Tri z 250 bottom of cylinder leaking

[Paul G]

wondering if the bottom of the tri z 250 cylindrs warp? Has anyone used the ceramic gaskets? Photo is my son's yamahonki it has the tri z 250 in it.

[Doug8765]

wondering if the bottom of the tri z 250 cylindrs warp? Has anyone used the ceramic gaskets? Photo is my son's yamahonki it has the tri z 250 in it.

Crank seals leaking?


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[Paul G]

No I was cleaning some gas off the engine with brake clean and it died when I sprayed at bottom of cylinder where it meets the case. That's why I asked if the bottom of the cylinder warp and the gasket leaks. Or I have a bad gasket.

[El Camexican]

They do warp and so do the cases, or maybe they come like that , I don't know, but none of my 4 sets was flat when I checked them. To the point that no gasket would seal one of them, but Yamabond might have (not suggesting this, but in a pinch maybe?)

I got mine nice and flat by removing the cylinder studs and carefully lapping the cylinder to the cases with valve grinding compound. Never done it on an assembled engine though.

If you do go this route and happen to have the engine apart, make sure you put in every singe case bolt and dowel pin and tighten them before you start as the cases change shape as the bolts are tightened. I did my first one with only 4 bolts and the dowels in place and it was noticeable out of whack when I did the final assembly. Live and learn.

[Paul G]

Well I be checking the surfaces and see what I come up with and order a gasket kit thanks for your info

[Paul G]

Got top end off head has little dents all over in it. Cylinder is smooth and shinney and piston is sloppy. Previous owner probably didn't use a air filter i think. Also who ever had the cylinder off really messed up the surface of the case! There for that's why it started leaking. There are some big divits and scratches. I mean deep ones. I want to fill them in. Was thinking of jb weld. Anybody have a better idea ??

[El Camexican]

I don’t know how bad yours are, but I had some gouges on a set of cases that needed to be fixed. I used a punch to raise the areas around the edges of the sealing surface and then filed and lapped them flat. The low points from the punch don’t hurt anything, some guys do it intentionally to help hold the base gasket in place.

If it’s really bad you can get the cases skimmed on a mill and have the base of the cylinder turned on a lathe. Then just determine how much material was removed in total and add another base gasket or two to get the port heights back where they were.

[Paul G]

Great info i know I can do that on all but one spot. Have another qusetion. There are two hose ports one on the right side of cylinder bottom and one on the left in the case. When I bought the bike they had short hoses on them with screws in them. Where do they go. Or can the stay blocked off ? Any pics would help to . Thanks

[El Camexican]

The one on the cylinder is a vacuum for the stock fuel pump. If it’s blocked you must have a fuel tank mounted above the carb that feeds by gravity, if so it needs to remain blocked off. Please don’t remove the stock elbow and thread a bolt onto the hole in the cylinder. Someday that cylinder might be needed for a stock Tri Z and that would devalue it. Plus those elbows are hard to find.

The case vent on the left should not be blocked. Put a hose on it and run it behind the engine to just below the frame. It keeps you from blowing out oil seals when the engine gets hit.

On that “one spot” JB weld will probably work, just clean the heck out of the pocket before you fill it and make sure it’s perfecty flush with the rest of the mounting surface before you assemble.

Don’t forget the pressure test the engine after assembly!

[Paul G]

Cool thanks i have left them the way they were when I got it so I will get a cap for the pump ones an hose for the other . Thank you very much I'll order my parts Tuesday and get to work on the surfaces. Let you know how it goes.

[Paul G]

On have one more question have to bore cylinder. Going to get a 68.5 mm piston i found ones that say there light weight. Is that better for performance or should I go a regular weight piston?

[El Camexican]

On have one more question have to bore cylinder. Going to get a 68.5 mm piston i found ones that say there light weight. Is that better for performance or should I go a regular weight piston?

I suggest a Wiesco piston and no other brand. Hand it to the guy doing the bore job along with the papers it comes with. Make sure you get a Tri Z piston and NOT a YZ piston.

[Paul G]

Yeah copy that on the tri z not yz piston. Bought a wiesco for my 200x and 100 r haven't had a problem so buying same brand. Have all my surfaces cleaned and smooth jb welded the deep spot need to level it out parts are on there way. What do you think of a reed spacer. Have you ever use one.

[El Camexican]

What do you think of a reed spacer. Have you ever use one.

Never used one. Suggest you use the money for a fresh set of Boyseen reeds, or a complete V Force reed block instead.

If you’re looking for maximum power, get the transfer ports and the exhaust port raised a little. When you bore a two stroke it lowers the ports due to them being angled upward. The larger you bore, the lower they get and that is essentially like putting a smaller cam in a four stroke. You don’t have to do this, but if you’re looking for this thing to run as strong as possible it’s something that doesn’t cost a lot of money to get done.

[Paul G]

I've read on the porting and polishing the cylinder in one of the forums raising them 2 mmm for performance and adding another boost port. I do have a port and polishing kit. Do have time to go that route just need to make a measuring tool not to hard i think. Don't want to go over board my son rides the bike got to keep the front end on the ground.!!!