduc]

Thanks GGman. I was just talking to my Duc mechanic and he suggested the "dingleberry" hone too. That would be cool if I didn't already have a new Vertex "A" piston. At least I think I'd need an oversized one and prolly end up costing me as much as the replate.

Glenn, or anyone, I tore apart the powervalve and there is NO washer on the left side as shown in the exploded view in the Powervalve info in Tech Articles. There is a shoulder machined onto the end of the left side PV "cylinder" but no washer. I assume that would be that the missing washer you mentioned. Is that shoulder there on those bikes with washers too?

Also the "hat" shaped alu (?) bushing on the right side is SLOPPPPPY loose on the "shaft" and there is probably 2mm of end gap between it and the bearing if you push the bushing tight one way or the other. I hope that makes sense...

Oh, and one other stupid question. I pulled the nipple for the PV drain off and I thought it was plugged. It has just a pinhole opening in it. Like the drill didn't quite make it all the way through. Is that thing supposed to be that way or should I drill it out the rest of the way?

[GMP]

Ross,

OEM piston supplier has changed throughout the years. Up to '03 or '04 it was Wiseco, then Wossner, and Vertex. Thats why I said make sure you specify Vertex rings because "GasGas 250 rings" could be one of three parts! The same Vertex piston has always been OEM in the TM250, and I beleive available from Vertex through a distributor.

I was refering to the "hat" or shouldered washer on the right side. Those parts were hand press/tight slip fits on both of my previous bikes, the '07 PV has not been apart yet. I do not remember a shoulder on the left side drum valve, or the "hat" being a very sloppy fit on the right side drum. The drums do have a lot of axial play and clearance to the cylinder, so they can function when fouled, like a military firearm. I have a pic on my home PC somewhere of my '03 PV parts during disassambly, I'll try and find it tonight. I don't think any of this was changed from '03.

The drain nipple on the lower left side has a small hole from my experience, and some came with tubing zip tied shut, to limit oil dripping for emmisions reasons as the EC bike is street legal in Europe. The right side vent on the cylinder is also your trans vent and should have a larger hole.

[GMP]

Here are the PV parts from my '03: